Implement --extend-ignore

Refs PyCQA#365

docs/source/user/invocation.rst

@@ -107,6 +107,10 @@ And you should see something like:
       --ignore=errors       Comma-separated list of errors and warnings to ignore
                             (or skip). For example, ``--ignore=E4,E51,W234``.
                             (Default: E121,E123,E126,E226,E24,E704)
+      --extend-ignore=errors
+                            Comma-separated list of errors and warnings to add to
+                            the list of ignored ones. For example, ``--extend-
+                            ignore=E4,E51,W234``.
       --max-line-length=n   Maximum allowed line length for the entirety of this
                             run. (Default: 79)
       --select=errors       Comma-separated list of errors and warnings to enable.

docs/source/user/options.rst

@@ -56,6 +56,8 @@ Index of Options
 
 - :option:`flake8 --ignore`
 
+- :option:`flake8 --extend-ignore`
+
 - :option:`flake8 --max-line-length`
 
 - :option:`flake8 --select`
@@ -416,6 +418,37 @@ Options and their Descriptions
         ignore = E121,E123
 
 
+.. option:: --extend-ignore=<errors>
+
+    :ref:`Go back to index <top>`
+
+    Specify a list of codes to add to the list of ignored ones. Similar
+    considerations as in :option:`--ignore` apply here with regard to the
+    value.
+
+    The difference to the :option:`--ignore` option is, that this option can be
+    used to selectively add individual codes without overriding the default
+    list entirely.
+
+    Command-line example:
+
+    .. prompt:: bash
+
+        flake8 --extend-ignore=E4,E51,W234 dir/
+
+    This **can** be specified in config files.
+
+    Example config file usage:
+
+    .. code-block:: ini
+
+        extend-ignore =
+            E4,
+            E51,
+            W234
+        extend-ignore = E4,E51,W234
+
+
 .. option:: --max-line-length=<n>
 
     :ref:`Go back to index <top>`

src/flake8/main/options.py

@@ -18,6 +18,7 @@ def register_default_options(option_manager):
     - ``--format``
     - ``--hang-closing``
     - ``--ignore``
+    - ``--extend-ignore``
     - ``--max-line-length``
     - ``--select``
     - ``--disable-noqa``
@@ -108,6 +109,13 @@ def register_default_options(option_manager):
              ' For example, ``--ignore=E4,E51,W234``. (Default: %default)',
     )
 
+    add_option(
+        '--extend-ignore', metavar='errors', default='',
+        parse_from_config=True, comma_separated_list=True,
+        help='Comma-separated list of errors and warnings to add to the list'
+             ' of ignored ones. For example, ``--extend-ignore=E4,E51,W234``.',
+    )
+
     add_option(
         '--max-line-length', type='int', metavar='n',
         default=defaults.MAX_LINE_LENGTH, parse_from_config=True,

src/flake8/style_guide.py

@@ -3,6 +3,7 @@
 import contextlib
 import enum
 import functools
+import itertools
 import linecache
 import logging
 
@@ -166,7 +167,10 @@ def __init__(self, options):
             self.selected + self.enabled_extensions,
             reverse=True,
         ))
-        self.ignored = tuple(sorted(options.ignore, reverse=True))
+        self.ignored = tuple(sorted(
+            itertools.chain(options.ignore, options.extend_ignore),
+            reverse=True,
+        ))
         self.using_default_ignore = set(self.ignored) == set(defaults.IGNORE)
         self.using_default_select = (
             set(self.selected) == set(defaults.SELECT)

tests/unit/test_decision_engine.py

@@ -12,6 +12,7 @@ def create_options(**kwargs):
     kwargs.setdefault('select', [])
     kwargs.setdefault('extended_default_select', [])
     kwargs.setdefault('ignore', [])
+    kwargs.setdefault('extend_ignore', [])
     kwargs.setdefault('disable_noqa', False)
     kwargs.setdefault('enable_extensions', [])
     return optparse.Values(kwargs)

tests/unit/test_style_guide.py

@@ -14,6 +14,7 @@ def create_options(**kwargs):
     kwargs.setdefault('select', [])
     kwargs.setdefault('extended_default_select', [])
     kwargs.setdefault('ignore', [])
+    kwargs.setdefault('extend_ignore', [])
     kwargs.setdefault('disable_noqa', False)
     kwargs.setdefault('enable_extensions', [])
     return optparse.Values(kwargs)