mypy now passes

32 files changed, 255 insertions, 212 deletions

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 4b1c28d..cbccb83 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -35,7 +35,7 @@ python36:
   script: tox -e py36
 
 python37:
-  image: python:3.7-rc
+  image: python:3.7
   stage: test
   script: tox -e py37
 
@@ -44,6 +44,11 @@ linters:
   stage: test
   script: tox -e linters
 
+pre-commit:
+  image: python:3.7
+  stage: test
+  script: tox -e pre-commit
+
 docs:
   stage: test
   script: tox -e docs
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
new file mode 100644
index 0000000..48f8a1d
--- /dev/null
+++ b/.pre-commit-config.yaml
@@ -0,0 +1,6 @@
+repos:
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.701
+    hooks:
+    -   id: mypy
+        exclude: ^(docs/|example-plugin/|tests/fixtures)
diff --git a/mypy.ini b/mypy.ini
deleted file mode 100644
index 3d1cd43..0000000
--- a/mypy.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[mypy]
-ignore_missing_imports = true
diff --git a/setup.cfg b/setup.cfg
index e887ea8..109a253 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -16,3 +16,28 @@ requires-dist =
     pyflakes >= 2.1.0, < 2.2.0
     pycodestyle >= 2.5.0, < 2.6.0
     mccabe >= 0.6.0, < 0.7.0
+
+[mypy]
+check_untyped_defs = true
+disallow_any_generics = true
+disallow_incomplete_defs = true
+# TODO: disallow_untyped_defs = true
+no_implicit_optional = true
+warn_unused_ignores = true
+
+# TODO: until we opt in all the modules
+[mypy-flake8.defaults]
+disallow_untyped_defs = true
+[mypy-flake8.exceptions]
+disallow_untyped_defs = true
+[mypy-flake8.formatting.*]
+disallow_untyped_defs = true
+[mypy-flake8.main.cli]
+disallow_untyped_defs = true
+[mypy-flake8.statistics]
+disallow_untyped_defs = true
+[mypy-flake8.utils]
+disallow_untyped_defs = true
+
+[mypy-tests.*]
+disallow_untyped_defs = false
diff --git a/src/flake8/__init__.py b/src/flake8/__init__.py
index 73ec63a..8394276 100644
--- a/src/flake8/__init__.py
+++ b/src/flake8/__init__.py
@@ -12,6 +12,9 @@ This module
 import logging
 import sys
 
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from typing import Type  # `typing.Type` was introduced in 3.5.2
+
 LOG = logging.getLogger(__name__)
 LOG.addHandler(logging.NullHandler())
 
@@ -61,7 +64,7 @@ def configure_logging(verbosity, filename=None, logformat=LOG_FORMAT):
 
     if not filename or filename in ("stderr", "stdout"):
         fileobj = getattr(sys, filename or "stderr")
-        handler_cls = logging.StreamHandler
+        handler_cls = logging.StreamHandler  # type: Type[logging.Handler]
     else:
         fileobj = filename
         handler_cls = logging.FileHandler
diff --git a/src/flake8/checker.py b/src/flake8/checker.py
index 4fb106e..fd5658a 100644
--- a/src/flake8/checker.py
+++ b/src/flake8/checker.py
@@ -3,14 +3,13 @@ import collections
 import errno
 import logging
 import signal
-import sys
 import tokenize
-from typing import List, Optional, Tuple
+from typing import Dict, List, Optional, Tuple
 
 try:
     import multiprocessing
 except ImportError:
-    multiprocessing = None
+    multiprocessing = None  # type: ignore
 
 from flake8 import defaults
 from flake8 import exceptions
@@ -73,8 +72,7 @@ class Manager(object):
         self.options = style_guide.options
         self.checks = checker_plugins
         self.jobs = self._job_count()
-        self.processes = []
-        self.checkers = []
+        self.checkers = []  # type: List[FileChecker]
         self.statistics = {
             "files": 0,
             "logical lines": 0,
@@ -195,7 +193,7 @@ class Manager(object):
         )
 
