Implementing no_cov feature for #131

README.rst

@@ -312,6 +312,19 @@ subprocess.  The python used by the subprocess must have pytest-cov installed.
 do normal site initialisation so that the environment variables can be detected and coverage
 started.
 
+Coverage and debuggers
+----------------------
+
+When it comes to TDD one obviously would like to debug tests. Debuggers in Python use mostly the sys.settrace function
+to gain access to context. Coverage uses the same technique to get access to the lines executed. Coverage does not play
+well with other tracers simultaneously running. This manifests itself in behaviour that PyCharm might not hit a
+breakpoint no matter what the user does. Since it is common practice to have coverage configuration in the pytest.ini
+file and pytest does not support removeopts or similar the `--no-cov` flag can disable coverage completely.
+
+At the reporting part a warning message will show on screen
+
+    Coverage disabled via --no-cov switch!
+
 Acknowledgements
 ================
 

src/pytest_cov/plugin.py

@@ -68,6 +68,9 @@ def pytest_addoption(parser):
     group.addoption('--no-cov-on-fail', action='store_true', default=False,
                     help='do not report coverage if test run fails, '
                          'default: False')
+    group.addoption('--no-cov', action='store_true', default=False,
+                    help='Disable coverage report completely (useful for debuggers) '
+                         'default: False')
     group.addoption('--cov-fail-under', action='store', metavar='MIN', type=int,
                     help='Fail if the total coverage is less than MIN.')
     group.addoption('--cov-append', action='store_true', default=False,
@@ -131,11 +134,16 @@ def __init__(self, options, pluginmanager, start=True):
         self.cov_total = None
         self.failed = False
         self._started = False
+        self._disabled = False
         self.options = options
 
         is_dist = (getattr(options, 'numprocesses', False) or
                    getattr(options, 'distload', False) or
                    getattr(options, 'dist', 'no') != 'no')
+        if options.no_cov:
+            self._disabled = True
+            return
+
         if is_dist and start:
             self.start(engine.DistMaster)
         elif start:
@@ -144,6 +152,7 @@ def __init__(self, options, pluginmanager, start=True):
         # slave is started in pytest hook
 
     def start(self, controller_cls, config=None, nodeid=None):
+
         if config is None:
             # fake config option for engine
             class Config(object):
@@ -170,6 +179,12 @@ def _is_slave(self, session):
 
     def pytest_sessionstart(self, session):
         """At session start determine our implementation and delegate to it."""
+
+        if self.options.no_cov:
+            # Coverage can be disabled because it does not cooperate with debuggers well.py
+            self._disabled = True
+            return
+
         self.pid = os.getpid()
         if self._is_slave(session):
             nodeid = session.config.slaveinput.get('slaveid',
@@ -207,6 +222,9 @@ def _failed_cov_total(self):
     def pytest_runtestloop(self, session):
         yield
 
+        if self._disabled:
+            return
+
         compat_session = compat.SessionWrapper(session)
 
         self.failed = bool(compat_session.testsfailed)
@@ -226,6 +244,12 @@ def pytest_runtestloop(self, session):
                 compat_session.testsfailed += 1
 
     def pytest_terminal_summary(self, terminalreporter):
+        if self._disabled:
+            msg = (
+                'Coverage disabled via --no-cov switch!'
+            )
+            terminalreporter.config.warn(code='COV-U1', message=msg)
+            return
         if self.cov_controller is None:
             return
 

tests/test_pytest_cov.py

@@ -420,6 +420,20 @@ def test_no_cov_on_fail(testdir):
     result.stdout.fnmatch_lines(['*1 failed*'])
 
 
+def test_no_cov(testdir):
+    script = testdir.makepyfile(SCRIPT)
+
+    result = testdir.runpytest('-v',
+                               '--cov=%s' % script.dirpath(),
+                               '--cov-report=term-missing',
+                               '--no-cov',
+                               '-rw',
+                               script)
+
+    assert 'WCOV-U1 None Coverage disabled via --no-cov switch!' in result.stdout.str()
+    assert result.ret == 0
+
+
 def test_cov_and_failure_report_on_fail(testdir):
     script = testdir.makepyfile(SCRIPT + SCRIPT_FAIL)