From 2b64353632a6f70638843c52e8a14fd105f9a79a Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 15:31:55 +0100
Subject: [PATCH 01/10] Fix deprecation warning

---
 jupyterlab/commands.py               | 6 +++---
 jupyterlab/handlers/build_handler.py | 2 +-
 jupyterlab/labapp.py                 | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/jupyterlab/commands.py b/jupyterlab/commands.py
index da2ecfc009c0..42cb8412f57f 100644
--- a/jupyterlab/commands.py
+++ b/jupyterlab/commands.py
@@ -775,7 +775,7 @@ def update_extension(self, name):
         Returns `True` if a rebuild is recommended, `False` otherwise.
         """
         if name not in self.info['extensions']:
-            self.logger.warn('No labextension named "%s" installed' % name)
+            self.logger.warning('No labextension named "%s" installed' % name)
             return False
         return self._update_extension(name)
 
@@ -815,8 +815,8 @@ def link_package(self, path):
             return self.install_extension(path)
 
         # Warn that it is a linked package.
-        self.logger.warn('Installing %s as a linked package:', path)
-        [self.logger.warn(m) for m in messages]
+        self.logger.warning('Installing %s as a linked package:', path)
+        [self.logger.warning(m) for m in messages]
 
         # Add to metadata.
         config = self._read_build_config()
diff --git a/jupyterlab/handlers/build_handler.py b/jupyterlab/handlers/build_handler.py
index 80cf27e5fa89..912840c9be18 100644
--- a/jupyterlab/handlers/build_handler.py
+++ b/jupyterlab/handlers/build_handler.py
@@ -114,7 +114,7 @@ def get(self):
     @web.authenticated
     @gen.coroutine
     def delete(self):
-        self.log.warn('Canceling build')
+        self.log.warning('Canceling build')
         try:
             yield self.builder.cancel()
         except Exception as e:
diff --git a/jupyterlab/labapp.py b/jupyterlab/labapp.py
index a79cf8ff7e7b..2c54e934981a 100644
--- a/jupyterlab/labapp.py
+++ b/jupyterlab/labapp.py
@@ -411,7 +411,7 @@ def init_server_extensions(self):
         super(LabApp, self).init_server_extensions()
         msg = 'JupyterLab server extension not enabled, manually loading...'
         if not self.nbserver_extensions.get('jupyterlab', False):
-            self.log.warn(msg)
+            self.log.warning(msg)
             load_jupyter_server_extension(self)
 
 

From 6a3c03451478b1804fa0a68963f3173208297a17 Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 15:33:31 +0100
Subject: [PATCH 02/10] Add a CoreConfig object to python

---
 jupyterlab/commands.py                        |  86 +++++------
 jupyterlab/coreconfig.py                      | 139 ++++++++++++++++++
 jupyterlab/handlers/build_handler.py          |  20 ++-
 .../handlers/extension_manager_handler.py     |  20 ++-
 jupyterlab/labapp.py                          |  14 +-
 jupyterlab/labextensions.py                   |  45 ++++--
 jupyterlab/tests/test_jupyterlab.py           |   6 +-
 7 files changed, 254 insertions(+), 76 deletions(-)
 create mode 100644 jupyterlab/coreconfig.py

diff --git a/jupyterlab/commands.py b/jupyterlab/commands.py
index 42cb8412f57f..0db625380bdb 100644
--- a/jupyterlab/commands.py
+++ b/jupyterlab/commands.py
@@ -29,6 +29,7 @@
 
 from .semver import Range, gte, lt, lte, gt, make_semver
 from .jlpmapp import YARN_PATH, HERE
+from .coreconfig import CoreConfig
 
 
 # The regex for expecting the webpack output.
@@ -283,7 +284,7 @@ def watch_dev(logger=None):
     return package_procs + [wp_proc]
 
 
-def watch(app_dir=None, logger=None):
+def watch(app_dir=None, logger=None, core_config=None):
     """Watch the application.
 
     Parameters
@@ -299,11 +300,11 @@ def watch(app_dir=None, logger=None):
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.watch()
 
 
-def install_extension(extension, app_dir=None, logger=None):
+def install_extension(extension, app_dir=None, logger=None, core_config=None):
     """Install an extension package into JupyterLab.
 
     The extension is first validated.
@@ -312,24 +313,24 @@ def install_extension(extension, app_dir=None, logger=None):
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.install_extension(extension)
 
 
-def uninstall_extension(name=None, app_dir=None, logger=None, all_=False):
+def uninstall_extension(name=None, app_dir=None, logger=None, all_=False, core_config=None):
     """Uninstall an extension by name or path.
 
     Returns `True` if a rebuild is recommended, `False` otherwise.
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     if all_ is True:
         return handler.uninstall_all_extensions()
     return handler.uninstall_extension(name)
 
 
-def update_extension(name=None, all_=False, app_dir=None, logger=None):
+def update_extension(name=None, all_=False, app_dir=None, logger=None, core_config=None):
     """Update an extension by name, or all extensions.
 
     Either `name` must be given as a string, or `all_` must be `True`.
@@ -339,7 +340,7 @@ def update_extension(name=None, all_=False, app_dir=None, logger=None):
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     if all_ is True:
         return handler.update_all_extensions()
     return handler.update_extension(name)
@@ -363,96 +364,96 @@ def clean(app_dir=None, logger=None):
 
 def build(app_dir=None, name=None, version=None, static_url=None,
           logger=None, command='build:prod', kill_event=None,
-          clean_staging=False):
+          clean_staging=False, core_config=None):
     """Build the JupyterLab application.
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger, kill_event=kill_event)
+    handler = _AppHandler(app_dir, logger, kill_event=kill_event, core_config=core_config)
     return handler.build(name=name, version=version, static_url=static_url,
                          command=command, clean_staging=clean_staging)
 
 
-def get_app_info(app_dir=None, logger=None):
+def get_app_info(app_dir=None, logger=None, core_config=None):
     """Get a dictionary of information about the app.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.info
 
 
-def enable_extension(extension, app_dir=None, logger=None):
+def enable_extension(extension, app_dir=None, logger=None, core_config=None):
     """Enable a JupyterLab extension.
 