     def make_checkers(self, paths=None):
-        # type: (List[str]) -> None
+        # type: (Optional[List[str]]) -> None
         """Create checkers for each file."""
         if paths is None:
             paths = self.arguments
@@ -270,8 +268,10 @@ class Manager(object):
 
     def run_parallel(self):
         """Run the checkers in parallel."""
-        final_results = collections.defaultdict(list)
-        final_statistics = collections.defaultdict(dict)
+        # fmt: off
+        final_results = collections.defaultdict(list)  # type: Dict[str, List[Tuple[str, int, int, str, Optional[str]]]]  # noqa: E501
+        final_statistics = collections.defaultdict(dict)  # type: Dict[str, Dict[str, None]]  # noqa: E501
+        # fmt: on
 
         try:
             pool = multiprocessing.Pool(self.jobs, _pool_init)
@@ -281,6 +281,7 @@ class Manager(object):
             self.run_serial()
             return
 
+        pool_closed = False
         try:
             pool_map = pool.imap_unordered(
                 _run_checks,
@@ -295,9 +296,9 @@ class Manager(object):
                 final_statistics[filename] = statistics
             pool.close()
             pool.join()
-            pool = None
+            pool_closed = True
         finally:
-            if pool is not None:
+            if not pool_closed:
                 pool.terminate()
                 pool.join()
 
@@ -345,9 +346,6 @@ class Manager(object):
     def stop(self):
         """Stop checking files."""
         self._process_statistics()
-        for proc in self.processes:
-            LOG.info("Joining %s to the main process", proc.name)
-            proc.join()
 
 
 class FileChecker(object):
@@ -370,7 +368,9 @@ class FileChecker(object):
         self.options = options
         self.filename = filename
         self.checks = checks
-        self.results = []
+        # fmt: off
+        self.results = []  # type: List[Tuple[str, int, int, str, Optional[str]]]  # noqa: E501
+        # fmt: on
         self.statistics = {
             "tokens": 0,
             "logical lines": 0,
@@ -389,17 +389,17 @@ class FileChecker(object):
         return "FileChecker for {}".format(self.filename)
 
     def _make_processor(self):
+        # type: () -> Optional[processor.FileProcessor]
         try:
             return processor.FileProcessor(self.filename, self.options)
-        except IOError:
+        except IOError as e:
             # If we can not read the file due to an IOError (e.g., the file
             # does not exist or we do not have the permissions to open it)
             # then we need to format that exception for the user.
             # NOTE(sigmavirus24): Historically, pep8 has always reported this
             # as an E902. We probably *want* a better error code for this
             # going forward.
-            (exc_type, exception) = sys.exc_info()[:2]
-            message = "{0}: {1}".format(exc_type.__name__, exception)
+            message = "{0}: {1}".format(type(e).__name__, e)
             self.report("E902", 0, 0, message)
             return None
 
@@ -446,7 +446,7 @@ class FileChecker(object):
         token = ()
         if len(exception.args) > 1:
             token = exception.args[1]
-            if len(token) > 2:
+            if token and len(token) > 2:
                 row, column = token[1:3]
         else:
             row, column = (1, 0)
@@ -482,14 +482,10 @@ class FileChecker(object):
         """Run all checks expecting an abstract syntax tree."""
         try:
             ast = self.processor.build_ast()
-        except (ValueError, SyntaxError, TypeError):
-            (exc_type, exception) = sys.exc_info()[:2]
-            row, column = self._extract_syntax_information(exception)
+        except (ValueError, SyntaxError, TypeError) as e:
+            row, column = self._extract_syntax_information(e)
             self.report(
-                "E999",
-                row,
-                column,
-                "%s: %s" % (exc_type.__name__, exception.args[0]),
+                "E999", row, column, "%s: %s" % (type(e).__name__, e.args[0])
             )
             return
 
diff --git a/src/flake8/exceptions.py b/src/flake8/exceptions.py
index bc44d8f..53ca4b7 100644
--- a/src/flake8/exceptions.py
+++ b/src/flake8/exceptions.py
@@ -1,5 +1,10 @@
 """Exception classes for all of Flake8."""
 
