No longer override existing warning filters during warnings capture

Fix pytest-dev#2430

Author: nicoddemus

CHANGELOG.rst

@@ -1,6 +1,11 @@
 3.1.1 (unreleased)
 ==================
 
+* pytest warning capture no longer override existing warning filters. The previous
+  behaviour would override all filters and caused regressions in test suites which configure warning
+  filters to match their needs. Note as a side-effect of this is that ``DeprecationWarning``
+  and ``PendingDeprecationWarning`` are no longer shown by default. (towncrier: 2430.bugfix)
+
 * Fix encoding errors for unicode warnings in Python 2. (towncrier: 2436.bugfix)
 
 * Fix issue with non-ascii contents in doctest text files. (towncrier: 2434.bugfix)

_pytest/warnings.py

@@ -53,7 +53,6 @@ def catch_warnings_for_item(item):
     args = item.config.getoption('pythonwarnings') or []
     inifilters = item.config.getini("filterwarnings")
     with warnings.catch_warnings(record=True) as log:
-        warnings.simplefilter('once')
         for arg in args:
             warnings._setoption(arg)
 

doc/en/warnings.rst

@@ -5,32 +5,18 @@ Warnings Capture
 
 .. versionadded:: 3.1
 
-.. warning::
-    pytest captures all warnings between tests, which prevents custom warning
-    filters in existing test suites from working. If this causes problems to your test suite,
-    this plugin can be disabled in your ``pytest.ini`` file with:
-
-    .. code-block:: ini
-
-        [pytest]
-        addopts = -p no:warnings
-
-    There's an ongoing discussion about this on `#2430
-    <https://github.com/pytest-dev/pytest/issues/2430>`_.
-
-
 Starting from version ``3.1``, pytest now automatically catches all warnings during test execution
 and displays them at the end of the session::
 
     # content of test_show_warnings.py
     import warnings
 
-    def deprecated_function():
-        warnings.warn("this function is deprecated, use another_function()", DeprecationWarning)
+    def api_v1():
+        warnings.warn(UserWarning("api v1, should use functions from v2"))
         return 1
 
     def test_one():
-        assert deprecated_function() == 1
+        assert api_v1() == 1
 
 Running pytest now produces this output::
 
@@ -53,7 +39,7 @@ Running pytest now produces this output::
 The ``-W`` flag can be passed to control which warnings will be displayed or even turn
 them into errors::
 
-    $ pytest -q test_show_warnings.py -W error::DeprecationWarning
+    $ pytest -q test_show_warnings.py -W error::UserWarning
     F
     ======= FAILURES ========
     _______ test_one ________
@@ -72,15 +58,15 @@ them into errors::
     1 failed in 0.12 seconds
 
 The same option can be set in the ``pytest.ini`` file using the ``filterwarnings`` ini option.
-For example, the configuration below will ignore all deprecation warnings, but will transform
+For example, the configuration below will ignore all user warnings, but will transform
 all other warnings into errors.
 
 .. code-block:: ini
 
     [pytest]
     filterwarnings =
         error
-        ignore::DeprecationWarning
+        ignore::UserWarning
 
 
 When a warning matches more than one option in the list, the action for the last matching option
@@ -90,13 +76,39 @@ Both ``-W`` command-line option and ``filterwarnings`` ini option are based on P
 `-W option`_ and `warnings.simplefilter`_, so please refer to those sections in the Python
 documentation for other examples and advanced usage.
 
+.. note::
+
+    ``DeprecationWarning`` and ``PendingDeprecationWarning`` are hidden by the standard library
+    by default so you have to explicitly configure them to be displayed in your ``pytest.ini``:
+
+    .. code-block:: ini
+
+        [pytest]
+        filterwarnings =
+            once::DeprecationWarning
+            once::PendingDeprecationWarning
+
+
 *Credits go to Florian Schulze for the reference implementation in the* `pytest-warnings`_
 *plugin.*
 
 .. _`-W option`: https://docs.python.org/3/using/cmdline.html?highlight=#cmdoption-W
 .. _warnings.simplefilter: https://docs.python.org/3/library/warnings.html#warnings.simplefilter
 .. _`pytest-warnings`: https://github.com/fschulze/pytest-warnings
 
+
+Disabling warning capture
+-------------------------
+
+This feature is enabled by default but can be disabled entirely in your ``pytest.ini`` file with:
+
+    .. code-block:: ini
+
+        [pytest]
+        addopts = -p no:warnings
+
+Or passing ``-p no:warnings`` in the command-line.
+
 .. _`asserting warnings`:
 