     Returns `True` if a rebuild is recommended, `False` otherwise.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.toggle_extension(extension, False)
 
 
-def disable_extension(extension, app_dir=None, logger=None):
+def disable_extension(extension, app_dir=None, logger=None, core_config=None):
     """Disable a JupyterLab package.
 
     Returns `True` if a rebuild is recommended, `False` otherwise.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.toggle_extension(extension, True)
 
 
-def check_extension(extension, app_dir=None, installed=False, logger=None):
+def check_extension(extension, app_dir=None, installed=False, logger=None, core_config=None):
     """Check if a JupyterLab extension is enabled or disabled.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.check_extension(extension, installed)
 
 
-def build_check(app_dir=None, logger=None):
+def build_check(app_dir=None, logger=None, core_config=None):
     """Determine whether JupyterLab should be built.
 
     Returns a list of messages.
     """
     logger = _ensure_logger(logger)
     _node_check(logger)
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.build_check()
 
 
-def list_extensions(app_dir=None, logger=None):
+def list_extensions(app_dir=None, logger=None, core_config=None):
     """List the extensions.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.list_extensions()
 
 
-def link_package(path, app_dir=None, logger=None):
+def link_package(path, app_dir=None, logger=None, core_config=None):
     """Link a package against the JupyterLab build.
 
     Returns `True` if a rebuild is recommended, `False` otherwise.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.link_package(path)
 
 
-def unlink_package(package, app_dir=None, logger=None):
+def unlink_package(package, app_dir=None, logger=None, core_config=None):
     """Unlink a package from JupyterLab by path or name.
 
     Returns `True` if a rebuild is recommended, `False` otherwise.
     """
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.unlink_package(package)
 
 
-def get_app_version(app_dir=None):
+def get_app_version(app_dir=None, core_config=None):
     """Get the application version."""
     app_dir = app_dir or get_app_dir()
-    handler = _AppHandler(app_dir)
+    handler = _AppHandler(app_dir, core_config=core_config)
     return handler.info['version']
 
 
-def get_latest_compatible_package_versions(names, app_dir=None, logger=None):
+def get_latest_compatible_package_versions(names, app_dir=None, logger=None, core_config=None):
     """Get the latest compatible version of a list of packages.
     """
     app_dir = app_dir or get_app_dir()
-    handler = _AppHandler(app_dir, logger)
+    handler = _AppHandler(app_dir, logger, core_config=core_config)
     return handler.latest_compatible_package_versions(names)
 
 
@@ -476,12 +477,13 @@ def read_package(target):
 
 class _AppHandler(object):
 
-    def __init__(self, app_dir, logger=None, kill_event=None):
+    def __init__(self, app_dir, logger=None, kill_event=None, core_config=None):
         """Create a new _AppHandler object
         """
         self.app_dir = app_dir or get_app_dir()
         self.sys_dir = get_app_dir()
         self.logger = _ensure_logger(logger)
+        self.core_data = (core_config or CoreConfig()).data
         self.info = self._get_app_info()
         self.kill_event = kill_event or Event()
         # TODO: Make this configurable
@@ -713,7 +715,6 @@ def uninstall_extension(self, name):
         Returns `True` if a rebuild is recommended, `False` otherwise.
         """
         # Allow for uninstalled core extensions.
-        data = self.info['core_data']
         if name in self.info['core_extensions']:
             config = self._read_build_config()
             uninstalled = config.get('uninstalled_core_extensions', [])
@@ -938,7 +939,7 @@ def _get_app_info(self):
         """
 
         info = dict()
-        info['core_data'] = core_data = _get_core_data()
+        info['core_data'] = core_data = self.core_data
         info['extensions'] = extensions = self._get_extensions(core_data)
         page_config = self._read_page_config()
         info['disabled'] = page_config.get('disabledExtensions', [])
@@ -963,7 +964,8 @@ def _get_app_info(self):
         info['sys_dir'] = self.sys_dir
         info['app_dir'] = self.app_dir
 
-        info['core_extensions'] = core_extensions = _get_core_extensions()
+        info['core_extensions'] = core_extensions = _get_core_extensions(
+            self.core_data)
 
         disabled_core = []
         for key in core_extensions:
@@ -1390,9 +1392,8 @@ def _install_extension(self, extension, tempdir):
             raise ValueError(msg % (extension, '\n'.join(messages)))
 
         # Verify package compatibility.
-        core_data = _get_core_data()
         deps = data.get('dependencies', dict())
-        errors = _validate_compatibility(extension, deps, core_data)
+        errors = _validate_compatibility(extension, deps, self.core_data)
         if errors:
             msg = _format_compatibility_errors(
                 data['name'], data['version'], errors
@@ -1628,7 +1629,7 @@ def _node_check(logger):
         output = subprocess.check_output([node, 'node-version-check.js'], cwd=HERE)
         logger.debug(output.decode('utf-8'))
     except Exception:
-        data = _get_core_data()
+        data = CoreConfig().data
         ver = data['engines']['node']
         msg = 'Please install nodejs %s before continuing. nodejs may be installed using conda or directly from the nodejs website.' % ver
         raise ValueError(msg)
@@ -1728,13 +1729,6 @@ def _tarsum(input_file):
     return h.hexdigest()
 
 
-def _get_core_data():
-    """Get the data for the app template.
-    """
-    with open(pjoin(HERE, 'staging', 'package.json')) as fid:
-        return json.load(fid)
-
-
 def _get_static_data(app_dir):
     """Get the data for the app static dir.
     """
@@ -1931,10 +1925,10 @@ def _compat_error_age(errors):
     return 0
 
