Make a first pass at a git pre-commit hook

4 files changed, 219 insertions, 5 deletions

diff --git a/flake8/api/git.py b/flake8/api/git.py
new file mode 100644
index 0000000..087e31b
--- /dev/null
+++ b/flake8/api/git.py
@@ -0,0 +1,192 @@
+"""Module containing the main git hook interface and helpers.
+
+.. autofunction:: hook
+
+"""
+import contextlib
+import os
+import shutil
+import stat
+import subprocess
+import tempdir
+
+from flake8 import defaults
+from flake8 import exceptions
+from flake8.main import cli
+
+__all__ = ('hook', 'install')
+
+
+def hook(lazy=False, strict=False):
+    """Execute Flake8 on the files in git's index.
+
+    Determine which files are about to be committed and run Flake8 over them
+    to check for violations.
+
+    :param bool lazy:
+        Find files not added to the index prior to committing. This is useful
+        if you frequently use ``git commit -a`` for example. This defaults to
+        False since it will otherwise include files not in the index.
+    :param bool strict:
+        If True, return the total number of errors/violations found by Flake8.
+        This will cause the hook to fail.
+    :returns:
+        Total number of errors found during the run.
+    :rtype:
+        int
+    """
+    app = cli.Application()
+    with make_temporary_directory() as tempdir:
+        filepaths = list(copy_indexed_files_to(tempdir, lazy))
+        app.initialize(filepaths)
+        app.run_checks()
+
+    app.report_errors()
+    if strict:
+        return app.result_count
+    return 0
+
+
+def install():
+    """Install the git hook script.
+
+    This searches for the ``.git`` directory and will install an executable
+    pre-commit python script in the hooks sub-directory if one does not
+    already exist.
+
+    :returns:
+        True if successful, False if the git directory doesn't exist.
+    :rtype:
+        bool
+    :raises:
+        flake8.exceptions.GitHookAlreadyExists
+    """
+    git_directory = find_git_directory()
+    if git_directory is None or not os.path.exists(git_directory):
+        return False
+
+    hooks_directory = os.path.join(git_directory, 'hooks')
+    if not os.path.exists(hooks_directory):
+        os.mkdir(hooks_directory)
+
+    pre_commit_file = os.path.join(hooks_directory, 'hooks', 'pre-commit')
+    if os.path.exists(pre_commit_file):
+        raise exceptions.GitHookAlreadyExists(
+            'File already exists',
+            path=pre_commit_file,
+        )
+
+    with open(pre_commit_file, 'w') as fd:
+        fd.write(_HOOK_TEMPLATE)
+
+    # NOTE(sigmavirus24): The following sets:
+    # - read, write, and execute permissions for the owner
+    # - read permissions for people in the group
+    # - read permissions for other people
+    # The owner needs the file to be readable, writable, and executable
+    # so that git can actually execute it as a hook.
+    pre_commit_permissions = stat.S_IRWXU | stat.S_IRGRP | stat.IROTH
+    os.chmod(pre_commit_file, pre_commit_permissions)
+    return True
+
+
+def find_git_directory():
+    rev_parse = piped_process(['git', 'rev-parse', '--git-dir'])
+
+    (stdout, _) = rev_parse.communicate()
+    stdout = to_text(stdout)
+
+    if rev_parse.returncode == 0:
+        return stdout.strip()
+    return None
+
+
+def copy_indexed_files_to(temporary_directory, lazy):
+    modified_files = find_modified_files(lazy)
+    for filename in modified_files:
+        contents = get_staged_contents_from(filename)
+        yield copy_file_to(temporary_directory, filename, contents)
+
+
+def copy_file_to(destination_directory, filepath, contents):
+    directory, filename = os.path.split(os.path.abspath(filepath))
+    temporary_directory = make_temporary_directory_from(destination_directory,
+                                                        directory)
+    temporary_filepath = os.path.join(temporary_directory, filename)
+    with open(temporary_filepath, 'wb') as fd:
+        fd.write(contents)
+    return temporary_filepath
+
+
+def make_temporary_directory_from(destination, directory):
+    prefix = os.path.commonprefix([directory, destination])
+    common_directory_path = os.path.relpath(directory, start=prefix)
+    return os.path.join(destination, common_directory_path)
+
+
+def find_modified_files(lazy):
+    diff_index = piped_process(
+        ['git', 'diff-index', '--cached', '--name-only',
+         '--diff-filter=ACMRTUXB', 'HEAD'],
+    )
+
+    (stdout, _) = diff_index.communicate()
+    stdout = to_text(stdout)
+    return stdout.splitlines()
+
+
+def get_staged_contents_from(filename):
+    git_show = piped_process(['git', 'show', ':{0}'.format(filename)])
+    (stdout, _) = git_show.communicate()
+    return stdout
+
+
+@contextlib.contextmanager
+def make_temporary_directory():
+    temporary_directory = tempdir.mkdtemp()
+    yield temporary_directory
+    shutil.rmtree(temporary_directory, ignore_errors=True)
+
+
+def to_text(string):
+    """Ensure that the string is text."""
+    if callable(getattr(string, 'decode', None)):
+        return string.decode('utf-8')
+    return string
+
+
+def piped_process(command):
+    return subprocess.Popen(
+        command,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+
+
+def git_config_for(parameter):
+    config = piped_process(['git', 'config', '--get', '--bool', parameter])
+    (stdout, ) = config.communicate()
+    return to_text(stdout)
+
+
+def config_for(parameter):
+    environment_variable = 'flake8_{0}'.format(parameter).upper()
+    git_variable = 'flake8.{0}'.format(parameter)
+    value = os.environ.get(environment_variable, git_config_for(git_variable))
+    return value.lower() in defaults.TRUTHY_VALUES
+
+
+_HOOK_TEMPLATE = """#!/usr/bin/env python
+import os
+import sys
+
+from flake8.api import git
+
+if __name__ == '__main__':
+    sys.exit(
+        git.hook(
+            strict=git.config_for('strict'),
+            lazy=git.config_for('lazy'),
+        )
+    )
+"""
diff --git a/flake8/defaults.py b/flake8/defaults.py
index d484f4c..2007eed 100644
--- a/flake8/defaults.py
+++ b/flake8/defaults.py
@@ -4,5 +4,7 @@ EXCLUDE = '.svn,CVS,.bzr,.hg,.git,__pycache__,.tox'
 IGNORE = 'E121,E123,E126,E226,E24,E704,W503,W504'
 MAX_LINE_LENGTH = 79
 