 .. _assertwarnings:

testing/test_recwarn.py

@@ -148,7 +148,7 @@ def test_two():
         result = testdir.runpytest()
         # the 2 tests must pass, but the call to test_one() will generate a warning
         # in pytest's summary
-        result.stdout.fnmatch_lines('*=== 2 passed, 1 warnings in *===')
+        result.stdout.fnmatch_lines('*=== 2 passed in *===')
 
 
 class TestWarns(object):

testing/test_warnings.py

@@ -20,8 +20,8 @@ def pyfile_with_warnings(testdir, request):
         module_name: '''
             import warnings
             def foo():
-                warnings.warn(PendingDeprecationWarning("functionality is pending deprecation"))
-                warnings.warn(DeprecationWarning("functionality is deprecated"))
+                warnings.warn(UserWarning("user warning"))
+                warnings.warn(RuntimeWarning("runtime warning"))
                 return 1
         ''',
         test_name: '''
@@ -43,11 +43,11 @@ def test_normal_flow(testdir, pyfile_with_warnings):
 
         '*test_normal_flow.py::test_func',
 
-        '*normal_flow_module.py:3: PendingDeprecationWarning: functionality is pending deprecation',
-        '*  warnings.warn(PendingDeprecationWarning("functionality is pending deprecation"))',
+        '*normal_flow_module.py:3: UserWarning: user warning',
+        '*  warnings.warn(UserWarning("user warning"))',
 
-        '*normal_flow_module.py:4: DeprecationWarning: functionality is deprecated',
-        '*  warnings.warn(DeprecationWarning("functionality is deprecated"))',
+        '*normal_flow_module.py:4: RuntimeWarning: runtime warning',
+        '*  warnings.warn(RuntimeWarning("runtime warning"))',
         '* 1 passed, 2 warnings*',
     ])
     assert result.stdout.str().count('test_normal_flow.py::test_func') == 1
@@ -90,8 +90,8 @@ def test_as_errors(testdir, pyfile_with_warnings, method):
             ''')
     result = testdir.runpytest(*args)
     result.stdout.fnmatch_lines([
-        'E       PendingDeprecationWarning: functionality is pending deprecation',
-        'as_errors_module.py:3: PendingDeprecationWarning',
+        'E       UserWarning: user warning',
+        'as_errors_module.py:3: UserWarning',
         '* 1 failed in *',
     ])
 
@@ -133,9 +133,7 @@ def test_func(fix):
     result = testdir.runpytest()
     result.stdout.fnmatch_lines([
         '*== %s ==*' % WARNINGS_SUMMARY_HEADER,
-
-        '*test_unicode.py:8: UserWarning: \u6d4b\u8bd5',
-        '*warnings.warn(u"\u6d4b\u8bd5")',
+        '*test_unicode.py:8: UserWarning: \u6d4b\u8bd5*',
         '* 1 passed, 1 warnings*',
     ])
 
@@ -163,6 +161,30 @@ def test_func(fix):
 
         '*test_py2_unicode.py:8: UserWarning: \u6d4b\u8bd5',
         '*warnings.warn(u"\u6d4b\u8bd5")',
-        '*warnings.py:82: UnicodeWarning: This warning*\u6d4b\u8bd5',
+        '*warnings.py:*: UnicodeWarning: This warning*\u6d4b\u8bd5',
         '* 1 passed, 2 warnings*',
     ])
+
+
+def test_works_with_filterwarnings(testdir):
+    """Ensure our warnings capture does not mess with pre-installed filters (#2430)."""
+    testdir.makepyfile('''
+        import warnings
+
+        class MyWarning(Warning):
+            pass
+        
+        warnings.filterwarnings("error", category=MyWarning)
+        
+        class TestWarnings(object):
+            def test_my_warning(self):
+                try:
+                    warnings.warn(MyWarning("warn!"))
+                    assert False
+                except MyWarning:
+                    assert True
+    ''')
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines([
+        '*== 1 passed in *',
+    ])

tox.ini

@@ -184,7 +184,7 @@ python_files=test_*.py *_test.py testing/*/*.py
 python_classes=Test Acceptance
 python_functions=test
 norecursedirs = .tox ja .hg cx_freeze_source
-filterwarnings= error
+filterwarnings=
                 # produced by path.local
                 ignore:bad escape.*:DeprecationWarning:re
                 # produced by path.readlines