Make pytest warnings show as from 'pytest' module instead of '_pytest.warning_types'

When we configure warnings as errors, users see error messages like this:

        def test():
    >       warnings.warn(pytest.PytestWarning("some warning"))
    E       _pytest.warning_types.PytestWarning: some warning

This is a problem because suggests the user should use `_pytest.warning_types.PytestWarning` to configure
their warning filters, which is not nice.

This commit changes the message to:

        def test():
    >       warnings.warn(pytest.PytestWarning("some warning"))
    E       pytest.PytestWarning: some warning

changelog/5452.feature.rst

@@ -0,0 +1 @@
+When warnings are configured as errors, pytest warnings now appear as originating from ``pytest.`` instead of the internal ``_pytest.warning_types.`` module.

src/_pytest/warning_types.py

@@ -8,6 +8,8 @@ class PytestWarning(UserWarning):
     Base class for all warnings emitted by pytest.
     """
 
+    __module__ = "pytest"
+
 
 class PytestAssertRewriteWarning(PytestWarning):
     """
@@ -16,6 +18,8 @@ class PytestAssertRewriteWarning(PytestWarning):
     Warning emitted by the pytest assert rewrite module.
     """
 
+    __module__ = "pytest"
+
 
 class PytestCacheWarning(PytestWarning):
     """
@@ -24,6 +28,8 @@ class PytestCacheWarning(PytestWarning):
     Warning emitted by the cache plugin in various situations.
     """
 
+    __module__ = "pytest"
+
 
 class PytestConfigWarning(PytestWarning):
     """
@@ -32,6 +38,8 @@ class PytestConfigWarning(PytestWarning):
     Warning emitted for configuration issues.
     """
 
+    __module__ = "pytest"
+
 
 class PytestCollectionWarning(PytestWarning):
     """
@@ -40,6 +48,8 @@ class PytestCollectionWarning(PytestWarning):
     Warning emitted when pytest is not able to collect a file or symbol in a module.
     """
 
+    __module__ = "pytest"
+
 
 class PytestDeprecationWarning(PytestWarning, DeprecationWarning):
     """
@@ -48,6 +58,8 @@ class PytestDeprecationWarning(PytestWarning, DeprecationWarning):
     Warning class for features that will be removed in a future version.
     """
 
+    __module__ = "pytest"
+
 
 class PytestExperimentalApiWarning(PytestWarning, FutureWarning):
     """
@@ -57,6 +69,8 @@ class PytestExperimentalApiWarning(PytestWarning, FutureWarning):
     removed completely in future version
     """
 
+    __module__ = "pytest"
+
     @classmethod
     def simple(cls, apiname):
         return cls(
@@ -75,6 +89,8 @@ class PytestUnhandledCoroutineWarning(PytestWarning):
     are not natively supported.
     """
 
+    __module__ = "pytest"
+
 
 class PytestUnknownMarkWarning(PytestWarning):
     """
@@ -84,6 +100,8 @@ class PytestUnknownMarkWarning(PytestWarning):
     See https://docs.pytest.org/en/latest/mark.html for details.
     """
 
+    __module__ = "pytest"
+
 
 class RemovedInPytest4Warning(PytestDeprecationWarning):
     """
@@ -92,6 +110,8 @@ class RemovedInPytest4Warning(PytestDeprecationWarning):
     Warning class for features scheduled to be removed in pytest 4.0.
     """
 
+    __module__ = "pytest"
+
 
 @attr.s
 class UnformattedWarning:

testing/test_warning_types.py

@@ -0,0 +1,37 @@
+import inspect
+
+import _pytest.warning_types
+import pytest
+
+
+@pytest.mark.parametrize(
+    "warning_class",
+    [
+        w
+        for n, w in vars(_pytest.warning_types).items()
+        if inspect.isclass(w) and issubclass(w, Warning)
+    ],
+)
+def test_warning_types(warning_class):
+    """Make sure all warnings declared in _pytest.warning_types are displayed as coming
+    from 'pytest' instead of the internal module (#5452).
+    """
+    assert warning_class.__module__ == "pytest"
+
+
+@pytest.mark.filterwarnings("error::pytest.PytestWarning")
+def test_pytest_warnings_repr_integration_test(testdir):
+    """Small integration test to ensure our small hack of setting the __module__ attribute
+    of our warnings actually works (#5452).
+    """
+    testdir.makepyfile(
+        """
+        import pytest
+        import warnings
+
+        def test():
+            warnings.warn(pytest.PytestWarning("some warning"))
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(["E       pytest.PytestWarning: some warning"])