client.py

# Copyright 2015 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Client for interacting with the Google Cloud Storage API."""

import base64
import binascii
import collections
import datetime
import functools
import json
import warnings
import google.api_core.client_options

from google.auth.credentials import AnonymousCredentials

from google import resumable_media

from google.api_core import page_iterator
from google.cloud._helpers import _LocalStack, _NOW
from google.cloud.client import ClientWithProject
from google.cloud.exceptions import NotFound
from google.cloud.storage._helpers import _get_default_headers
from google.cloud.storage._helpers import _get_environ_project
from google.cloud.storage._helpers import _get_storage_host
from google.cloud.storage._helpers import _BASE_STORAGE_URI
from google.cloud.storage._helpers import _DEFAULT_STORAGE_HOST
from google.cloud.storage._helpers import _bucket_bound_hostname_url
from google.cloud.storage._helpers import _add_etag_match_headers
from google.cloud.storage._http import Connection
from google.cloud.storage._signing import (
    get_expiration_seconds_v4,
    get_v4_now_dtstamps,
    ensure_signed_credentials,
    _sign_message,
)
from google.cloud.storage.batch import Batch
from google.cloud.storage.bucket import Bucket, _item_to_blob, _blobs_page_start
from google.cloud.storage.blob import (
    Blob,
    _get_encryption_headers,
    _raise_from_invalid_response,
)
from google.cloud.storage.hmac_key import HMACKeyMetadata
from google.cloud.storage.acl import BucketACL
from google.cloud.storage.acl import DefaultObjectACL
from google.cloud.storage.constants import _DEFAULT_TIMEOUT
from google.cloud.storage.retry import DEFAULT_RETRY
from google.cloud.storage.retry import ConditionalRetryPolicy


_marker = object()


class Client(ClientWithProject):
    """Client to bundle configuration needed for API requests.

    :type project: str or None
    :param project: the project which the client acts on behalf of. Will be
                    passed when creating a topic.  If not passed,
                    falls back to the default inferred from the environment.

    :type credentials: :class:`~google.auth.credentials.Credentials`
    :param credentials: (Optional) The OAuth2 Credentials to use for this
                        client. If not passed (and if no ``_http`` object is
                        passed), falls back to the default inferred from the
                        environment.

    :type _http: :class:`~requests.Session`
    :param _http: (Optional) HTTP object to make requests. Can be any object
                  that defines ``request()`` with the same interface as
                  :meth:`requests.Session.request`. If not passed, an
                  ``_http`` object is created that is bound to the
                  ``credentials`` for the current object.
                  This parameter should be considered private, and could
                  change in the future.

    :type client_info: :class:`~google.api_core.client_info.ClientInfo`
    :param client_info:
        The client info used to send a user-agent string along with API
        requests. If ``None``, then default info will be used. Generally,
        you only need to set this if you're developing your own library
        or partner tool.

    :type client_options: :class:`~google.api_core.client_options.ClientOptions` or :class:`dict`
    :param client_options: (Optional) Client options used to set user options on the client.
        API Endpoint should be set through client_options.
    """

    SCOPE = (
        "https://www.googleapis.com/auth/devstorage.full_control",
        "https://www.googleapis.com/auth/devstorage.read_only",
        "https://www.googleapis.com/auth/devstorage.read_write",
    )
    """The scopes required for authenticating as a Cloud Storage consumer."""

    def __init__(
        self,
        project=_marker,
        credentials=None,
        _http=None,
        client_info=None,
        client_options=None,
    ):
        self._base_connection = None

        if project is None:
            no_project = True
            project = "<none>"
        else:
            no_project = False

        if project is _marker:
            project = None

        kw_args = {"client_info": client_info}

        # `api_endpoint` should be only set by the user via `client_options`,
        # or if the _get_storage_host() returns a non-default value.
        # `api_endpoint` plays an important role for mTLS, if it is not set,
        # then mTLS logic will be applied to decide which endpoint will be used.
        storage_host = _get_storage_host()
        kw_args["api_endpoint"] = (
            storage_host if storage_host != _DEFAULT_STORAGE_HOST else None
        )

        if client_options:
            if type(client_options) == dict:
                client_options = google.api_core.client_options.from_dict(
                    client_options
                )
            if client_options.api_endpoint:
                api_endpoint = client_options.api_endpoint
                kw_args["api_endpoint"] = api_endpoint

        # Use anonymous credentials and no project when
        # STORAGE_EMULATOR_HOST or a non-default api_endpoint is set.
        if (
            kw_args["api_endpoint"] is not None
            and _BASE_STORAGE_URI not in kw_args["api_endpoint"]
        ):
            if credentials is None:
                credentials = AnonymousCredentials()
            if project is None:
                project = _get_environ_project()
            if project is None:
                no_project = True
                project = "<none>"

        super(Client, self).__init__(
            project=project,
            credentials=credentials,
            client_options=client_options,
            _http=_http,
        )

        if no_project:
            self.project = None

        self._connection = Connection(self, **kw_args)
        self._batch_stack = _LocalStack()

    @classmethod
    def create_anonymous_client(cls):
        """Factory: return client with anonymous credentials.

        .. note::

           Such a client has only limited access to "public" buckets:
           listing their contents and downloading their blobs.

        :rtype: :class:`google.cloud.storage.client.Client`
        :returns: Instance w/ anonymous credentials and no project.
        """
        client = cls(project="<none>", credentials=AnonymousCredentials())
        client.project = None
        return client

    @property
    def _connection(self):
        """Get connection or batch on the client.

