Normalize 'per-file-ignores' file paths

Leverage the `type` keyword argument to `ArgumentParser.add_option()` for
converting the raw string, either from a configuration file or the
command-line, into a data structure for later use.

Now with `OptionManager.parse_args()` feeding configuration file values
as default values to be considered by the `ArgumentParser.parse_args()`
framework, normalization of the file paths occur in single place.

src/flake8/main/options.py

@@ -1,11 +1,25 @@
 """Contains the logic for all of the default options for Flake8."""
+import argparse
 import functools
+from typing import List, Tuple
 
 from flake8 import defaults
+from flake8 import exceptions
+from flake8 import utils
 from flake8.main import debug
 from flake8.main import vcs
 
 
+def _convert_per_file_ignores(value):
+    # type: (str) -> List[Tuple[str, List[str]]]
+    try:
+        mappings = utils.parse_files_to_codes_mapping(value)
+    except exceptions.ExecutionError as e:
+        raise argparse.ArgumentTypeError(e)
+
+    return [(utils.normalize_path(path), codes) for path, codes in mappings]
+
+
 def register_default_options(option_manager):
     """Register the default options on our OptionManager.
 
@@ -159,6 +173,7 @@ def register_default_options(option_manager):
 
     add_option(
         "--per-file-ignores",
+        type=_convert_per_file_ignores,
         default="",
         parse_from_config=True,
         help="A pairing of filenames and violation codes that defines which "

src/flake8/style_guide.py

@@ -347,7 +347,7 @@ class StyleGuideManager(object):
         """Generate style guides from the per-file-ignores option.
 
         :param options:
-            The original options parsed from the CLI and config file.
+            The normalized options parsed from the CLI and config file.
         :type options:
             :class:`~argparse.Namespace`
         :returns:
@@ -355,10 +355,7 @@ class StyleGuideManager(object):
         :rtype:
             :class:`~flake8.style_guide.StyleGuide`
         """
-        per_file = utils.parse_files_to_codes_mapping(
-            options.per_file_ignores
-        )
-        for filename, violations in per_file:
+        for filename, violations in options.per_file_ignores:
             yield self.default_style_guide.copy(
                 filename=filename, extend_ignore_with=violations
             )

src/flake8/utils.py

@@ -116,11 +116,9 @@ def parse_files_to_codes_mapping(value_):  # noqa: C901
             return "    " + s.strip().replace("\n", "\n    ")
 
         return exceptions.ExecutionError(
-            "Expected `per-file-ignores` to be a mapping from file exclude "
-            "patterns to ignore codes.\n\n"
-            "Configured `per-file-ignores` setting:\n\n{}".format(
-                _indent(value)
-            )
+            "Expected a mapping from file exclude patterns to ignore "
+            "codes.\n\n"
+            "Found:\n\n{}".format(_indent(value))
         )
 
     for token in _tokenize_files_to_codes_mapping(value):

tests/integration/test_main.py

@@ -94,14 +94,14 @@ per-file-ignores =
 
     with tmpdir.as_cwd():
         tmpdir.join('setup.cfg').write(setup_cfg)
-        _call_main(['.'], retv=1)
+        _call_main(['.'], retv=2)
 
     out, err = capsys.readouterr()
-    assert out == '''\
-There was a critical error during execution of Flake8:
-Expected `per-file-ignores` to be a mapping from file exclude patterns to ignore codes.
+    assert err == '''\
+usage: flake8 [options] file file ...
+flake8: error: argument --per-file-ignores: Expected a mapping from file exclude patterns to ignore codes.
 
-Configured `per-file-ignores` setting:
+Found:
 
     incorrect/*
     values/*

tests/unit/test_style_guide.py

@@ -45,11 +45,11 @@ def test_style_guide_manager():
     assert len(guide.style_guides) == 1
 
 
-PER_FILE_IGNORES_UNPARSED = [
-    "first_file.py:W9",
-    "second_file.py:F4,F9",
-    "third_file.py:E3",
-    "sub_dir/*:F4",
+PER_FILE_IGNORES_PARSED = [
+    ("first_file.py", ["W9"]),
+    ("second_file.py", ["F4", "F9"]),
+    ("third_file.py", ["E3"]),
+    ("sub_dir/*", ["F4"]),
 ]
 
 
@@ -75,7 +75,7 @@ def test_style_guide_applies_to(style_guide_file, filename, expected):
 def test_style_guide_manager_pre_file_ignores_parsing():
     """Verify how the StyleGuideManager creates a default style guide."""
     formatter = mock.create_autospec(base.BaseFormatter, instance=True)
-    options = create_options(per_file_ignores=PER_FILE_IGNORES_UNPARSED)
+    options = create_options(per_file_ignores=PER_FILE_IGNORES_PARSED)
     guide = style_guide.StyleGuideManager(options, formatter=formatter)
     assert len(guide.style_guides) == 5
     assert list(map(utils.normalize_path,
@@ -98,7 +98,7 @@ def test_style_guide_manager_pre_file_ignores(ignores, violation, filename,
     formatter = mock.create_autospec(base.BaseFormatter, instance=True)
     options = create_options(ignore=ignores,
                              select=['E', 'F', 'W'],
-                             per_file_ignores=PER_FILE_IGNORES_UNPARSED)
+                             per_file_ignores=PER_FILE_IGNORES_PARSED)
     guide = style_guide.StyleGuideManager(options, formatter=formatter)
     assert (guide.handle_error(violation, filename, 1, 1, "Fake text")
             == handle_error_return)
@@ -115,7 +115,7 @@ def test_style_guide_manager_pre_file_ignores(ignores, violation, filename,
 def test_style_guide_manager_style_guide_for(filename, expected):
     """Verify the style guide selection function."""
     formatter = mock.create_autospec(base.BaseFormatter, instance=True)
-    options = create_options(per_file_ignores=PER_FILE_IGNORES_UNPARSED)
+    options = create_options(per_file_ignores=PER_FILE_IGNORES_PARSED)
     guide = style_guide.StyleGuideManager(options, formatter=formatter)
 
     file_guide = guide.style_guide_for(filename)