+TRUTHY_VALUES = set(['true', '1', 't'])
+
 # Other consants
 WHITESPACE = frozenset(' \t')
diff --git a/flake8/exceptions.py b/flake8/exceptions.py
index 18ee90c..c5f3c5a 100644
--- a/flake8/exceptions.py
+++ b/flake8/exceptions.py
@@ -41,3 +41,12 @@ class InvalidSyntax(Flake8Exception):
             # strerror attribute instead of a message attribute
             self.error_message = self.original_exception.strerror
         super(InvalidSyntax, self).__init__(*args, **kwargs)
+
+
+class GitHookAlreadyExists(Flake8Exception):
+    """Exception raised when the git pre-commit hook file already exists."""
+
+    def __init__(self, *args, **kwargs):
+        """Initialize the path attribute."""
+        self.path = kwargs.pop('path')
+        super(GitHookAlreadyExists, self).__init__(*args, **kwargs)
diff --git a/flake8/main/cli.py b/flake8/main/cli.py
index 93c4a14..23135c4 100644
--- a/flake8/main/cli.py
+++ b/flake8/main/cli.py
@@ -271,8 +271,11 @@ class Application(object):
         #: :attr:`option_manager`
         self.args = None
         #: The number of errors, warnings, and other messages after running
-        #: flake8
+        #: flake8 and taking into account ignored errors and lines.
         self.result_count = 0
+        #: The total number of errors before accounting for ignored errors and
+        #: lines.
+        self.total_result_count = 0
 
         #: Whether the program is processing a diff or not
         self.running_against_diff = False
@@ -406,11 +409,15 @@ class Application(object):
         results = self.file_checker_manager.report()
         self.total_result_count, self.result_count = results
         LOG.info('Found a total of %d results and reported %d',
-                 self.total_result_count,
-                 self.result_count)
+                 self.total_result_count, self.result_count)
 
-    def _run(self, argv):
-        # type: (Union[NoneType, List[str]]) -> NoneType
+    def initialize(self, argv):
+        # type: () -> NoneType
+        """Initialize the application to be run.
+
+        This finds the plugins, registers their options, and parses the
+        command-line arguments.
+        """
         self.find_plugins()
         self.register_plugin_options()
         self.parse_configuration_and_cli(argv)
@@ -418,6 +425,10 @@ class Application(object):
         self.make_notifier()
         self.make_guide()
         self.make_file_checker_manager()
+
+    def _run(self, argv):
+        # type: (Union[NoneType, List[str]]) -> NoneType
+        self.initialize(argv)
         self.run_checks()
         self.report_errors()