        :rtype: :class:`google.cloud.storage._http.Connection`
        :returns: The connection set on the client, or the batch
                  if one is set.
        """
        if self.current_batch is not None:
            return self.current_batch
        else:
            return self._base_connection

    @_connection.setter
    def _connection(self, value):
        """Set connection on the client.

        Intended to be used by constructor (since the base class calls)
            self._connection = connection
        Will raise if the connection is set more than once.

        :type value: :class:`google.cloud.storage._http.Connection`
        :param value: The connection set on the client.

        :raises: :class:`ValueError` if connection has already been set.
        """
        if self._base_connection is not None:
            raise ValueError("Connection already set on client")
        self._base_connection = value

    def _push_batch(self, batch):
        """Push a batch onto our stack.

        "Protected", intended for use by batch context mgrs.

        :type batch: :class:`google.cloud.storage.batch.Batch`
        :param batch: newly-active batch
        """
        self._batch_stack.push(batch)

    def _pop_batch(self):
        """Pop a batch from our stack.

        "Protected", intended for use by batch context mgrs.

        :raises: IndexError if the stack is empty.
        :rtype: :class:`google.cloud.storage.batch.Batch`
        :returns: the top-most batch/transaction, after removing it.
        """
        return self._batch_stack.pop()

    @property
    def current_batch(self):
        """Currently-active batch.

        :rtype: :class:`google.cloud.storage.batch.Batch` or ``NoneType`` (if
                no batch is active).
        :returns: The batch at the top of the batch stack.
        """
        return self._batch_stack.top

    def get_service_account_email(
        self, project=None, timeout=_DEFAULT_TIMEOUT, retry=DEFAULT_RETRY
    ):
        """Get the email address of the project's GCS service account

        :type project: str
        :param project:
            (Optional) Project ID to use for retreiving GCS service account
            email address.  Defaults to the client's project.
        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type retry: google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy
        :param retry:
            (Optional) How to retry the RPC. See: :ref:`configuring_retries`

        :rtype: str
        :returns: service account email address
        """
        if project is None:
            project = self.project

        path = f"/projects/{project}/serviceAccount"
        api_response = self._get_resource(path, timeout=timeout, retry=retry)
        return api_response["email_address"]

    def bucket(self, bucket_name, user_project=None):
        """Factory constructor for bucket object.

        .. note::
          This will not make an HTTP request; it simply instantiates
          a bucket object owned by this client.

        :type bucket_name: str
        :param bucket_name: The name of the bucket to be instantiated.

        :type user_project: str
        :param user_project: (Optional) The project ID to be billed for API
                             requests made via the bucket.

        :rtype: :class:`google.cloud.storage.bucket.Bucket`
        :returns: The bucket object created.
        """
        return Bucket(client=self, name=bucket_name, user_project=user_project)

    def batch(self):
        """Factory constructor for batch object.

        .. note::
          This will not make an HTTP request; it simply instantiates
          a batch object owned by this client.

        :rtype: :class:`google.cloud.storage.batch.Batch`
        :returns: The batch object created.
        """
        return Batch(client=self)

    def _get_resource(
        self,
        path,
        query_params=None,
        headers=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
        _target_object=None,
    ):
        """Helper for bucket / blob methods making API 'GET' calls.

        Args:
            path str:
                The path of the resource to fetch.

            query_params Optional[dict]:
                HTTP query parameters to be passed

            headers Optional[dict]:
                HTTP headers to be passed

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

            _target_object (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                :class:`~google.cloud.storage.bucket.blob`, \
            ]):
                Object to which future data is to be applied -- only relevant
                in the context of a batch.

        Returns:
            dict
                The JSON resource fetched

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.
        """
        return self._connection.api_request(
            method="GET",
            path=path,
            query_params=query_params,
            headers=headers,
            timeout=timeout,
            retry=retry,
            _target_object=_target_object,
        )

    def _list_resource(
        self,
        path,
        item_to_value,
        page_token=None,
        max_results=None,
        extra_params=None,
        page_start=page_iterator._do_nothing_page_start,
        page_size=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
    ):
        api_request = functools.partial(
            self._connection.api_request, timeout=timeout, retry=retry
        )
        return page_iterator.HTTPIterator(
            client=self,
            api_request=api_request,
            path=path,
            item_to_value=item_to_value,
            page_token=page_token,
            max_results=max_results,
            extra_params=extra_params,
            page_start=page_start,
            page_size=page_size,
        )

    def _patch_resource(
        self,
        path,
        data,
        query_params=None,
        headers=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=None,
        _target_object=None,
    ):
        """Helper for bucket / blob methods making API 'PATCH' calls.

        Args:
            path str:
                The path of the resource to fetch.

            data dict:
                The data to be patched.

            query_params Optional[dict]:
                HTTP query parameters to be passed

            headers Optional[dict]:
                HTTP headers to be passed

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

            _target_object (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                :class:`~google.cloud.storage.bucket.blob`, \
            ]):
                Object to which future data is to be applied -- only relevant
                in the context of a batch.

        Returns:
            dict
                The JSON resource fetched

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.
        """
        return self._connection.api_request(
            method="PATCH",
            path=path,
            data=data,
            query_params=query_params,
            headers=headers,
            timeout=timeout,
            retry=retry,
            _target_object=_target_object,
        )

    def _put_resource(
        self,
        path,
        data,
        query_params=None,
        headers=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=None,
        _target_object=None,
    ):
        """Helper for bucket / blob methods making API 'PUT' calls.

