Coverage(data_file=None) means no data file at all. #871

CHANGES.rst

@@ -29,6 +29,11 @@ Unreleased
   one (or many) environments, and then report in another.  It has not had much
   real-world testing, so it may change in incompatible ways in the future.
 
+- When constructing a :class:`coverage.Coverage` object, `data_file` can be
+  specified as None to prevent writing any data file at all.  In previous
+  versions, an explicit `data_file=None` argument would use the default of
+  ".coverage". Fixes `issue 871`_.
+
 - Python files run with ``-m`` now have ``__spec__`` defined properly.  This
   fixes `issue 745`_ (about not being able to run unittest tests that spawn
   subprocesses), and `issue 838`_, which described the problem directly.
@@ -46,6 +51,7 @@ Unreleased
 .. _issue 649: https://github.com/nedbat/coveragepy/issues/649
 .. _issue 745: https://github.com/nedbat/coveragepy/issues/745
 .. _issue 838: https://github.com/nedbat/coveragepy/issues/838
+.. _issue 871: https://github.com/nedbat/coveragepy/issues/871
 
 
 .. _changes_50b1:

coverage/control.py

@@ -25,7 +25,7 @@
 from coverage.inorout import InOrOut
 from coverage.jsonreport import JsonReporter
 from coverage.misc import CoverageException, bool_or_none, join_regex
-from coverage.misc import ensure_dir_for_file, isolate_module
+from coverage.misc import DefaultValue, ensure_dir_for_file, isolate_module
 from coverage.plugin import FileReporter
 from coverage.plugin_support import Plugins
 from coverage.python import PythonFileReporter
@@ -58,6 +58,8 @@ def override_config(cov, **kwargs):
         cov.config = original_config
 
 
+_DEFAULT_DATAFILE = DefaultValue("MISSING")
+
 class Coverage(object):
     """Programmatic access to coverage.py.
 
@@ -91,16 +93,21 @@ def current(cls):
             return None
 
     def __init__(
-        self, data_file=None, data_suffix=None, cover_pylib=None,
+        self, data_file=_DEFAULT_DATAFILE, data_suffix=None, cover_pylib=None,
         auto_data=False, timid=None, branch=None, config_file=True,
         source=None, omit=None, include=None, debug=None,
         concurrency=None, check_preimported=False, context=None,
     ):
         """
-        `data_file` is the base name of the data file to use, defaulting to
-        ".coverage".  `data_suffix` is appended (with a dot) to `data_file` to
-        create the final file name.  If `data_suffix` is simply True, then a
-        suffix is created with the machine and process identity included.
+        Many of these arguments duplicate and override values that can be
+        provided in a configuration file.  Parameters that are missing here
+        will use values from the config file.
+
+        `data_file` is the base name of the data file to use. The config value
+        defaults to ".coverage".  None can be provied to prevent writing a data
+        file.  `data_suffix` is appended (with a dot) to `data_file` to create
+        the final file name.  If `data_suffix` is simply True, then a suffix is
+        created with the machine and process identity included.
 
         `cover_pylib` is a boolean determining whether Python code installed
         with the Python interpreter is measured.  This includes the Python
@@ -166,6 +173,12 @@ def __init__(
             The `check_preimported` and `context` parameters.
 
         """
+        # data_file=None means no disk file at all. data_file missing means
+        # use the value from the config file.
+        self._no_disk = data_file is None
+        if data_file is _DEFAULT_DATAFILE:
+            data_file = None
+
         # Build our configuration from a number of sources.
         self.config = read_coverage_config(
             config_file=config_file,
@@ -460,6 +473,7 @@ def _init_data(self, suffix):
                 suffix=suffix,
                 warn=self._warn,
                 debug=self._debug,
+                no_disk=self._no_disk,
             )
 
     def start(self):
@@ -526,6 +540,7 @@ def erase(self):
         self._init_data(suffix=None)
         self._data.erase(parallel=self.config.parallel)
         self._data = None
+        self._inited_for_start = False
 
     def switch_context(self, new_context):
         """Switch to a new dynamic context.

coverage/misc.py

@@ -254,6 +254,20 @@ def _needs_to_implement(that, func_name):
         )
 
 
+class DefaultValue(object):
+    """A sentinel object to use for unusual default-value needs.
+
+    Construct with a string that will be used as the repr, for display in help
+    and Sphinx output.
+
+    """
+    def __init__(self, display_as):
+        self.display_as = display_as
+
+    def __repr__(self):
+        return self.display_as
+
+
 def substitute_variables(text, variables):
     """Substitute ``${VAR}`` variables in `text` with their values.
 

doc/whatsnew5x.rst

@@ -33,6 +33,11 @@ Backward Incompatibilities
   circumstances, you may need to use ``parallel=true`` to avoid multiple
   processes overwriting each others' data.
 
+- When constructing a :class:`coverage.Coverage` object, `data_file` can be
+  specified as None to prevent writing any data file at all.  In previous
+  versions, an explicit `data_file=None` argument would use the default of
+  ".coverage". Fixes :github:`871`.
+
 - The ``[run] note`` setting has been deprecated. Using it will result in a
   warning, and the note will not be written to the data file.  The
   corresponding :class:`.CoverageData` methods have been removed.

tests/test_api.py

@@ -16,7 +16,7 @@
 
 import coverage
 from coverage import env
-from coverage.backward import StringIO, import_local_file
+from coverage.backward import code_object, import_local_file, StringIO
 from coverage.data import line_counts
 from coverage.files import abs_file
 from coverage.misc import CoverageException
@@ -263,6 +263,31 @@ def test_deep_datafile(self):
         self.assertFiles(["datatest5.py", "deep"])
         self.assert_exists("deep/sub/cov.data")
 
+    def test_datafile_none(self):
+        cov = coverage.Coverage(data_file=None)
+
+        def f1():
+            a = 1       # pylint: disable=unused-variable
+
+        one_line_number = code_object(f1).co_firstlineno + 1
+        lines = []
+
+        def run_one_function(f):
+            cov.erase()
+            cov.start()
+            f()
+            cov.stop()
+
+            fs = cov.get_data().measured_files()
+            lines.append(cov.get_data().lines(list(fs)[0]))
+
+        run_one_function(f1)
+        run_one_function(f1)
+        run_one_function(f1)
+        assert lines == [[one_line_number]] * 3
+        self.assert_doesnt_exist(".coverage")
+        assert os.listdir(".") == []
+
     def test_empty_reporting(self):
         # empty summary reports raise exception, just like the xml report
         cov = coverage.Coverage()