 
-def _get_core_extensions():
+def _get_core_extensions(core_data):
     """Get the core extensions.
     """
-    data = _get_core_data()['jupyterlab']
+    data = core_data['jupyterlab']
     return list(data['extensions']) + list(data['mimeExtensions'])
 
 
diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
new file mode 100644
index 000000000000..1b35d145ba57
--- /dev/null
+++ b/jupyterlab/coreconfig.py
@@ -0,0 +1,139 @@
+# coding: utf-8
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+from itertools import filterfalse
+import json
+import os.path as osp
+
+from .jlpmapp import HERE
+
+
+def pjoin(*args):
+    """Join paths to create a real path.
+    """
+    return osp.abspath(osp.join(*args))
+
+
+def _get_core_data():
+    """Get the data for the app template.
+    """
+    with open(pjoin(HERE, 'staging', 'package.json')) as fid:
+        return json.load(fid)
+
+
+def _is_lab_package(name):
+    """Whether a package name is in the lab namespace"""
+    return name.startswith('@jupyterlab/')
+
+
+def _only_nonlab(collection):
+    """Filter a dict/sequence to remove all lab packages"""
+    if isinstance(collection, dict):
+        return dict(
+            (k, v) for (k, v) in collection.items()
+            if not _is_lab_package(k)
+        )
+    elif isinstance(collection, (list, tuple)):
+        return list(filterfalse(_is_lab_package, collection))
+    raise TypeError('collection arg should be either dict or list/tuple')
+
+
+class CoreConfig:
+    """An object representing a core-mode package/extension configuration.
+
+    This enables custom lab application to change
+    """
+
+    def __init__(self):
+        self._data = _get_core_data()
+
+    def clear_defaults(self, lab_only=True):
+        """Clear the default packages/extensions.
+
+        lab_only: string
+            Whether to remove all packages, or only those from
+            JupyterLab. Defaults to True (only lab packages).
+            This will leave dependencies like phosphor, react
+            etc untouched.
+        """
+        data = self._data
+        if lab_only:
+            # Clear all "@jupyterlab/" dependencies
+            data['dependencies'] = _only_nonlab(data['dependencies'])
+            data['resolutions'] = _only_nonlab(data['resolutions'])
+            data['jupyterlab']['extensions'] = _only_nonlab(
+                data['jupyterlab']['extensions'])
+            data['jupyterlab']['mimeExtensions'] = _only_nonlab(
+                data['jupyterlab']['mimeExtensions'])
+            data['jupyterlab']['singletonPackages'] = _only_nonlab(
+                data['jupyterlab']['singletonPackages'])
+        else:
+            # Clear all dependencies
+            data['dependencies'] = {}
+            data['resolutions'] = {}
+            data['jupyterlab']['extensions'] = {}
+            data['jupyterlab']['mimeExtensions'] = {}
+            data['jupyterlab']['singletonPackages'] = []
+
+    def add(self, name, semver, extension=False, mimeExtension=False):
+        """Remove a package/extension.
+
+        name: string
+            The npm package name
+        semver: string
+            The semver range for the package
+        extension: bool
+            Whether the package is an extension
+        mimeExtension: bool
+            Whether the package is a MIME extension
+        """
+        data = self._data
+        if name in self._data['resolutions']:
+            raise ValueError('Package already present: %r' % (name,))
+        data['resolutions'][name] = semver
+
+        # If both mimeExtension and extensions are True, treat
+        # as mime extension
+        if mimeExtension:
+            data['jupyterlab']['mimeExtensions'][name] = ""
+            data['dependencies'][name] = semver
+        elif extension:
+            data['jupyterlab']['extensions'][name] = ""
+            data['dependencies'][name] = semver
+        else:
+            data['singletonPackages'].append(name)
+
+    def remove(self, name):
+        """Remove a package/extension.
+
+        name: string
+            The npm package name
+        """
+        data = self._data
+        maps = (
+            data['dependencies'],
+            data['resolutions'],
+            data['jupyterlab']['extensions'],
+            data['jupyterlab']['mimeExtensions'],
+        )
+        for m in maps:
+            try:
+                del m[name]
+            except KeyError:
+                pass
+
+        data['jupyterlab']['singletonPackages'].remove(name)
+
+
+    def set_static_dir(self, static_dir):
+        self._data['jupyterlab']['staticDir'] = static_dir
+
+    @property
+    def data(self):
+        """Returns the raw core data.
+
+        Its content should be considered an internal implementation
+        detail of lab, and should not be relied upon outide of lab.
+        """
+        return self._data
diff --git a/jupyterlab/handlers/build_handler.py b/jupyterlab/handlers/build_handler.py
index 912840c9be18..c8bfbfec15db 100644
--- a/jupyterlab/handlers/build_handler.py
+++ b/jupyterlab/handlers/build_handler.py
@@ -21,10 +21,11 @@ class Builder(object):
     _kill_event = None
     _future = None
 
-    def __init__(self, log, core_mode, app_dir):
+    def __init__(self, log, core_mode, app_dir, core_config=None):
         self.log = log
         self.core_mode = core_mode
         self.app_dir = app_dir
+        self.core_config = core_config
 
     @gen.coroutine
     def get_status(self):
@@ -34,7 +35,8 @@ def get_status(self):
             raise gen.Return(dict(status='building', message=''))
 
         try:
-            messages = yield self._run_build_check(self.app_dir, self.log)
+            messages = yield self._run_build_check(
+                self.app_dir, self.log, self.core_config)
             status = 'needed' if messages else 'stable'
             if messages:
                 self.log.warn('Build recommended')
@@ -60,7 +62,8 @@ def build(self):
             self.building = True
             self._kill_event = evt = Event()
             try:
-                yield self._run_build(self.app_dir, self.log, evt)
+                yield self._run_build(
+                    self.app_dir, self.log, evt, self.core_config)
                 future.set_result(True)
             except Exception as e:
                 if str(e) == 'Aborted':
@@ -84,12 +87,15 @@ def cancel(self):
         self.canceled = True
 
     @run_on_executor
-    def _run_build_check(self, app_dir, logger):
-        return build_check(app_dir=app_dir, logger=logger)
+    def _run_build_check(self, app_dir, logger, core_config):
+        return build_check(
+            app_dir=app_dir, logger=logger, core_config=core_config)
 