        Args:
            path str:
                The path of the resource to fetch.

            data dict:
                The data to be patched.

            query_params Optional[dict]:
                HTTP query parameters to be passed

            headers Optional[dict]:
                HTTP headers to be passed

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

            _target_object (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                :class:`~google.cloud.storage.bucket.blob`, \
            ]):
                Object to which future data is to be applied -- only relevant
                in the context of a batch.

        Returns:
            dict
                The JSON resource fetched

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.
        """
        return self._connection.api_request(
            method="PUT",
            path=path,
            data=data,
            query_params=query_params,
            headers=headers,
            timeout=timeout,
            retry=retry,
            _target_object=_target_object,
        )

    def _post_resource(
        self,
        path,
        data,
        query_params=None,
        headers=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=None,
        _target_object=None,
    ):
        """Helper for bucket / blob methods making API 'POST' calls.

        Args:
            path str:
                The path of the resource to which to post.

            data dict:
                The data to be posted.

            query_params Optional[dict]:
                HTTP query parameters to be passed

            headers Optional[dict]:
                HTTP headers to be passed

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

            _target_object (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                :class:`~google.cloud.storage.bucket.blob`, \
            ]):
                Object to which future data is to be applied -- only relevant
                in the context of a batch.

        Returns:
            dict
                The JSON resource returned from the post.

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.
        """

        return self._connection.api_request(
            method="POST",
            path=path,
            data=data,
            query_params=query_params,
            headers=headers,
            timeout=timeout,
            retry=retry,
            _target_object=_target_object,
        )

    def _delete_resource(
        self,
        path,
        query_params=None,
        headers=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
        _target_object=None,
    ):
        """Helper for bucket / blob methods making API 'DELETE' calls.

        Args:
            path str:
                The path of the resource to delete.

            query_params Optional[dict]:
                HTTP query parameters to be passed

            headers Optional[dict]:
                HTTP headers to be passed

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

            _target_object (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                :class:`~google.cloud.storage.bucket.blob`, \
            ]):
                Object to which future data is to be applied -- only relevant
                in the context of a batch.

        Returns:
            dict
                The JSON resource fetched

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.
        """
        return self._connection.api_request(
            method="DELETE",
            path=path,
            query_params=query_params,
            headers=headers,
            timeout=timeout,
            retry=retry,
            _target_object=_target_object,
        )

    def _bucket_arg_to_bucket(self, bucket_or_name):
        """Helper to return given bucket or create new by name.

        Args:
            bucket_or_name (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                 str, \
            ]):
                The bucket resource to pass or name to create.

        Returns:
            google.cloud.storage.bucket.Bucket
                The newly created bucket or the given one.
        """
        if isinstance(bucket_or_name, Bucket):
            bucket = bucket_or_name
            if bucket.client is None:
                bucket._client = self
        else:
            bucket = Bucket(self, name=bucket_or_name)
        return bucket

    def get_bucket(
        self,
        bucket_or_name,
        timeout=_DEFAULT_TIMEOUT,
        if_metageneration_match=None,
        if_metageneration_not_match=None,
        retry=DEFAULT_RETRY,
    ):
        """API call: retrieve a bucket via a GET request.

        See
        https://cloud.google.com/storage/docs/json_api/v1/buckets/get

        Args:
            bucket_or_name (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                 str, \
            ]):
                The bucket resource to pass or name to create.

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            if_metageneration_match (Optional[long]):
                Make the operation conditional on whether the
                blob's current metageneration matches the given value.

            if_metageneration_not_match (Optional[long]):
                Make the operation conditional on whether the blob's
                current metageneration does not match the given value.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

        Returns:
            google.cloud.storage.bucket.Bucket
                The bucket matching the name provided.

        Raises:
            google.cloud.exceptions.NotFound
                If the bucket is not found.

        Examples:
            Retrieve a bucket using a string.

            .. literalinclude:: snippets.py
                :start-after: START get_bucket
                :end-before: END get_bucket
                :dedent: 4

            Get a bucket using a resource.

            >>> from google.cloud import storage
            >>> client = storage.Client()

            >>> # Set properties on a plain resource object.
            >>> bucket = client.get_bucket("my-bucket-name")

            >>> # Time passes. Another program may have modified the bucket
            ... # in the meantime, so you want to get the latest state.
            >>> bucket = client.get_bucket(bucket)  # API request.

        """
        bucket = self._bucket_arg_to_bucket(bucket_or_name)
        bucket.reload(
            client=self,
            timeout=timeout,
            if_metageneration_match=if_metageneration_match,
            if_metageneration_not_match=if_metageneration_not_match,
            retry=retry,
        )
        return bucket

    def lookup_bucket(
        self,
        bucket_name,
        timeout=_DEFAULT_TIMEOUT,
        if_metageneration_match=None,
        if_metageneration_not_match=None,
        retry=DEFAULT_RETRY,
    ):
        """Get a bucket by name, returning None if not found.

        You can use this if you would rather check for a None value
        than catching an exception:

        .. literalinclude:: snippets.py
            :start-after: START lookup_bucket
            :end-before: END lookup_bucket
            :dedent: 4

        :type bucket_name: str
        :param bucket_name: The name of the bucket to get.

        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type if_metageneration_match: long
        :param if_metageneration_match: (Optional) Make the operation conditional on whether the
                                        blob's current metageneration matches the given value.

        :type if_metageneration_not_match: long
        :param if_metageneration_not_match: (Optional) Make the operation conditional on whether the
                                            blob's current metageneration does not match the given value.

