Open our output file in append mode always

This avoid overwriting portions of our log output when using Flake8
in verbose mode.

Closes #193

docs/source/release-notes/3.0.3.rst

@@ -16,12 +16,18 @@
   not updated for the current version of flake8. This is nicer than raising a
   `PicklingError` about failing to pickle a module (See also `GitLab#164`_)
 
+- Always open our output file in append mode so we do not overwrite log
+  messages. (See also `GitLab#193`_)
+
+
 .. links
+.. _GitLab#164:
+    https://gitlab.com/pycqa/flake8/issues/164
 .. _GitLab#178:
     https://gitlab.com/pycqa/flake8/issues/178
+.. _GitLab#193:
+    https://gitlab.com/pycqa/flake8/issues/193
 .. _GitLab#195:
     https://gitlab.com/pycqa/flake8/issues/195
 .. _this Python bug report:
     https://bugs.python.org/issue27649
-.. _GitLab#164:
-    https://gitlab.com/pycqa/flake8/issues/164

src/flake8/formatting/base.py

@@ -50,7 +50,7 @@ class BaseFormatter(object):
         This defaults to initializing :attr:`output_fd` if :attr:`filename`
         """
         if self.filename:
-            self.output_fd = open(self.filename, 'w')
+            self.output_fd = open(self.filename, 'a')
 
     def handle(self, error):
         """Handle an error reported by Flake8.

tests/unit/test_base_formatter.py

@@ -25,7 +25,7 @@ def test_start(filename):
     if filename is None:
         assert mock_open.called is False
     else:
-        mock_open.assert_called_once_with(filename, 'w')
+        mock_open.assert_called_once_with(filename, 'a')
 
 
 def test_stop():