     @run_on_executor
-    def _run_build(self, app_dir, logger, kill_event):
-        kwargs = dict(app_dir=app_dir, logger=logger, kill_event=kill_event, command='build')
+    def _run_build(self, app_dir, logger, kill_event, core_config):
+        kwargs = dict(
+            app_dir=app_dir, logger=logger, kill_event=kill_event,
+            core_config=core_config, command='build')
         try:
             return build(**kwargs)
         except Exception as e:
diff --git a/jupyterlab/handlers/extension_manager_handler.py b/jupyterlab/handlers/extension_manager_handler.py
index 3ef873e357ba..df02348df367 100644
--- a/jupyterlab/handlers/extension_manager_handler.py
+++ b/jupyterlab/handlers/extension_manager_handler.py
@@ -66,9 +66,10 @@ def _build_check_info(app_dir, logger):
 class ExtensionManager(object):
     executor = ThreadPoolExecutor(max_workers=1)
 
-    def __init__(self, log, app_dir):
+    def __init__(self, log, app_dir, core_config=None):
         self.log = log
         self.app_dir = app_dir
+        self.core_config = core_config
         self._outdated = None
         # To start fetching data on outdated extensions immediately, uncomment:
         # IOLoop.current().spawn_callback(self._get_outdated)
@@ -122,7 +123,9 @@ def list_extensions(self):
     def install(self, extension):
         """Handle an install/update request"""
         try:
-            install_extension(extension, app_dir=self.app_dir, logger=self.log)
+            install_extension(
+                extension, app_dir=self.app_dir, logger=self.log,
+                core_config=self.core_config)
         except ValueError as e:
             raise gen.Return(dict(status='error', message=str(e)))
         raise gen.Return(dict(status='ok',))
@@ -130,19 +133,25 @@ def install(self, extension):
     @gen.coroutine
     def uninstall(self, extension):
         """Handle an uninstall request"""
-        did_uninstall = uninstall_extension(extension, app_dir=self.app_dir, logger=self.log)
+        did_uninstall = uninstall_extension(
+            extension, app_dir=self.app_dir, logger=self.log,
+            core_config=self.core_config)
         raise gen.Return(dict(status='ok' if did_uninstall else 'error',))
 
     @gen.coroutine
     def enable(self, extension):
         """Handle an enable request"""
-        enable_extension(extension, app_dir=self.app_dir, logger=self.log)
+        enable_extension(
+            extension, app_dir=self.app_dir, logger=self.log,
+            core_config=self.core_config)
         raise gen.Return(dict(status='ok',))
 
     @gen.coroutine
     def disable(self, extension):
         """Handle a disable request"""
-        disable_extension(extension, app_dir=self.app_dir, logger=self.log)
+        disable_extension(
+            extension, app_dir=self.app_dir, logger=self.log,
+            core_config=self.core_config)
         raise gen.Return(dict(status='ok',))
 
     @gen.coroutine
@@ -187,6 +196,7 @@ def _load_outdated(self):
             names,
             app_dir=self.app_dir,
             logger=self.log,
+            core_config=self.core_config,
         )
         raise gen.Return(data)
 
diff --git a/jupyterlab/labapp.py b/jupyterlab/labapp.py
index 2c54e934981a..288b6a0afc86 100644
--- a/jupyterlab/labapp.py
+++ b/jupyterlab/labapp.py
@@ -14,7 +14,7 @@
 from jupyterlab_server import slugify, WORKSPACE_EXTENSION
 from notebook.notebookapp import NotebookApp, aliases, flags
 from notebook.utils import url_path_join as ujoin
-from traitlets import Bool, Unicode
+from traitlets import Bool, Instance, Unicode
 
 from ._version import __version__
 from .debuglog import DebugLogFileMixin
@@ -23,6 +23,7 @@
     build, clean, get_app_dir, get_app_version, get_user_settings_dir,
     get_workspaces_dir
 )
+from .coreconfig import CoreConfig
 
 
 build_aliases = dict(base_aliases)
@@ -53,6 +54,9 @@ class LabBuildApp(JupyterApp, DebugLogFileMixin):
     aliases = build_aliases
     flags = build_flags
 
+    # Not configurable!
+    core_config = Instance(CoreConfig, allow_none=True)
+
     app_dir = Unicode('', config=True,
         help="The app directory to build in")
 
@@ -88,7 +92,8 @@ def start(self):
                 clean(self.app_dir)
             self.log.info('Building in %s', app_dir)
             build(app_dir=app_dir, name=self.name, version=self.version,
-                  command=command, logger=self.log)
+                  command=command, logger=self.log,
+                  core_config=self.core_config)
 
 
 clean_aliases = dict(base_aliases)
@@ -105,10 +110,13 @@ class LabCleanApp(JupyterApp):
     """
     aliases = clean_aliases
 
+    # Not configurable!
+    core_config = Instance(CoreConfig, allow_none=True)
+
     app_dir = Unicode('', config=True, help='The app directory to clean')
 
     def start(self):
-        clean(self.app_dir, logger=self.log)
+        clean(self.app_dir, logger=self.log, core_config=self.core_config)
 
 
 class LabPathApp(JupyterApp):
diff --git a/jupyterlab/labextensions.py b/jupyterlab/labextensions.py
index bc5071a2c030..6d1984737eb4 100644
--- a/jupyterlab/labextensions.py
+++ b/jupyterlab/labextensions.py
@@ -11,7 +11,7 @@
 
 from jupyter_core.application import JupyterApp, base_flags, base_aliases
 
-from traitlets import Bool, Unicode
+from traitlets import Bool, Instance, Unicode
 
 from .commands import (
     install_extension, uninstall_extension, list_extensions,
@@ -19,6 +19,7 @@
     link_package, unlink_package, build, get_app_version, HERE,
     update_extension,
 )
+from .coreconfig import CoreConfig
 from .debuglog import DebugLogFileMixin
 
 
@@ -65,6 +66,9 @@ class BaseExtensionApp(JupyterApp, DebugLogFileMixin):
     flags = flags
     aliases = aliases
 
+    # Not configurable!
+    core_config = Instance(CoreConfig, allow_none=True)
+
     app_dir = Unicode('', config=True,
         help="The app directory to target")
 
@@ -95,7 +99,8 @@ def start(self):
                 command = ':'.join(parts)
 
                 build(app_dir=self.app_dir, clean_staging=self.should_clean,
-                      logger=self.log, command=command)
+                      logger=self.log, command=command,
+                      core_config=self.core_config)
 
     def run_task(self):
         pass
@@ -111,7 +116,9 @@ class InstallLabExtensionApp(BaseExtensionApp):
     def run_task(self):
         self.extra_args = self.extra_args or [os.getcwd()]
         return any([
-            install_extension(arg, self.app_dir, logger=self.log)
+            install_extension(
+                arg, self.app_dir, logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args
         ])
 