        :type retry: google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy
        :param retry:
            (Optional) How to retry the RPC. See: :ref:`configuring_retries`

        :rtype: :class:`google.cloud.storage.bucket.Bucket`
        :returns: The bucket matching the name provided or None if not found.
        """
        try:
            return self.get_bucket(
                bucket_name,
                timeout=timeout,
                if_metageneration_match=if_metageneration_match,
                if_metageneration_not_match=if_metageneration_not_match,
                retry=retry,
            )
        except NotFound:
            return None

    def create_bucket(
        self,
        bucket_or_name,
        requester_pays=None,
        project=None,
        user_project=None,
        location=None,
        data_locations=None,
        predefined_acl=None,
        predefined_default_object_acl=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
    ):
        """API call: create a new bucket via a POST request.

        See
        https://cloud.google.com/storage/docs/json_api/v1/buckets/insert

        Args:
            bucket_or_name (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                 str, \
            ]):
                The bucket resource to pass or name to create.
            requester_pays (bool):
                DEPRECATED. Use Bucket().requester_pays instead.
                (Optional) Whether requester pays for API requests for
                this bucket and its blobs.
            project (str):
                (Optional) The project under which the bucket is to be created.
                If not passed, uses the project set on the client.
            user_project (str):
                (Optional) The project ID to be billed for API requests
                made via created bucket.
            location (str):
                (Optional) The location of the bucket. If not passed,
                the default location, US, will be used. If specifying a dual-region,
                `data_locations` should be set in conjunction.. See:
                https://cloud.google.com/storage/docs/locations
            data_locations (list of str):
                (Optional) The list of regional locations of a custom dual-region bucket.
                Dual-regions require exactly 2 regional locations. See:
                https://cloud.google.com/storage/docs/locations
            predefined_acl (str):
                (Optional) Name of predefined ACL to apply to bucket. See:
                https://cloud.google.com/storage/docs/access-control/lists#predefined-acl
            predefined_default_object_acl (str):
                (Optional) Name of predefined ACL to apply to bucket's objects. See:
                https://cloud.google.com/storage/docs/access-control/lists#predefined-acl
            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

        Returns:
            google.cloud.storage.bucket.Bucket
                The newly created bucket.

        Raises:
            google.cloud.exceptions.Conflict
                If the bucket already exists.

        Examples:
            Create a bucket using a string.

            .. literalinclude:: snippets.py
                :start-after: START create_bucket
                :end-before: END create_bucket
                :dedent: 4

            Create a bucket using a resource.

            >>> from google.cloud import storage
            >>> client = storage.Client()

            >>> # Set properties on a plain resource object.
            >>> bucket = storage.Bucket("my-bucket-name")
            >>> bucket.location = "europe-west6"
            >>> bucket.storage_class = "COLDLINE"

            >>> # Pass that resource object to the client.
            >>> bucket = client.create_bucket(bucket)  # API request.

        """
        bucket = self._bucket_arg_to_bucket(bucket_or_name)
        query_params = {}

        if project is None:
            project = self.project

        # Use no project if STORAGE_EMULATOR_HOST is set
        if _BASE_STORAGE_URI not in _get_storage_host():
            if project is None:
                project = _get_environ_project()
            if project is None:
                project = "<none>"

        # Only include the project parameter if a project is set.
        # If a project is not set, falls back to API validation (BadRequest).
        if project is not None:
            query_params = {"project": project}

        if requester_pays is not None:
            warnings.warn(
                "requester_pays arg is deprecated. Use Bucket().requester_pays instead.",
                PendingDeprecationWarning,
                stacklevel=1,
            )
            bucket.requester_pays = requester_pays

        if predefined_acl is not None:
            predefined_acl = BucketACL.validate_predefined(predefined_acl)
            query_params["predefinedAcl"] = predefined_acl

        if predefined_default_object_acl is not None:
            predefined_default_object_acl = DefaultObjectACL.validate_predefined(
                predefined_default_object_acl
            )
            query_params["predefinedDefaultObjectAcl"] = predefined_default_object_acl

        if user_project is not None:
            query_params["userProject"] = user_project

        properties = {key: bucket._properties[key] for key in bucket._changes}
        properties["name"] = bucket.name

        if location is not None:
            properties["location"] = location

        if data_locations is not None:
            properties["customPlacementConfig"] = {"dataLocations": data_locations}

        api_response = self._post_resource(
            "/b",
            properties,
            query_params=query_params,
            timeout=timeout,
            retry=retry,
            _target_object=bucket,
        )

        bucket._set_properties(api_response)
        return bucket

    def download_blob_to_file(
        self,
        blob_or_uri,
        file_obj,
        start=None,
        end=None,
        raw_download=False,
        if_etag_match=None,
        if_etag_not_match=None,
        if_generation_match=None,
        if_generation_not_match=None,
        if_metageneration_match=None,
        if_metageneration_not_match=None,
        timeout=_DEFAULT_TIMEOUT,
        checksum="md5",
        retry=DEFAULT_RETRY,
    ):
        """Download the contents of a blob object or blob URI into a file-like object.

