Allow stdin and directly named files to be excluded from check

For the sake of IDEs, check filename for exclusion even if the file is directly
named in the command line.

Also, if the filename is "-" (stdin) check the provided display name for
exclusion.

Also, avoid calling path checking functions on the "-" filename:

 * fnmatch.fnmatch()
 * os.path.isdir()
 * os.path.exists()

2 files changed, 24 insertions, 17 deletions

diff --git a/src/flake8/checker.py b/src/flake8/checker.py
index f7bd540..d5132ff 100644
--- a/src/flake8/checker.py
+++ b/src/flake8/checker.py
@@ -237,6 +237,12 @@ class Manager(object):
         exclude = self.options.exclude
         if not exclude:
             return False
+        if path == '-':
+            # stdin, use display name to check exclusion, if present
+            path = self.options.stdin_display_name
+            if path is None:
+                LOG.debug("unnamed stdin has not been excluded")
+                return False
         basename = os.path.basename(path)
         if utils.fnmatch(basename, exclude):
             LOG.debug('"%s" has been excluded', basename)
@@ -263,12 +269,11 @@ class Manager(object):
         # best solution right now.
         def should_create_file_checker(filename):
             """Determine if we should create a file checker."""
-            matches_filename_patterns = utils.fnmatch(
-                filename, filename_patterns
+            return (
+                filename == '-' or  # stdin
+                utils.fnmatch(filename, filename_patterns) and
+                os.path.exists(filename)
             )
-            is_stdin = filename == '-'
-            file_exists = os.path.exists(filename)
-            return (file_exists and matches_filename_patterns) or is_stdin
 
         self.checkers = [
             FileChecker(filename, self.checks, self.style_guide)
diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index 68ed530..52417fc 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -208,24 +208,26 @@ def filenames_from(arg, predicate=None):
     """
     if predicate is None:
         predicate = _default_predicate
-    if os.path.isdir(arg):
-        for root, sub_directories, files in os.walk(arg):
-            if predicate(root):
-                sub_directories[:] = []
-                continue
 
+    if predicate(arg):
+        return
+
+    if arg == "-":
+        # stdin, don't call isdir()
+        yield arg
+    elif os.path.isdir(arg):
+        for root, sub_directories, files in os.walk(arg):
             # NOTE(sigmavirus24): os.walk() will skip a directory if you
             # remove it from the list of sub-directories.
-            for directory in sub_directories:
-                joined = os.path.join(root, directory)
-                if predicate(directory) or predicate(joined):
-                    sub_directories.remove(directory)
+            sub_directories[:] = [
+                directory for directory in sub_directories
+                if not predicate(os.path.join(root, directory))
+            ]
 
             for filename in files:
                 joined = os.path.join(root, filename)
-                if predicate(joined) or predicate(filename):
-                    continue
-                yield joined
+                if not predicate(joined):
+                    yield joined
     else:
         yield arg