Fix warning about non-ascii warnings even when they are ascii

Fix #2809

_pytest/warnings.py

@@ -72,8 +72,8 @@ def catch_warnings_for_item(item):
             unicode_warning = False
 
             if compat._PY2 and any(isinstance(m, compat.UNICODE_TYPES) for m in warn_msg.args):
-                new_args = [compat.safe_str(m) for m in warn_msg.args]
-                unicode_warning = warn_msg.args != new_args
+                new_args = [m.encode('ascii', 'replace') for m in warn_msg.args]
+                unicode_warning = list(warn_msg.args) != new_args
                 warn_msg.args = new_args
 
             msg = warnings.formatwarning(

changelog/2809.bugfix

@@ -0,0 +1 @@
+Pytest no longer complains about warnings with unicode messages being non-ascii compatible even for ascii-compatible messages. As a result of this, warnings with unicode messages are converted first to an ascii representation for safety.

testing/test_warnings.py

@@ -163,13 +163,33 @@ def test_py2_unicode(testdir, pyfile_with_warnings):
     result.stdout.fnmatch_lines([
         '*== %s ==*' % WARNINGS_SUMMARY_HEADER,
 
-        '*test_py2_unicode.py:8: UserWarning: \u6d4b\u8bd5',
+        '*test_py2_unicode.py:8: UserWarning: ??',
         '*warnings.warn(u"\u6d4b\u8bd5")',
         '*warnings.py:*: UnicodeWarning: Warning is using unicode non*',
         '* 1 passed, 2 warnings*',
     ])
 
 
+def test_py2_unicode_ascii(testdir):
+    """Ensure that our warning about 'unicode warnings containing non-ascii messages'
+    does not trigger with ascii-convertible messages"""
+    testdir.makeini('[pytest]')
+    testdir.makepyfile('''
+        import pytest
+        import warnings
+
+        @pytest.mark.filterwarnings('always')
+        def test_func():
+            warnings.warn(u"hello")
+    ''')
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines([
+        '*== %s ==*' % WARNINGS_SUMMARY_HEADER,
+        '*warnings.warn(u"hello")',
+        '* 1 passed, 1 warnings in*'
+    ])
+
+
 def test_works_with_filterwarnings(testdir):
     """Ensure our warnings capture does not mess with pre-installed filters (#2430)."""
     testdir.makepyfile('''