        Args:
            blob_or_uri (Union[ \
            :class:`~google.cloud.storage.blob.Blob`, \
             str, \
            ]):
                The blob resource to pass or URI to download.

            file_obj (file):
                A file handle to which to write the blob's data.

            start (int):
                (Optional) The first byte in a range to be downloaded.

            end (int):
                (Optional) The last byte in a range to be downloaded.

            raw_download (bool):
                (Optional) If true, download the object without any expansion.

            if_etag_match (Union[str, Set[str]]):
                (Optional) See :ref:`using-if-etag-match`

            if_etag_not_match (Union[str, Set[str]]):
                (Optional) See :ref:`using-if-etag-not-match`

            if_generation_match (long):
                (Optional) See :ref:`using-if-generation-match`

            if_generation_not_match (long):
                (Optional) See :ref:`using-if-generation-not-match`

            if_metageneration_match (long):
                (Optional) See :ref:`using-if-metageneration-match`

            if_metageneration_not_match (long):
                (Optional) See :ref:`using-if-metageneration-not-match`

            timeout ([Union[float, Tuple[float, float]]]):
                (Optional) The amount of time, in seconds, to wait
                for the server response.  See: :ref:`configuring_timeouts`

            checksum (str):
                (Optional) The type of checksum to compute to verify the integrity
                of the object. The response headers must contain a checksum of the
                requested type. If the headers lack an appropriate checksum (for
                instance in the case of transcoded or ranged downloads where the
                remote service does not know the correct checksum, including
                downloads where chunk_size is set) an INFO-level log will be
                emitted. Supported values are "md5", "crc32c" and None. The default
                is "md5".
            retry (google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy)
                (Optional) How to retry the RPC. A None value will disable
                retries. A google.api_core.retry.Retry value will enable retries,
                and the object will define retriable response codes and errors and
                configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a
                Retry object and activates it only if certain conditions are met.
                This class exists to provide safe defaults for RPC calls that are
                not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a
                condition such as if_metageneration_match is set.

                See the retry.py source code and docstrings in this package
                (google.cloud.storage.retry) for information on retry types and how
                to configure them.

                Media operations (downloads and uploads) do not support non-default
                predicates in a Retry object. The default will always be used. Other
                configuration changes for Retry objects such as delays and deadlines
                are respected.

        Examples:
            Download a blob using a blob resource.

            >>> from google.cloud import storage
            >>> client = storage.Client()

            >>> bucket = client.get_bucket('my-bucket-name')
            >>> blob = storage.Blob('path/to/blob', bucket)

            >>> with open('file-to-download-to', 'w') as file_obj:
            >>>     client.download_blob_to_file(blob, file_obj)  # API request.


            Download a blob using a URI.

            >>> from google.cloud import storage
            >>> client = storage.Client()

            >>> with open('file-to-download-to', 'wb') as file_obj:
            >>>     client.download_blob_to_file(
            >>>         'gs://bucket_name/path/to/blob', file_obj)


        """

        # Handle ConditionalRetryPolicy.
        if isinstance(retry, ConditionalRetryPolicy):
            # Conditional retries are designed for non-media calls, which change
            # arguments into query_params dictionaries. Media operations work
            # differently, so here we make a "fake" query_params to feed to the
            # ConditionalRetryPolicy.
            query_params = {
                "ifGenerationMatch": if_generation_match,
                "ifMetagenerationMatch": if_metageneration_match,
            }
            retry = retry.get_retry_policy_if_conditions_met(query_params=query_params)

        if not isinstance(blob_or_uri, Blob):
            blob_or_uri = Blob.from_string(blob_or_uri)
        download_url = blob_or_uri._get_download_url(
            self,
            if_generation_match=if_generation_match,
            if_generation_not_match=if_generation_not_match,
            if_metageneration_match=if_metageneration_match,
            if_metageneration_not_match=if_metageneration_not_match,
        )
        headers = _get_encryption_headers(blob_or_uri._encryption_key)
        headers["accept-encoding"] = "gzip"
        _add_etag_match_headers(
            headers,
            if_etag_match=if_etag_match,
            if_etag_not_match=if_etag_not_match,
        )
        headers = {**_get_default_headers(self._connection.user_agent), **headers}

        transport = self._http
        try:
            blob_or_uri._do_download(
                transport,
                file_obj,
                download_url,
                headers,
                start,
                end,
                raw_download,
                timeout=timeout,
                checksum=checksum,
                retry=retry,
            )
        except resumable_media.InvalidResponse as exc:
            _raise_from_invalid_response(exc)

    def list_blobs(
        self,
        bucket_or_name,
        max_results=None,
        page_token=None,
        prefix=None,
        delimiter=None,
        start_offset=None,
        end_offset=None,
        include_trailing_delimiter=None,
        versions=None,
        projection="noAcl",
        fields=None,
        page_size=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
    ):
        """Return an iterator used to find blobs in the bucket.

        If :attr:`user_project` is set, bills the API request to that project.

        Args:
            bucket_or_name (Union[ \
                :class:`~google.cloud.storage.bucket.Bucket`, \
                 str, \
            ]):
                The bucket resource to pass or name to create.

            max_results (int):
                (Optional) The maximum number of blobs to return.

            page_token (str):
                (Optional) If present, return the next batch of blobs, using the
                value, which must correspond to the ``nextPageToken`` value
                returned in the previous response.  Deprecated: use the ``pages``
                property of the returned iterator instead of manually passing the
                token.

            prefix (str):
                (Optional) Prefix used to filter blobs.

            delimiter (str):
                (Optional) Delimiter, used with ``prefix`` to
                emulate hierarchy.

            start_offset (str):
                (Optional) Filter results to objects whose names are
                lexicographically equal to or after ``startOffset``. If
                ``endOffset`` is also set, the objects listed will have names
                between ``startOffset`` (inclusive) and ``endOffset``
                (exclusive).

            end_offset (str):
                (Optional) Filter results to objects whose names are
                lexicographically before ``endOffset``. If ``startOffset`` is
                also set, the objects listed will have names between
                ``startOffset`` (inclusive) and ``endOffset`` (exclusive).

            include_trailing_delimiter (boolean):
                (Optional) If true, objects that end in exactly one instance of
                ``delimiter`` will have their metadata included in ``items`` in
                addition to ``prefixes``.

            versions (bool):
                (Optional) Whether object versions should be returned
                as separate blobs.

            projection (str):
                (Optional) If used, must be 'full' or 'noAcl'.
                Defaults to ``'noAcl'``. Specifies the set of
                properties to return.

            fields (str):
                (Optional) Selector specifying which fields to include
                in a partial response. Must be a list of fields. For
                example to get a partial response with just the next
                page token and the name and language of each blob returned:
                ``'items(name,contentLanguage),nextPageToken'``.
                See: https://cloud.google.com/storage/docs/json_api/v1/parameters#fields

            page_size (int):
                (Optional) Maximum number of blobs to return in each page.
                Defaults to a value set by the API.

            timeout (Optional[Union[float, Tuple[float, float]]]):
                The amount of time, in seconds, to wait for the server response.