+from typing import Dict
+
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from flake8.plugins.manager import Plugin
+
 
 class Flake8Exception(Exception):
     """Plain Flake8 exception."""
@@ -19,13 +24,14 @@ class FailedToLoadPlugin(Flake8Exception):
     FORMAT = 'Flake8 failed to load plugin "%(name)s" due to %(exc)s.'
 
     def __init__(self, plugin, exception):
+        # type: (Plugin, Exception) -> None
         """Initialize our FailedToLoadPlugin exception."""
         self.plugin = plugin
         self.ep_name = self.plugin.name
         self.original_exception = exception
         super(FailedToLoadPlugin, self).__init__(plugin, exception)
 
-    def __str__(self):
+    def __str__(self):  # type: () -> str
         """Format our exception message."""
         return self.FORMAT % {
             "name": self.ep_name,
@@ -47,7 +53,7 @@ class InvalidSyntax(Flake8Exception):
         self.column_number = 0
         super(InvalidSyntax, self).__init__(exception)
 
-    def __str__(self):
+    def __str__(self):  # type: () -> str
         """Format our exception message."""
         return self.error_message
 
@@ -58,6 +64,7 @@ class PluginRequestedUnknownParameters(Flake8Exception):
     FORMAT = '"%(name)s" requested unknown parameters causing %(exc)s'
 
     def __init__(self, plugin, exception):
+        # type: (Dict[str, str], Exception) -> None
         """Pop certain keyword arguments for initialization."""
         self.plugin = plugin
         self.original_exception = exception
@@ -65,7 +72,7 @@ class PluginRequestedUnknownParameters(Flake8Exception):
             plugin, exception
         )
 
-    def __str__(self):
+    def __str__(self):  # type: () -> str
         """Format our exception message."""
         return self.FORMAT % {
             "name": self.plugin["plugin_name"],
@@ -79,12 +86,13 @@ class PluginExecutionFailed(Flake8Exception):
     FORMAT = '"%(name)s" failed during execution due to "%(exc)s"'
 
     def __init__(self, plugin, exception):
+        # type: (Dict[str, str], Exception) -> None
         """Utilize keyword arguments for message generation."""
         self.plugin = plugin
         self.original_exception = exception
         super(PluginExecutionFailed, self).__init__(plugin, exception)
 
-    def __str__(self):
+    def __str__(self):  # type: () -> str
         """Format our exception message."""
         return self.FORMAT % {
             "name": self.plugin["plugin_name"],
@@ -99,40 +107,33 @@ class HookInstallationError(Flake8Exception):
 class GitHookAlreadyExists(HookInstallationError):
     """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)
-
-    def __str__(self):
-        """Provide a nice message regarding the exception."""
-        msg = (
+    def __init__(self, path):  # type: (str) -> None
+        """Initialize the exception message from the `path`."""
+        self.path = path
+        tmpl = (
             "The Git pre-commit hook ({0}) already exists. To convince "
             "Flake8 to install the hook, please remove the existing "
             "hook."
         )
-        return msg.format(self.path)
+        super(GitHookAlreadyExists, self).__init__(tmpl.format(self.path))
 
 
 class MercurialHookAlreadyExists(HookInstallationError):
     """Exception raised when a mercurial hook is already configured."""
 
