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).

docs/source/user/invocation.rst

@@ -125,6 +125,7 @@ And you should see something like:
                             available to use. (Default: auto)
       --output-file=OUTPUT_FILE
                             Redirect report to a file.
+      --tee                 Write to stdout and output-file.
       --append-config=APPEND_CONFIG
                             Provide extra config files to parse in addition to the
                             files found by Flake8 by default. These files are the

docs/source/user/options.rst

@@ -563,6 +563,26 @@
         output_file = output.txt
 
 
+.. option:: --tee
+
+    Also print output to stdout if output-file has ben configured.
+
+    Command-line example:
+
+    .. prompt:: bash
+
+        flake8 --tee --output-file=output.txt dir/
+
+    This **can** be specified in config files.
+
+    Example config file usage:
+
+    .. code-block:: ini
+
+        output-file = output.txt
+        tee = True
+
+
 .. option:: --append-config=<config>
 
     Provide extra config files to parse in after and in addition to the files

src/flake8/formatting/base.py

@@ -148,7 +148,7 @@ class BaseFormatter(object):
         """Handle logic of whether to use an output file or print()."""
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)
-        else:
+        if self.output_fd is None or self.options.tee:
             print(output)
 
     def write(self, line, source):

src/flake8/main/options.py

@@ -26,6 +26,7 @@ def register_default_options(option_manager):
     - ``--exit-zero``
     - ``-j``/``--jobs``
     - ``--output-file``
+    - ``--tee``
     - ``--append-config``
     - ``--config``
     - ``--isolated``
@@ -171,6 +172,11 @@ def register_default_options(option_manager):
         help='Redirect report to a file.',
     )
 
+    add_option(
+        '--tee', default=False, parse_from_config=True, action='store_true',
+        help='Write to stdout and output-file.',
+    )
+
     # Config file options
 
     add_option(

tests/unit/test_base_formatter.py

@@ -1,4 +1,5 @@
 """Tests for the BaseFormatter object."""
+import io
 import optparse
 
 import mock
@@ -11,6 +12,7 @@ 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 +78,22 @@ def test_show_source_updates_physical_line_appropriately(line, column):
     assert pointer.count(' ') == column
 
 
-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'
+    line = u'Something to write'
+    source = u'source'
     filemock = mock.Mock()
 
-    formatter = base.BaseFormatter(options())
-    formatter.output_fd = filemock
-    formatter.write(line, source)
+    with mock.patch('sys.stdout', new_callable=io.StringIO) as stdout:
+        formatter = base.BaseFormatter(options(tee=tee))
+        formatter.output_fd = filemock
+        formatter.write(line, source)
+        if tee:
+            output = line + formatter.newline + source + formatter.newline
+        else:
+            output = ''
+        assert stdout.getvalue() == output
 
     assert filemock.write.called is True
     assert filemock.write.call_count == 2