                Can also be passed as a tuple (connect_timeout, read_timeout).
                See :meth:`requests.Session.request` documentation for details.

            retry (Optional[Union[google.api_core.retry.Retry, google.cloud.storage.retry.ConditionalRetryPolicy]]):
                How to retry the RPC. A None value will disable retries.
                A google.api_core.retry.Retry value will enable retries, and the object will
                define retriable response codes and errors and configure backoff and timeout options.

                A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
                activates it only if certain conditions are met. This class exists to provide safe defaults
                for RPC calls that are not technically safe to retry normally (due to potential data
                duplication or other side-effects) but become safe to retry if a condition such as
                if_metageneration_match is set.

                See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
                information on retry types and how to configure them.

        Returns:
            Iterator of all :class:`~google.cloud.storage.blob.Blob`
            in this bucket matching the arguments.

        Example:
            List blobs in the bucket with user_project.

            >>> from google.cloud import storage
            >>> client = storage.Client()

            >>> bucket = storage.Bucket(client, "my-bucket-name", user_project="my-project")
            >>> all_blobs = list(client.list_blobs(bucket))
        """
        bucket = self._bucket_arg_to_bucket(bucket_or_name)

        extra_params = {"projection": projection}

        if prefix is not None:
            extra_params["prefix"] = prefix

        if delimiter is not None:
            extra_params["delimiter"] = delimiter

        if start_offset is not None:
            extra_params["startOffset"] = start_offset

        if end_offset is not None:
            extra_params["endOffset"] = end_offset

        if include_trailing_delimiter is not None:
            extra_params["includeTrailingDelimiter"] = include_trailing_delimiter

        if versions is not None:
            extra_params["versions"] = versions

        if fields is not None:
            extra_params["fields"] = fields

        if bucket.user_project is not None:
            extra_params["userProject"] = bucket.user_project

        path = bucket.path + "/o"
        iterator = self._list_resource(
            path,
            _item_to_blob,
            page_token=page_token,
            max_results=max_results,
            extra_params=extra_params,
            page_start=_blobs_page_start,
            page_size=page_size,
            timeout=timeout,
            retry=retry,
        )
        iterator.bucket = bucket
        iterator.prefixes = set()
        return iterator

    def list_buckets(
        self,
        max_results=None,
        page_token=None,
        prefix=None,
        projection="noAcl",
        fields=None,
        project=None,
        page_size=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
    ):
        """Get all buckets in the project associated to the client.

        This will not populate the list of blobs available in each
        bucket.

        .. literalinclude:: snippets.py
            :start-after: START list_buckets
            :end-before: END list_buckets
            :dedent: 4

        This implements "storage.buckets.list".

        :type max_results: int
        :param max_results: (Optional) The maximum number of buckets to return.

        :type page_token: str
        :param page_token:
            (Optional) If present, return the next batch of buckets, using the
            value, which must correspond to the ``nextPageToken`` value
            returned in the previous response.  Deprecated: use the ``pages``
            property of the returned iterator instead of manually passing the
            token.

        :type prefix: str
        :param prefix: (Optional) Filter results to buckets whose names begin
                       with this prefix.

        :type projection: str
        :param projection:
            (Optional) Specifies the set of properties to return. If used, must
            be 'full' or 'noAcl'. Defaults to 'noAcl'.

        :type fields: str
        :param fields:
            (Optional) Selector specifying which fields to include in a partial
            response. Must be a list of fields. For example to get a partial
            response with just the next page token and the language of each
            bucket returned: 'items/id,nextPageToken'

        :type project: str
        :param project: (Optional) The project whose buckets are to be listed.
                        If not passed, uses the project set on the client.

        :type page_size: int
        :param page_size: (Optional) Maximum number of buckets to return in each page.
            Defaults to a value set by the API.

        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type retry: google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy
        :param retry:
            (Optional) How to retry the RPC. See: :ref:`configuring_retries`

        :rtype: :class:`~google.api_core.page_iterator.Iterator`
        :raises ValueError: if both ``project`` is ``None`` and the client's
                            project is also ``None``.
        :returns: Iterator of all :class:`~google.cloud.storage.bucket.Bucket`
                  belonging to this project.
        """
        extra_params = {}

        if project is None:
            project = self.project

        # Use no project if STORAGE_EMULATOR_HOST is set
        if _BASE_STORAGE_URI not in _get_storage_host():
            if project is None:
                project = _get_environ_project()
            if project is None:
                project = "<none>"

        # Only include the project parameter if a project is set.
        # If a project is not set, falls back to API validation (BadRequest).
        if project is not None:
            extra_params = {"project": project}

        if prefix is not None:
            extra_params["prefix"] = prefix

        extra_params["projection"] = projection

        if fields is not None:
            extra_params["fields"] = fields

        return self._list_resource(
            "/b",
            _item_to_bucket,
            page_token=page_token,
            max_results=max_results,
            extra_params=extra_params,
            page_size=page_size,
            timeout=timeout,
            retry=retry,
        )

    def create_hmac_key(
        self,
        service_account_email,
        project_id=None,
        user_project=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=None,
    ):
        """Create an HMAC key for a service account.