@@ -128,9 +135,13 @@ def run_task(self):
             self.log.warn('Specify an extension to update, or use --all to update all extensions')
             return False
         if self.all:
-            return update_extension(all_=True, app_dir=self.app_dir, logger=self.log)
+            return update_extension(
+                all_=True, app_dir=self.app_dir, logger=self.log,
+                core_config=self.core_config)
         return any([
-            update_extension(name=arg, app_dir=self.app_dir, logger=self.log)
+            update_extension(
+                name=arg, app_dir=self.app_dir, logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args
         ])
 
@@ -149,7 +160,9 @@ class LinkLabExtensionApp(BaseExtensionApp):
     def run_task(self):
         self.extra_args = self.extra_args or [os.getcwd()]
         return any([
-            link_package(arg, self.app_dir, logger=self.log)
+            link_package(
+                arg, self.app_dir, logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args
         ])
 
@@ -160,7 +173,9 @@ class UnlinkLabExtensionApp(BaseExtensionApp):
     def run_task(self):
         self.extra_args = self.extra_args or [os.getcwd()]
         return any([
-            unlink_package(arg, self.app_dir, logger=self.log)
+            unlink_package(
+                arg, self.app_dir, logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args
         ])
 
@@ -175,7 +190,9 @@ class UninstallLabExtensionApp(BaseExtensionApp):
     def run_task(self):
         self.extra_args = self.extra_args or [os.getcwd()]
         return any([
-            uninstall_extension(arg, all_=self.all, app_dir=self.app_dir, logger=self.log)
+            uninstall_extension(
+                arg, all_=self.all, app_dir=self.app_dir, logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args
         ])
 
@@ -184,14 +201,16 @@ class ListLabExtensionsApp(BaseExtensionApp):
     description = "List the installed labextensions"
 
     def run_task(self):
-        list_extensions(self.app_dir, logger=self.log)
+        list_extensions(
+            self.app_dir, logger=self.log, core_config=self.core_config)
 
 
 class EnableLabExtensionsApp(BaseExtensionApp):
     description = "Enable labextension(s) by name"
 
     def run_task(self):
-        [enable_extension(arg, self.app_dir, logger=self.log)
+        [enable_extension(
+            arg, self.app_dir, logger=self.log, core_config=self.core_config)
          for arg in self.extra_args]
 
 
@@ -199,7 +218,8 @@ class DisableLabExtensionsApp(BaseExtensionApp):
     description = "Disable labextension(s) by name"
 
     def run_task(self):
-        [disable_extension(arg, self.app_dir, logger=self.log)
+        [disable_extension(
+            arg, self.app_dir, logger=self.log, core_config=self.core_config)
          for arg in self.extra_args]
 
 
@@ -215,7 +235,8 @@ def run_task(self):
             check_extension(
                 arg, self.app_dir,
                 self.should_check_installed_only,
-                logger=self.log)
+                logger=self.log,
+                core_config=self.core_config)
             for arg in self.extra_args)
         if not all_enabled:
             self.exit(1)
diff --git a/jupyterlab/tests/test_jupyterlab.py b/jupyterlab/tests/test_jupyterlab.py
index 415b19c23fd7..518b95b6d361 100644
--- a/jupyterlab/tests/test_jupyterlab.py
+++ b/jupyterlab/tests/test_jupyterlab.py
@@ -23,9 +23,9 @@
     install_extension, uninstall_extension, list_extensions,
     build, link_package, unlink_package, build_check,
     disable_extension, enable_extension, get_app_info,
-    check_extension, _test_overlap, _get_core_data,
-    update_extension
+    check_extension, _test_overlap, update_extension
 )
+from jupyterlab.coreconfig import CoreConfig
 
 here = os.path.dirname(os.path.abspath(__file__))
 
@@ -477,7 +477,7 @@ def test_compatibility(self):
         assert _test_overlap('<0.6', '0.1') is None
 
     def test_install_compatible(self):
-        core_data = _get_core_data()
+        core_data = CoreConfig().data
         current_app_dep = core_data['dependencies']['@jupyterlab/application']
         def _gen_dep(ver):
             return { "dependencies": {

From 29a40e01ed6c58ff29158207d309156399e478ae Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 16:41:00 +0100
Subject: [PATCH 03/10] fix clean

---
 jupyterlab/labapp.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/jupyterlab/labapp.py b/jupyterlab/labapp.py
index 288b6a0afc86..992784d02560 100644
--- a/jupyterlab/labapp.py
+++ b/jupyterlab/labapp.py
@@ -116,7 +116,7 @@ class LabCleanApp(JupyterApp):
     app_dir = Unicode('', config=True, help='The app directory to clean')
 
     def start(self):
-        clean(self.app_dir, logger=self.log, core_config=self.core_config)
+        clean(self.app_dir, logger=self.log)
 
 
 class LabPathApp(JupyterApp):

From 849244d282fd154fa492d241ebe8bede33009dd2 Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 19:12:15 +0100
Subject: [PATCH 04/10] Expose current info on coreconfig

---
 jupyterlab/coreconfig.py | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index 1b35d145ba57..deeb81b1d304 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -77,7 +77,10 @@ def clear_defaults(self, lab_only=True):
             data['jupyterlab']['singletonPackages'] = []
 
     def add(self, name, semver, extension=False, mimeExtension=False):
-        """Remove a package/extension.
+        """Remove an extension/singleton.
+
+        If neither extension or mimeExtension is True (the default)
+        the package is added as a singleton dependency.
 
         name: string
             The npm package name
@@ -104,6 +107,24 @@ def add(self, name, semver, extension=False, mimeExtension=False):
         else:
             data['singletonPackages'].append(name)
 
+    @property
+    def extensions(self):
+        """A dict mapping all extension names to their semver"""
+        return dict(self._data['jupyterlab']['extensions'])
+
+    @property
+    def mimeExtensions(self):
+        """A dict mapping all MIME extension names to their semver"""
+        return dict(self._data['jupyterlab']['mimeExtensions'])
+
+    @property
+    def singletons(self):
+        """A dict mapping all singleton names to their semver"""
+        return dict(
+            (k, self._data['resolutions'][k])
+            for k in self._data['singletonPackages']
+        )
+
     def remove(self, name):
         """Remove a package/extension.
 

