Lint Python files with a shebang

When a pattern is not passed to flake8 (--filename), look for all files
that end with .py as well as extension-less files that start with a
Python shebang. Helps project lint scripts that may not have an
extension.

Fixes #409

src/flake8/checker.py

@@ -3,6 +3,7 @@ import collections
 import errno
 import logging
 import os
+import re
 import signal
 import sys
 import tokenize
@@ -54,6 +55,8 @@ class Manager(object):
       together and make our output deterministic.
     """
 
+    SHEBANG_RE = re.compile(br'^#!.*\bpython[23w]?\b')
+
     def __init__(self, style_guide, arguments, checker_plugins):
         """Initialize our Manager instance.
 
@@ -199,9 +202,26 @@ class Manager(object):
             paths = ['.']
 
         filename_patterns = self.options.filename
+        patterns_exist = True
+        if not filename_patterns:
+            filename_patterns = ['*.py']
+            patterns_exist = False
         running_from_vcs = self.options._running_from_vcs
         running_from_diff = self.options.diff
 
+        def has_shebang(filename):
+            if patterns_exist:
+                # If a user explicitly specifies something, e.g. ``*.py``,
+                # don't inspect the shebang.
+                return False
+
+            if not os.path.isfile(filename):
+                return False
+
+            with open(filename, 'rb') as fp:
+                line = fp.readline(100)
+            return bool(self.SHEBANG_RE.match(line))
+
         # NOTE(sigmavirus24): Yes this is a little unsightly, but it's our
         # best solution right now.
         def should_create_file_checker(filename, argument):
@@ -220,8 +240,8 @@ class Manager(object):
             explicitly_provided = (not running_from_vcs and
                                    not running_from_diff and
                                    (argument == filename))
-            return ((explicitly_provided or matches_filename_patterns) or
-                    is_stdin)
+            return (explicitly_provided or matches_filename_patterns or
+                    is_stdin or has_shebang(filename))
 
         checks = self.checks.to_dictionary()
         checkers = (

src/flake8/main/options.py

@@ -72,7 +72,7 @@ def register_default_options(option_manager):
     )
 
     add_option(
-        '--filename', metavar='patterns', default='*.py',
+        '--filename', metavar='patterns',
         parse_from_config=True, comma_separated_list=True,
         help='Only check for filenames matching the patterns in this comma-'
              'separated list. (Default: %default)',

tests/fixtures/example-code/script

@@ -0,0 +1 @@
+#!/usr/bin/env python

tests/unit/test_checker_manager.py

@@ -1,5 +1,6 @@
 """Tests for the Manager object for FileCheckers."""
 import errno
+import os
 
 import mock
 import pytest
@@ -70,3 +71,57 @@ def test_make_checkers():
 
     for file_checker in manager.checkers:
         assert file_checker.filename in files
+
+
+def test_make_checkers_shebang():
+    """Verify that extension-less files with a Python shebang are checked."""
+    style_guide = style_guide_mock(
+        filename=[],
+        exclude=[],
+    )
+    checkplugins = mock.Mock()
+    checkplugins.to_dictionary.return_value = {
+        'ast_plugins': [],
+        'logical_line_plugins': [],
+        'physical_line_plugins': [],
+    }
+    with mock.patch('flake8.checker.multiprocessing', None):
+        manager = checker.Manager(style_guide, [], checkplugins)
+
+    path = os.path.abspath(
+        os.path.join(os.path.dirname(__file__), '..', 'fixtures')
+    )
+    manager.make_checkers([path])
+
+    filenames = [
+        os.path.relpath(file_checker.filename, path)
+        for file_checker in manager.checkers
+    ]
+    assert 'example-code/script' in filenames
+
+
+def test_make_checkers_explicit_pattern_ignore_shebang():
+    """Verify that shebangs are ignored when passing a pattern."""
+    style_guide = style_guide_mock(
+        filename=['*.py'],
+        exclude=[],
+    )
+    checkplugins = mock.Mock()
+    checkplugins.to_dictionary.return_value = {
+        'ast_plugins': [],
+        'logical_line_plugins': [],
+        'physical_line_plugins': [],
+    }
+    with mock.patch('flake8.checker.multiprocessing', None):
+        manager = checker.Manager(style_guide, [], checkplugins)
+
+    path = os.path.abspath(
+        os.path.join(os.path.dirname(__file__), '..', 'fixtures')
+    )
+    manager.make_checkers([path])
+
+    filenames = [
+        os.path.relpath(file_checker.filename, path)
+        for file_checker in manager.checkers
+    ]
+    assert 'example-code/script' not in filenames