        :type service_account_email: str
        :param service_account_email: e-mail address of the service account

        :type project_id: str
        :param project_id: (Optional) Explicit project ID for the key.
            Defaults to the client's project.

        :type user_project: str
        :param user_project: (Optional) This parameter is currently ignored.

        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type retry: google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy
        :param retry: (Optional) How to retry the RPC. A None value will disable retries.
            A google.api_core.retry.Retry value will enable retries, and the object will
            define retriable response codes and errors and configure backoff and timeout options.

            A google.cloud.storage.retry.ConditionalRetryPolicy value wraps a Retry object and
            activates it only if certain conditions are met. This class exists to provide safe defaults
            for RPC calls that are not technically safe to retry normally (due to potential data
            duplication or other side-effects) but become safe to retry if a condition such as
            if_metageneration_match is set.

            See the retry.py source code and docstrings in this package (google.cloud.storage.retry) for
            information on retry types and how to configure them.

        :rtype:
            Tuple[:class:`~google.cloud.storage.hmac_key.HMACKeyMetadata`, str]
        :returns: metadata for the created key, plus the bytes of the key's secret, which is an 40-character base64-encoded string.
        """
        if project_id is None:
            project_id = self.project

        path = f"/projects/{project_id}/hmacKeys"
        qs_params = {"serviceAccountEmail": service_account_email}

        if user_project is not None:
            qs_params["userProject"] = user_project

        api_response = self._post_resource(
            path,
            None,
            query_params=qs_params,
            timeout=timeout,
            retry=retry,
        )
        metadata = HMACKeyMetadata(self)
        metadata._properties = api_response["metadata"]
        secret = api_response["secret"]
        return metadata, secret

    def list_hmac_keys(
        self,
        max_results=None,
        service_account_email=None,
        show_deleted_keys=None,
        project_id=None,
        user_project=None,
        timeout=_DEFAULT_TIMEOUT,
        retry=DEFAULT_RETRY,
    ):
        """List HMAC keys for a project.

        :type max_results: int
        :param max_results:
            (Optional) Max number of keys to return in a given page.

        :type service_account_email: str
        :param service_account_email:
            (Optional) Limit keys to those created by the given service account.

        :type show_deleted_keys: bool
        :param show_deleted_keys:
            (Optional) Included deleted keys in the list. Default is to
            exclude them.

        :type project_id: str
        :param project_id: (Optional) Explicit project ID for the key.
            Defaults to the client's project.

        :type user_project: str
        :param user_project: (Optional) This parameter is currently ignored.

        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type retry: google.api_core.retry.Retry or google.cloud.storage.retry.ConditionalRetryPolicy
        :param retry:
            (Optional) How to retry the RPC. See: :ref:`configuring_retries`

        :rtype:
            Tuple[:class:`~google.cloud.storage.hmac_key.HMACKeyMetadata`, str]
        :returns: metadata for the created key, plus the bytes of the key's secret, which is an 40-character base64-encoded string.
        """
        if project_id is None:
            project_id = self.project

        path = f"/projects/{project_id}/hmacKeys"
        extra_params = {}

        if service_account_email is not None:
            extra_params["serviceAccountEmail"] = service_account_email

        if show_deleted_keys is not None:
            extra_params["showDeletedKeys"] = show_deleted_keys

        if user_project is not None:
            extra_params["userProject"] = user_project

        return self._list_resource(
            path,
            _item_to_hmac_key_metadata,
            max_results=max_results,
            extra_params=extra_params,
            timeout=timeout,
            retry=retry,
        )

    def get_hmac_key_metadata(
        self, access_id, project_id=None, user_project=None, timeout=_DEFAULT_TIMEOUT
    ):
        """Return a metadata instance for the given HMAC key.

        :type access_id: str
        :param access_id: Unique ID of an existing key.

        :type project_id: str
        :param project_id: (Optional) Project ID of an existing key.
            Defaults to client's project.

        :type timeout: float or tuple
        :param timeout:
            (Optional) The amount of time, in seconds, to wait
            for the server response.  See: :ref:`configuring_timeouts`

        :type user_project: str
        :param user_project: (Optional) This parameter is currently ignored.
        """
        metadata = HMACKeyMetadata(self, access_id, project_id, user_project)
        metadata.reload(timeout=timeout)  # raises NotFound for missing key
        return metadata

    def generate_signed_post_policy_v4(
        self,
        bucket_name,
        blob_name,
        expiration,
        conditions=None,
        fields=None,
        credentials=None,
        virtual_hosted_style=False,
        bucket_bound_hostname=None,
        scheme="http",
        service_account_email=None,
        access_token=None,
    ):
        """Generate a V4 signed policy object.

        .. note::

            Assumes ``credentials`` implements the
            :class:`google.auth.credentials.Signing` interface. Also assumes
            ``credentials`` has a ``service_account_email`` property which
            identifies the credentials.

        Generated policy object allows user to upload objects with a POST request.

        :type bucket_name: str
        :param bucket_name: Bucket name.

        :type blob_name: str
        :param blob_name: Object name.

        :type expiration: Union[Integer, datetime.datetime, datetime.timedelta]
        :param expiration: Policy expiration time. If a ``datetime`` instance is
                           passed without an explicit ``tzinfo`` set,  it will be
                           assumed to be ``UTC``.

