Use record.message field instead of getMessage() method

In case when a log record has mutable objects attached as arguments,
the getMessage() method may return different results each time it is
called. Instead, use the 'message' field initialized by a formatter
upon a record reaches a handler.

pytest_catchlog/fixture.py

@@ -61,7 +61,7 @@ def record_tuples(self):
 
             (logger_name, log_level, message)
         """
-        return [(r.name, r.levelno, r.getMessage()) for r in self.records]
+        return [(r.name, r.levelno, r.message) for r in self.records]
 
     def set_level(self, level, logger=None):
         """Sets the level for capturing of logs.

tests/test_fixture.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import sys
 import logging
+from fnmatch import fnmatch
 
 import pytest
 
@@ -65,3 +66,21 @@ def test_unicode(caplog):
     assert caplog.records[0].levelname == 'INFO'
     assert caplog.records[0].msg == u('bū')
     assert u('bū') in caplog.text
+
+
+def test_mutable_arg(caplog):
+    mutable = {}
+    logger.info("Mutable dict %r empty", mutable)
+    mutable['foo'] = 'bar'
+    logger.info("Mutable dict %r bar", mutable)
+    mutable['foo'] = 'baz'
+    logger.info("Mutable dict %r baz", mutable)
+
+    assert caplog.record_tuples == [
+        (__name__, logging.INFO, "Mutable dict {} empty"),
+        (__name__, logging.INFO, "Mutable dict {'foo': 'bar'} bar"),
+        (__name__, logging.INFO, "Mutable dict {'foo': 'baz'} baz"),
+    ]
+    assert fnmatch(caplog.text, ("*Mutable dict {} empty\n"
+                                 "*Mutable dict {'foo': 'bar'} bar\n"
+                                 "*Mutable dict {'foo': 'baz'} baz\n"))

tests/test_reporting.py

@@ -138,3 +138,26 @@ def test_foo():
                                  'text going to stderr'])
     with pytest.raises(pytest.fail.Exception):
         result.stdout.fnmatch_lines(['*- Captured *log call -*'])
+
+
+def test_mutable_arg(testdir):
+    testdir.makepyfile('''
+        import logging
+
+        logger = logging.getLogger(__name__)
+
+        def test_it():
+            mutable = {}
+            logger.info("Mutable dict %r empty", mutable)
+            mutable['foo'] = 'bar'
+            logger.info("Mutable dict %r bar", mutable)
+            mutable['foo'] = 'baz'
+            logger.info("Mutable dict %r baz", mutable)
+            assert False
+        ''')
+    result = testdir.runpytest()
+    assert result.ret == 1
+    result.stdout.fnmatch_lines(['*- Captured *log call -*',
+                                 "*Mutable dict {} empty",
+                                 "*Mutable dict {'foo': 'bar'} bar",
+                                 "*Mutable dict {'foo': 'baz'} baz"])