Refactor recwarn to use warnings.catch_warnings instead of custom code

Since we dropped 2.5, we can now use warnings.catch_warnings to do the
"catch warnings" magic for us, simplifying the code a bit.

_pytest/recwarn.py

@@ -6,11 +6,10 @@ import py
 import sys
 import warnings
 import pytest
-from collections import namedtuple
 
 
 @pytest.yield_fixture
-def recwarn(request):
+def recwarn():
     """Return a WarningsRecorder instance that provides these methods:
 
     * ``pop(category=None)``: return last warning matching the category.
@@ -115,19 +114,14 @@ def warns(expected_warning, *args, **kwargs):
             return func(*args[1:], **kwargs)
 
 
-RecordedWarning = namedtuple('RecordedWarning', (
-    'message', 'category', 'filename', 'lineno', 'file', 'line',
-))
-    
-
-class WarningsRecorder(object):
+class WarningsRecorder(warnings.catch_warnings):
     """A context manager to record raised warnings.
 
     Adapted from `warnings.catch_warnings`.
     """
 
-    def __init__(self, module=None):
-        self._module = sys.modules['warnings'] if module is None else module
+    def __init__(self):
+        super(WarningsRecorder, self).__init__(record=True)
         self._entered = False
         self._list = []
 
@@ -164,38 +158,20 @@ class WarningsRecorder(object):
         if self._entered:
             __tracebackhide__ = True
             raise RuntimeError("Cannot enter %r twice" % self)
-        self._entered = True
-        self._filters = self._module.filters
-        self._module.filters = self._filters[:]
-        self._showwarning = self._module.showwarning
-
-        def showwarning(message, category, filename, lineno,
-                        file=None, line=None):
-            self._list.append(RecordedWarning(
-                message, category, filename, lineno, file, line))
-
-            # still perform old showwarning functionality
-            self._showwarning(
-                message, category, filename, lineno, file=file, line=line)
-
-        self._module.showwarning = showwarning
-
-        # allow the same warning to be raised more than once
-
-        self._module.simplefilter('always')
+        self._list = super(WarningsRecorder, self).__enter__()
+        warnings.simplefilter('always')
         return self
 
     def __exit__(self, *exc_info):
         if not self._entered:
             __tracebackhide__ = True
             raise RuntimeError("Cannot exit %r without entering first" % self)
-        self._module.filters = self._filters
-        self._module.showwarning = self._showwarning
+        super(WarningsRecorder, self).__exit__(*exc_info)
 
 
 class WarningsChecker(WarningsRecorder):
-    def __init__(self, expected_warning=None, module=None):
-        super(WarningsChecker, self).__init__(module=module)
+    def __init__(self, expected_warning=None):
+        super(WarningsChecker, self).__init__()
 
         msg = ("exceptions must be old-style classes or "
                "derived from Warning, not %s")

testing/test_recwarn.py

@@ -217,7 +217,6 @@ class TestWarns(object):
         excinfo.match(re.escape(message_template.format(warning_classes,
                                                         [each.message for each in warninfo])))
 
-
     def test_record(self):
         with pytest.warns(UserWarning) as record:
             warnings.warn("user", UserWarning)
@@ -225,9 +224,6 @@ class TestWarns(object):
         assert len(record) == 1
         assert str(record[0].message) == "user"
 
-        print(repr(record[0]))
-        assert str(record[0].message) in repr(record[0])
-
     def test_record_only(self):
         with pytest.warns(None) as record:
             warnings.warn("user", UserWarning)