        :type conditions: list
        :param conditions: (Optional) List of POST policy conditions, which are
                           used to restrict what is allowed in the request.

        :type fields: dict
        :param fields: (Optional) Additional elements to include into request.

        :type credentials: :class:`google.auth.credentials.Signing`
        :param credentials: (Optional) Credentials object with an associated private
                            key to sign text.

        :type virtual_hosted_style: bool
        :param virtual_hosted_style: (Optional) If True, construct the URL relative to the bucket
                                     virtual hostname, e.g., '<bucket-name>.storage.googleapis.com'.

        :type bucket_bound_hostname: str
        :param bucket_bound_hostname:
            (Optional) If passed, construct the URL relative to the bucket-bound hostname.
            Value can be bare or with a scheme, e.g., 'example.com' or 'http://example.com'.
            See: https://cloud.google.com/storage/docs/request-endpoints#cname

        :type scheme: str
        :param scheme:
            (Optional) If ``bucket_bound_hostname`` is passed as a bare hostname, use
            this value as a scheme. ``https`` will work only when using a CDN.
            Defaults to ``"http"``.

        :type service_account_email: str
        :param service_account_email: (Optional) E-mail address of the service account.

        :type access_token: str
        :param access_token: (Optional) Access token for a service account.

        :rtype: dict
        :returns: Signed POST policy.

        Example:
            Generate signed POST policy and upload a file.

            >>> import datetime
            >>> from google.cloud import storage
            >>> client = storage.Client()
            >>> tz = datetime.timezone(datetime.timedelta(hours=1), 'CET')
            >>> policy = client.generate_signed_post_policy_v4(
                "bucket-name",
                "blob-name",
                expiration=datetime.datetime(2020, 3, 17, tzinfo=tz),
                conditions=[
                    ["content-length-range", 0, 255]
                ],
                fields=[
                    "x-goog-meta-hello" => "world"
                ],
            )
            >>> with open("bucket-name", "rb") as f:
                files = {"file": ("bucket-name", f)}
                requests.post(policy["url"], data=policy["fields"], files=files)
        """
        credentials = self._credentials if credentials is None else credentials
        ensure_signed_credentials(credentials)

        # prepare policy conditions and fields
        timestamp, datestamp = get_v4_now_dtstamps()

        x_goog_credential = "{email}/{datestamp}/auto/storage/goog4_request".format(
            email=credentials.signer_email, datestamp=datestamp
        )
        required_conditions = [
            {"bucket": bucket_name},
            {"key": blob_name},
            {"x-goog-date": timestamp},
            {"x-goog-credential": x_goog_credential},
            {"x-goog-algorithm": "GOOG4-RSA-SHA256"},
        ]

        conditions = conditions or []
        policy_fields = {}
        for key, value in sorted((fields or {}).items()):
            if not key.startswith("x-ignore-"):
                policy_fields[key] = value
                conditions.append({key: value})

        conditions += required_conditions

        # calculate policy expiration time
        now = _NOW()
        if expiration is None:
            expiration = now + datetime.timedelta(hours=1)

        policy_expires = now + datetime.timedelta(
            seconds=get_expiration_seconds_v4(expiration)
        )

        # encode policy for signing
        policy = json.dumps(
            collections.OrderedDict(
                sorted(
                    {
                        "conditions": conditions,
                        "expiration": policy_expires.isoformat() + "Z",
                    }.items()
                )
            ),
            separators=(",", ":"),
        )
        str_to_sign = base64.b64encode(policy.encode("utf-8"))

        # sign the policy and get its cryptographic signature
        if access_token and service_account_email:
            signature = _sign_message(str_to_sign, access_token, service_account_email)
            signature_bytes = base64.b64decode(signature)
        else:
            signature_bytes = credentials.sign_bytes(str_to_sign)

        # get hexadecimal representation of the signature
        signature = binascii.hexlify(signature_bytes).decode("utf-8")

        policy_fields.update(
            {
                "key": blob_name,
                "x-goog-algorithm": "GOOG4-RSA-SHA256",
                "x-goog-credential": x_goog_credential,
                "x-goog-date": timestamp,
                "x-goog-signature": signature,
                "policy": str_to_sign.decode("utf-8"),
            }
        )
        # designate URL
        if virtual_hosted_style:
            url = f"https://{bucket_name}.storage.googleapis.com/"
        elif bucket_bound_hostname:
            url = _bucket_bound_hostname_url(bucket_bound_hostname, scheme)
        else:
            url = f"https://storage.googleapis.com/{bucket_name}/"

        return {"url": url, "fields": policy_fields}


def _item_to_bucket(iterator, item):
    """Convert a JSON bucket to the native object.

    :type iterator: :class:`~google.api_core.page_iterator.Iterator`
    :param iterator: The iterator that has retrieved the item.

    :type item: dict
    :param item: An item to be converted to a bucket.

    :rtype: :class:`.Bucket`
    :returns: The next bucket in the page.
    """
    name = item.get("name")
    bucket = Bucket(iterator.client, name)
    bucket._set_properties(item)
    return bucket


def _item_to_hmac_key_metadata(iterator, item):
    """Convert a JSON key metadata resource to the native object.

    :type iterator: :class:`~google.api_core.page_iterator.Iterator`
    :param iterator: The iterator that has retrieved the item.

    :type item: dict
    :param item: An item to be converted to a key metadata instance.

    :rtype: :class:`~google.cloud.storage.hmac_key.HMACKeyMetadata`
    :returns: The next key metadata instance in the page.
    """
    metadata = HMACKeyMetadata(iterator.client)
    metadata._properties = item
    return metadata