Refactor to support the per-file-ignores

Most of the problems with our logic were due to not having the same
logic as our exclude parameter. This refactors that out into a separate
function so we can confidently achieve that.

src/flake8/checker.py

@@ -2,7 +2,6 @@
 import collections
 import errno
 import logging
-import os
 import signal
 import sys
 import tokenize
@@ -188,20 +187,12 @@ class Manager(object):
                 return False
             path = self.options.stdin_display_name
 
-        exclude = self.options.exclude
-        if not exclude:
-            return False
-        basename = os.path.basename(path)
-        if utils.fnmatch(basename, exclude):
-            LOG.debug('"%s" has been excluded', basename)
-            return True
-
-        absolute_path = os.path.abspath(path)
-        match = utils.fnmatch(absolute_path, exclude)
-        LOG.debug(
-            '"%s" has %sbeen excluded', absolute_path, "" if match else "not "
+        return utils.matches_filename(
+            path,
+            patterns=self.options.exclude,
+            log_message='"%(path)s" has %(whether)sbeen excluded',
+            logger=LOG,
         )
-        return match
 
     def make_checkers(self, paths=None):
         # type: (List[str]) -> NoneType

src/flake8/style_guide.py

@@ -459,7 +459,7 @@ class StyleGuide(object):
     def copy(self, filename=None, extend_ignore_with=None, **kwargs):
         """Create a copy of this style guide with different values."""
         filename = filename or self.filename
-        options = copy.copy(self.options)
+        options = copy.deepcopy(self.options)
         options.ignore.extend(extend_ignore_with or [])
         return StyleGuide(
             options, self.listener, self.formatter, filename=filename
@@ -485,11 +485,11 @@ class StyleGuide(object):
         """
         if self.filename is None:
             return True
-        normalized_filename = utils.normalize_path(filename)
-        return (
-            normalized_filename == self.filename
-            or utils.fnmatch(filename, self.filename)
-            or utils.fnmatch(normalized_filename, self.filename)
+        return utils.matches_filename(
+            filename,
+            patterns=[self.filename],
+            log_message='{!r} does %(whether)smatch "%(path)s"'.format(self),
+            logger=LOG,
         )
 
     def should_report_error(self, code):

src/flake8/utils.py

@@ -336,6 +336,38 @@ def parameters_for(plugin):
     return parameters
 
 
+def matches_filename(path, patterns, log_message, logger):
+    """Use fnmatch to discern if a path exists in patterns.
+
+    :param str path:
+        The path to the file under question
+    :param patterns:
+        The patterns to match the path against.
+    :type patterns:
+        list[str]
+    :param str log_message:
+        The message used for logging purposes.
+    :returns:
+        True if path matches patterns, False otherwise
+    :rtype:
+        bool
+    """
+    if not patterns:
+        return False
+    basename = os.path.basename(path)
+    if fnmatch(basename, patterns):
+        logger.debug(log_message, {"path": basename, "whether": ""})
+        return True
+
+    absolute_path = os.path.abspath(path)
+    match = fnmatch(absolute_path, patterns)
+    logger.debug(
+        log_message,
+        {"path": absolute_path, "whether": "" if match else "not "},
+    )
+    return match
+
+
 def get_python_version():
     """Find and format the python implementation and version.