From 64ba1ca1d50a0bac043e74c3774014fa222495e3 Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 19:19:34 +0100
Subject: [PATCH 05/10] Avoid camelCase in Python

---
 jupyterlab/coreconfig.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index deeb81b1d304..3e07f3bcba17 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -76,7 +76,7 @@ def clear_defaults(self, lab_only=True):
             data['jupyterlab']['mimeExtensions'] = {}
             data['jupyterlab']['singletonPackages'] = []
 
-    def add(self, name, semver, extension=False, mimeExtension=False):
+    def add(self, name, semver, extension=False, mime_extension=False):
         """Remove an extension/singleton.
 
         If neither extension or mimeExtension is True (the default)
@@ -88,7 +88,7 @@ def add(self, name, semver, extension=False, mimeExtension=False):
             The semver range for the package
         extension: bool
             Whether the package is an extension
-        mimeExtension: bool
+        mime_extension: bool
             Whether the package is a MIME extension
         """
         data = self._data
@@ -98,7 +98,7 @@ def add(self, name, semver, extension=False, mimeExtension=False):
 
         # If both mimeExtension and extensions are True, treat
         # as mime extension
-        if mimeExtension:
+        if mime_extension:
             data['jupyterlab']['mimeExtensions'][name] = ""
             data['dependencies'][name] = semver
         elif extension:
@@ -113,7 +113,7 @@ def extensions(self):
         return dict(self._data['jupyterlab']['extensions'])
 
     @property
-    def mimeExtensions(self):
+    def mime_extensions(self):
         """A dict mapping all MIME extension names to their semver"""
         return dict(self._data['jupyterlab']['mimeExtensions'])
 

From b937f18f161053751095e10c253bca9614656144 Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 19:27:59 +0100
Subject: [PATCH 06/10] Fix CoreConfig signleton location

---
 jupyterlab/coreconfig.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index 3e07f3bcba17..74b4ca32630a 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -105,7 +105,7 @@ def add(self, name, semver, extension=False, mime_extension=False):
             data['jupyterlab']['extensions'][name] = ""
             data['dependencies'][name] = semver
         else:
-            data['singletonPackages'].append(name)
+            data['jupyterlab']['singletonPackages'].append(name)
 
     @property
     def extensions(self):
@@ -122,7 +122,7 @@ def singletons(self):
         """A dict mapping all singleton names to their semver"""
         return dict(
             (k, self._data['resolutions'][k])
-            for k in self._data['singletonPackages']
+            for k in self._data['jupyterlab']['singletonPackages']
         )
 
     def remove(self, name):

From 3c6a864e0ee1cd7e56951ff8a8f40f91d98dacb9 Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 20:29:33 +0100
Subject: [PATCH 07/10] More fixes for CoreConfig

---
 jupyterlab/coreconfig.py | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index 74b4ca32630a..8ce0d2cdf1df 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -92,6 +92,10 @@ def add(self, name, semver, extension=False, mime_extension=False):
             Whether the package is a MIME extension
         """
         data = self._data
+        if not name:
+            raise ValueError('Missing package name')
+        if not semver:
+            raise ValueError('Missing package semver')
         if name in self._data['resolutions']:
             raise ValueError('Package already present: %r' % (name,))
         data['resolutions'][name] = semver
@@ -110,12 +114,16 @@ def add(self, name, semver, extension=False, mime_extension=False):
     @property
     def extensions(self):
         """A dict mapping all extension names to their semver"""
-        return dict(self._data['jupyterlab']['extensions'])
+        return dict(
+            (k, self._data['resolutions'][k])
+            for k in self._data['jupyterlab']['extensions'].keys())
 
     @property
     def mime_extensions(self):
         """A dict mapping all MIME extension names to their semver"""
-        return dict(self._data['jupyterlab']['mimeExtensions'])
+        return dict(
+            (k, self._data['resolutions'][k])
+            for k in self._data['jupyterlab']['mimeExtensions'].keys())
 
     @property
     def singletons(self):

From a99899e4976d4cf19e3abc62e4ff11691c612d3f Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 21:32:21 +0100
Subject: [PATCH 08/10] Lint COTRIBUTING.md

---
 CONTRIBUTING.md | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d3d3e772a5f4..6a8048827258 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -44,13 +44,14 @@ a keyboard shortcut or automatically on save.
 ## Submitting a Pull Request Contribution
 
 Generally, an issue should be opened describing a piece of proposed work and the
-issues it solves before a pull request is opened. 
+issues it solves before a pull request is opened.
 
 ### Issue Management
-Opening an issue lets community members participate in the design discussion, 
-makes others aware of work being done, and sets the stage for a fruitful community 
-interaction. A pull request should reference the issue it is addressing. Once the 
-pull request is merged, the issue related to it will also be closed. If there is 
+
+Opening an issue lets community members participate in the design discussion,
+makes others aware of work being done, and sets the stage for a fruitful community
+interaction. A pull request should reference the issue it is addressing. Once the
+pull request is merged, the issue related to it will also be closed. If there is
 additional discussion around implemementation the issue may be re-opened. Once 30 days
 have passed with no additional discussion, the [lock bot](https://github.com/apps/lock) will lock the issue. If
 additional discussion is desired, or if the pull request doesn't fully address the

From 900b84750ff84c040cf91da276e6a1b6e1e1bfdb Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Mon, 12 Aug 2019 21:36:46 +0100
Subject: [PATCH 09/10] Update coreconfig docstrings

---
 jupyterlab/coreconfig.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index 8ce0d2cdf1df..bdeeffe0aae2 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -42,7 +42,9 @@ def _only_nonlab(collection):
 class CoreConfig:
     """An object representing a core-mode package/extension configuration.
 
