Warn if two things change the dynamic context. #901

CHANGES.rst

@@ -30,7 +30,11 @@ Unreleased
   information about the config files read now shows absolute paths to the
   files.
 
+- A new warning ("dynamic-conflict") is issued if two mechanisms are trying to
+  change the dynamic context.  Closes `issue 901`_.
+
 .. _issue 890: https://github.com/nedbat/coveragepy/issues/890
+.. _issue 901: https://github.com/nedbat/coveragepy/issues/901
 
 
 .. _changes_501:

coverage/control.py

@@ -581,6 +581,10 @@ def switch_context(self, new_context):
             raise CoverageException(
                 "Cannot switch context, coverage is not started"
                 )
+
+        if self._collector.should_start_context:
+            self._warn("Conflicting dynamic contexts", slug="dynamic-conflict", once=True)
+
         self._collector.switch_context(new_context)
 
     def clear_exclude(self, which='exclude'):

doc/cmd.rst

@@ -179,6 +179,12 @@ could affect the measurement process.  The possible warnings include:
   are meant to focus measurement on a particular part of your source code, so
   ``--include`` is ignored in favor of ``--source``.
 
+* ``Conflicting dynamic contexts (dynamic-conflict)`` |br|
+  The ``[run] dynamic_context`` option is set in the configuration file, but
+  something (probably a test runner plugin) is also calling the
+  :meth:`.Coverage.switch_context` function to change the context. Only one of
+  these mechanisms should be in use at a time.
+
 Individual warnings can be disabled with the `disable_warnings
 <config_run_disable_warnings>`_ configuration setting.  To silence "No data was
 collected," add this to your .coveragerc file::

tests/test_api.py

@@ -672,6 +672,19 @@ def test_switch_context_with_static(self):
         data.set_query_context("mysuite|multiply_zero")
         self.assertEqual([2, 5], sorted(data.lines(suite_filename)))
 
+    def test_dynamic_context_conflict(self):
+        cov = coverage.Coverage(source=["."])
+        cov.set_option("run:dynamic_context", "test_function")
+        cov.start()
+        # Switch twice, but only get one warning.
+        cov.switch_context("test1")                                     # pragma: nested
+        cov.switch_context("test2")                                     # pragma: nested
+        self.assertEqual(                                               # pragma: nested
+            self.stderr(),
+            "Coverage.py warning: Conflicting dynamic contexts (dynamic-conflict)\n"
+        )
+        cov.stop()                                                      # pragma: nested
+
     def test_switch_context_unstarted(self):
         # Coverage must be started to switch context
         msg = "Cannot switch context, coverage is not started"