Merge branch 'master' into 'master'

Add --tee option to split report output stream.

The `--tee` option allows the linter report to be written to stdout, even
though it is being redirected to a file with the` --output-file` option.
This is useful if I want to store the report in a separate file for later
analysis but also be able to print the output on screen (e.g when running
in a CI environment).

See merge request !90

tests/unit/test_base_formatter.py

@@ -11,6 +11,7 @@ from flake8.formatting import base
 def options(**kwargs):
     """Create an optparse.Values instance."""
     kwargs.setdefault('output_file', None)
+    kwargs.setdefault('tee', False)
     return optparse.Values(kwargs)
 
 
@@ -76,15 +77,26 @@ def test_show_source_updates_physical_line_appropriately(line, column):
     assert pointer.count(' ') == (column - 1)
 
 
-def test_write_uses_an_output_file():
+@pytest.mark.parametrize('tee', [False, True])
+def test_write_uses_an_output_file(tee):
     """Verify that we use the output file when it's present."""
     line = 'Something to write'
     source = 'source'
     filemock = mock.Mock()
 
-    formatter = base.BaseFormatter(options())
+    formatter = base.BaseFormatter(options(tee=tee))
     formatter.output_fd = filemock
-    formatter.write(line, source)
+
+    with mock.patch('flake8.formatting.base.print') as print_func:
+        formatter.write(line, source)
+        if tee:
+            assert print_func.called
+            assert print_func.mock_calls == [
+                mock.call(line),
+                mock.call(source),
+            ]
+        else:
+            assert not print_func.called
 
     assert filemock.write.called is True
     assert filemock.write.call_count == 2