-    This enables custom lab application to change
+    This enables custom lab application to change the core configuration
+    of the various build system commands. See e.g. commands.py and
+    any apps that use these functions.
     """
 
     def __init__(self):
@@ -51,7 +53,7 @@ def __init__(self):
     def clear_defaults(self, lab_only=True):
         """Clear the default packages/extensions.
 
-        lab_only: string
+        lab_only: bool
             Whether to remove all packages, or only those from
             JupyterLab. Defaults to True (only lab packages).
             This will leave dependencies like phosphor, react

From 853a5f5775edf9966efd73d27e58450e366e01fd Mon Sep 17 00:00:00 2001
From: Vidar Tonaas Fauske <vidartf@gmail.com>
Date: Wed, 14 Aug 2019 16:35:41 +0100
Subject: [PATCH 10/10] Tweak and test

---
 jupyterlab/commands.py              |   8 +-
 jupyterlab/coreconfig.py            | 129 +++++++++++++---------------
 jupyterlab/tests/test_jupyterlab.py |  50 ++++++++++-
 3 files changed, 115 insertions(+), 72 deletions(-)

diff --git a/jupyterlab/commands.py b/jupyterlab/commands.py
index 0db625380bdb..64c781f5383c 100644
--- a/jupyterlab/commands.py
+++ b/jupyterlab/commands.py
@@ -29,7 +29,7 @@
 
 from .semver import Range, gte, lt, lte, gt, make_semver
 from .jlpmapp import YARN_PATH, HERE
-from .coreconfig import CoreConfig
+from .coreconfig import _get_default_core_data
 
 
 # The regex for expecting the webpack output.
@@ -483,7 +483,9 @@ def __init__(self, app_dir, logger=None, kill_event=None, core_config=None):
         self.app_dir = app_dir or get_app_dir()
         self.sys_dir = get_app_dir()
         self.logger = _ensure_logger(logger)
-        self.core_data = (core_config or CoreConfig()).data
+        self.core_data = (
+            core_config._data if core_config else _get_default_core_data()
+        )
         self.info = self._get_app_info()
         self.kill_event = kill_event or Event()
         # TODO: Make this configurable
@@ -1629,7 +1631,7 @@ def _node_check(logger):
         output = subprocess.check_output([node, 'node-version-check.js'], cwd=HERE)
         logger.debug(output.decode('utf-8'))
     except Exception:
-        data = CoreConfig().data
+        data = CoreConfig()._data
         ver = data['engines']['node']
         msg = 'Please install nodejs %s before continuing. nodejs may be installed using conda or directly from the nodejs website.' % ver
         raise ValueError(msg)
diff --git a/jupyterlab/coreconfig.py b/jupyterlab/coreconfig.py
index bdeeffe0aae2..af08f6cdd45b 100644
--- a/jupyterlab/coreconfig.py
+++ b/jupyterlab/coreconfig.py
@@ -2,6 +2,7 @@
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
+from collections import defaultdict
 from itertools import filterfalse
 import json
 import os.path as osp
@@ -15,7 +16,7 @@ def pjoin(*args):
     return osp.abspath(osp.join(*args))
 
 
-def _get_core_data():
+def _get_default_core_data():
     """Get the data for the app template.
     """
     with open(pjoin(HERE, 'staging', 'package.json')) as fid:
@@ -28,7 +29,12 @@ def _is_lab_package(name):
 
 
 def _only_nonlab(collection):
-    """Filter a dict/sequence to remove all lab packages"""
+    """Filter a dict/sequence to remove all lab packages
+
+    This is useful to take the default values of e.g. singletons and filter
+    away the '@jupyterlab/' namespace packages, but leave any others (e.g.
+    phosphor and react).
+    """
     if isinstance(collection, dict):
         return dict(
             (k, v) for (k, v) in collection.items()
@@ -40,43 +46,13 @@ def _only_nonlab(collection):
 
 
 class CoreConfig:
-    """An object representing a core-mode package/extension configuration.
+    """An object representing a core config.
 
-    This enables custom lab application to change the core configuration
-    of the various build system commands. See e.g. commands.py and
-    any apps that use these functions.
+    This enables custom lab application to override some parts of the core
+    configuration of the build system.
     """
-
     def __init__(self):
-        self._data = _get_core_data()
-
-    def clear_defaults(self, lab_only=True):
-        """Clear the default packages/extensions.
-
-        lab_only: bool
-            Whether to remove all packages, or only those from
-            JupyterLab. Defaults to True (only lab packages).
-            This will leave dependencies like phosphor, react
-            etc untouched.
-        """
-        data = self._data
-        if lab_only:
-            # Clear all "@jupyterlab/" dependencies
-            data['dependencies'] = _only_nonlab(data['dependencies'])
-            data['resolutions'] = _only_nonlab(data['resolutions'])
-            data['jupyterlab']['extensions'] = _only_nonlab(
-                data['jupyterlab']['extensions'])
-            data['jupyterlab']['mimeExtensions'] = _only_nonlab(
-                data['jupyterlab']['mimeExtensions'])
-            data['jupyterlab']['singletonPackages'] = _only_nonlab(
-                data['jupyterlab']['singletonPackages'])
-        else:
-            # Clear all dependencies
-            data['dependencies'] = {}
-            data['resolutions'] = {}
-            data['jupyterlab']['extensions'] = {}
-            data['jupyterlab']['mimeExtensions'] = {}
-            data['jupyterlab']['singletonPackages'] = []
+        self._data = _get_default_core_data()
 
     def add(self, name, semver, extension=False, mime_extension=False):
         """Remove an extension/singleton.
@@ -98,7 +74,7 @@ def add(self, name, semver, extension=False, mime_extension=False):
             raise ValueError('Missing package name')
         if not semver:
             raise ValueError('Missing package semver')
-        if name in self._data['resolutions']:
+        if name in data['resolutions']:
             raise ValueError('Package already present: %r' % (name,))
         data['resolutions'][name] = semver
 
@@ -113,28 +89,6 @@ def add(self, name, semver, extension=False, mime_extension=False):
         else:
             data['jupyterlab']['singletonPackages'].append(name)
 
-    @property
-    def extensions(self):
-        """A dict mapping all extension names to their semver"""
-        return dict(
-            (k, self._data['resolutions'][k])
-            for k in self._data['jupyterlab']['extensions'].keys())
-
-    @property
-    def mime_extensions(self):
-        """A dict mapping all MIME extension names to their semver"""
-        return dict(
-            (k, self._data['resolutions'][k])
-            for k in self._data['jupyterlab']['mimeExtensions'].keys())
-
-    @property
-    def singletons(self):
-        """A dict mapping all singleton names to their semver"""
-        return dict(
-            (k, self._data['resolutions'][k])
-            for k in self._data['jupyterlab']['singletonPackages']
-        )
-
     def remove(self, name):
         """Remove a package/extension.
 
