Make warnings plugin opt-in

CHANGELOG.rst

@@ -1,6 +1,28 @@
-3.1.1 (unreleased)
+3.2.0 (unreleased)
 ==================
 
+
+Changes
+-------
+
+* The warnings capture plugin introduced in 3.1 in now **opt-in**: users must explicitly enable it by adding this to their
+  ``pytest.ini`` file:
+
+  .. code-block:: ini
+
+        [pytest]
+        addopts = -p _pytest.warnings
+
+  This change was made because it being automatically active has broken some test suites and the team reconsidered
+  it being active by default (See `#2430`_ for a complete overview of the discussion).
+
+
+.. _#2430: https://github.com/pytest-dev/pytest/issues/2430
+
+
+Bug Fixes
+---------
+
 * 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/config.py

@@ -99,7 +99,7 @@ def directory_arg(path, optname):
      "mark main terminal runner python fixtures debugging unittest capture skipping "
      "tmpdir monkeypatch recwarn pastebin helpconfig nose assertion "
      "junitxml resultlog doctest cacheprovider freeze_support "
-     "setuponly setupplan warnings").split()
+     "setuponly setupplan").split()
 
 
 builtin_plugins = set(default_plugins)

doc/en/warnings.rst

@@ -4,22 +4,21 @@ Warnings Capture
 ================
 
 .. versionadded:: 3.1
+.. versionchanged:: 3.2
+    This plugin must now be explicitly enabled in your ``pytest.ini`` file or in the command line.
 
 .. 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:
+    filters in existing test suites from working. For this reason starting in pytest 3.2 this plugin
+    must be explicitly enabled in your ``pytest.ini`` file with:
 
     .. code-block:: ini
 
         [pytest]
-        addopts = -p no:warnings
+        addopts = -p _pytest.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
+This plugin makes pytest automatically catch all warnings during test execution
 and displays them at the end of the session::
 
     # content of test_show_warnings.py

testing/test_warnings.py

@@ -8,6 +8,15 @@
 
 WARNINGS_SUMMARY_HEADER = 'warnings summary'
 
+
+@pytest.fixture(autouse=True)
+def enable_warnings_plugin(monkeypatch):
+    """
+    Auto-use fixture for this module which enables the warnings plugin, because it is opt-in.
+    """
+    monkeypatch.setenv('PYTEST_ADDOPTS', '-p _pytest.warnings')
+
+
 @pytest.fixture
 def pyfile_with_warnings(testdir, request):
     """
@@ -53,6 +62,16 @@ def test_normal_flow(testdir, pyfile_with_warnings):
     assert result.stdout.str().count('test_normal_flow.py::test_func') == 1
 
 
+def test_plugin_is_opt_in(testdir, pyfile_with_warnings, monkeypatch):
+    """
+    Ensure warnings plugin is opt-in (#2430).
+    """
+    monkeypatch.delenv('PYTEST_ADDOPTS')
+    result = testdir.runpytest()
+    assert WARNINGS_SUMMARY_HEADER not in result.stdout.str()
+    assert 'DeprecationWarning' not in result.stdout.str()
+
+
 def test_setup_teardown_warnings(testdir, pyfile_with_warnings):
     testdir.makepyfile('''
         import warnings