@@ -156,15 +110,56 @@ def remove(self, name):
 
         data['jupyterlab']['singletonPackages'].remove(name)
 
+    def clear_packages(self, lab_only=True):
+        """Clear the packages/extensions.
+        """
+        data = self._data
+        # Clear all dependencies
+        if lab_only:
+            # Clear all "@jupyterlab/" dependencies
+            data['dependencies'] = _only_nonlab(data['dependencies'])
+            data['resolutions'] = _only_nonlab(data['resolutions'])
+            data['jupyterlab']['extensions'] = _only_nonlab(
+                data['jupyterlab']['extensions'])
+            data['jupyterlab']['mimeExtensions'] = _only_nonlab(
+                data['jupyterlab']['mimeExtensions'])
+            data['jupyterlab']['singletonPackages'] = _only_nonlab(
+                data['jupyterlab']['singletonPackages'])
+        else:
+            data['dependencies'] = {}
+            data['resolutions'] = {}
+            data['jupyterlab']['extensions'] = {}
+            data['jupyterlab']['mimeExtensions'] = {}
+            data['jupyterlab']['singletonPackages'] = []
 
-    def set_static_dir(self, static_dir):
-        self._data['jupyterlab']['staticDir'] = static_dir
+    @property
+    def extensions(self):
+        """A dict mapping all extension names to their semver"""
+        data = self._data
+        return dict(
+            (k, data['resolutions'][k])
+            for k in data['jupyterlab']['extensions'].keys())
 
     @property
-    def data(self):
-        """Returns the raw core data.
+    def mime_extensions(self):
+        """A dict mapping all MIME extension names to their semver"""
+        data = self._data
+        return dict(
+            (k, data['resolutions'][k])
+            for k in data['jupyterlab']['mimeExtensions'].keys())
 
-        Its content should be considered an internal implementation
-        detail of lab, and should not be relied upon outide of lab.
-        """
-        return self._data
+    @property
+    def singletons(self):
+        """A dict mapping all singleton names to their semver"""
+        data = self._data
+        return dict(
+            (k, data['resolutions'].get(k, None))
+            for k in data['jupyterlab']['singletonPackages'])
+
+    @property
+    def static_dir(self):
+        return self._data['jupyterlab']['staticDir']
+
+    @static_dir.setter
+    def static_dir(self, static_dir):
+        self._data['jupyterlab']['staticDir'] = static_dir
diff --git a/jupyterlab/tests/test_jupyterlab.py b/jupyterlab/tests/test_jupyterlab.py
index 518b95b6d361..73c95f6173a7 100644
--- a/jupyterlab/tests/test_jupyterlab.py
+++ b/jupyterlab/tests/test_jupyterlab.py
@@ -5,6 +5,7 @@
 # Distributed under the terms of the Modified BSD License.
 import glob
 import json
+import logging
 import os
 import shutil
 import sys
@@ -25,7 +26,7 @@
     disable_extension, enable_extension, get_app_info,
     check_extension, _test_overlap, update_extension
 )
-from jupyterlab.coreconfig import CoreConfig
+from jupyterlab.coreconfig import CoreConfig, _get_default_core_data
 
 here = os.path.dirname(os.path.abspath(__file__))
 
@@ -379,6 +380,51 @@ def test_build_custom(self):
         assert data['jupyterlab']['version'] == '1.0'
         assert data['jupyterlab']['staticUrl'] == 'bar'
 
+    def test_build_custom_minimal_core_config(self):
+        default_config = CoreConfig()
+        core_config = CoreConfig()
+        core_config.clear_packages()
+        logger = logging.getLogger('jupyterlab_test_logger')
+        logger.setLevel('DEBUG')
+        extensions = (
+            '@jupyterlab/application-extension',
+            '@jupyterlab/apputils-extension',
+        )
+        singletons = (
+            "@jupyterlab/application",
+            "@jupyterlab/apputils",
+            "@jupyterlab/coreutils",
+            "@jupyterlab/services",
+        )
+        for name in extensions:
+            semver = default_config.extensions[name]
+            core_config.add(name, semver, extension=True)
+        for name in singletons:
+            semver = default_config.singletons[name]
+            core_config.add(name, semver)
+
+        assert install_extension(self.mock_extension) is True
+        build(core_config=core_config, logger=logger)
+
+        # check static directory.
+        entry = pjoin(self.app_dir, 'static', 'index.out.js')
+        with open(entry) as fid:
+            data = fid.read()
+        assert self.pkg_names['extension'] in data
+
+        pkg = pjoin(self.app_dir, 'static', 'package.json')
+        with open(pkg) as fid:
+            data = json.load(fid)
+        assert list(data['jupyterlab']['extensions'].keys()) == [
+            '@jupyterlab/application-extension',
+            '@jupyterlab/apputils-extension',
+            self.pkg_names['extension'],
+        ]
+        assert data['jupyterlab']['mimeExtensions'] == {}
+        for pkg in data['jupyterlab']['singletonPackages']:
+            if pkg.startswith('@jupyterlab/'):
+                assert pkg in singletons
+
     def test_load_extension(self):
         app = NotebookApp()
         stderr = sys.stderr
@@ -477,7 +523,7 @@ def test_compatibility(self):
         assert _test_overlap('<0.6', '0.1') is None
 
     def test_install_compatible(self):
-        core_data = CoreConfig().data
+        core_data = _get_default_core_data()
         current_app_dep = core_data['dependencies']['@jupyterlab/application']
         def _gen_dep(ver):
             return { "dependencies": {