-    hook_name = None
+    hook_name = None  # type: str
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, path, value):  # type: (str, str) -> None
         """Initialize the relevant attributes."""
-        self.path = kwargs.pop("path")
-        self.value = kwargs.pop("value")
-        super(MercurialHookAlreadyExists, self).__init__(*args, **kwargs)
-
-    def __str__(self):
-        """Return a nicely formatted string for these errors."""
-        msg = (
+        self.path = path
+        self.value = value
+        tmpl = (
             'The Mercurial {0} hook already exists with "{1}" in {2}. '
             "To convince Flake8 to install the hook, please remove the "
             "{0} configuration from the [hooks] section of your hgrc."
         )
-        return msg.format(self.hook_name, self.value, self.path)
+        msg = tmpl.format(self.hook_name, self.value, self.path)
+        super(MercurialHookAlreadyExists, self).__init__(msg)
 
 
 class MercurialCommitHookAlreadyExists(MercurialHookAlreadyExists):
diff --git a/src/flake8/formatting/base.py b/src/flake8/formatting/base.py
index 259d03f..388ab0e 100644
--- a/src/flake8/formatting/base.py
+++ b/src/flake8/formatting/base.py
@@ -1,6 +1,13 @@
 """The base class and interface for all formatting plugins."""
 from __future__ import print_function
 
+import optparse
+from typing import IO, List, Optional, Tuple
+
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from flake8.statistics import Statistics
+    from flake8.style_guide import Violation
+
 
 class BaseFormatter(object):
     """Class defining the formatter interface.
@@ -25,6 +32,7 @@ class BaseFormatter(object):
     """
 
     def __init__(self, options):
+        # type: (optparse.Values) -> None
         """Initialize with the options parsed from config and cli.
 
         This also calls a hook, :meth:`after_init`, so subclasses do not need
@@ -36,33 +44,30 @@ class BaseFormatter(object):
         """
         self.options = options
         self.filename = options.output_file
-        self.output_fd = None
+        self.output_fd = None  # type: Optional[IO[str]]
         self.newline = "\n"
         self.after_init()
 
-    def after_init(self):
+    def after_init(self):  # type: () -> None
         """Initialize the formatter further."""
-        pass
 
-    def beginning(self, filename):
+    def beginning(self, filename):  # type: (str) -> None
         """Notify the formatter that we're starting to process a file.
 
         :param str filename:
             The name of the file that Flake8 is beginning to report results
             from.
         """
-        pass
 
-    def finished(self, filename):
+    def finished(self, filename):  # type: (str) -> None
         """Notify the formatter that we've finished processing a file.
 
         :param str filename:
             The name of the file that Flake8 has finished reporting results
             from.
         """
-        pass
 
-    def start(self):
+    def start(self):  # type: () -> None
         """Prepare the formatter to receive input.
 
         This defaults to initializing :attr:`output_fd` if :attr:`filename`
@@ -70,7 +75,7 @@ class BaseFormatter(object):
         if self.filename:
             self.output_fd = open(self.filename, "a")
 
-    def handle(self, error):
+    def handle(self, error):  # type: (Violation) -> None
         """Handle an error reported by Flake8.
 
         This defaults to calling :meth:`format`, :meth:`show_source`, and
@@ -87,7 +92,7 @@ class BaseFormatter(object):
         source = self.show_source(error)
         self.write(line, source)
 
-    def format(self, error):
+    def format(self, error):  # type: (Violation) -> Optional[str]
         """Format an error reported by Flake8.
 
         This method **must** be implemented by subclasses.
@@ -106,7 +111,7 @@ class BaseFormatter(object):
             "Subclass of BaseFormatter did not implement" " format."
         )
 
-    def show_statistics(self, statistics):
+    def show_statistics(self, statistics):  # type: (Statistics) -> None
         """Format and print the statistics."""
         for error_code in statistics.error_codes():
             stats_for_error_code = statistics.statistics_for(error_code)
@@ -122,6 +127,7 @@ class BaseFormatter(object):
             )
 
     def show_benchmarks(self, benchmarks):
+        # type: (List[Tuple[str, float]]) -> None
         """Format and print the benchmarks."""
         # NOTE(sigmavirus24): The format strings are a little confusing, even
         # to me, so here's a quick explanation:
@@ -142,7 +148,7 @@ class BaseFormatter(object):
                 benchmark = float_format(statistic=statistic, value=value)
             self._write(benchmark)
 
-    def show_source(self, error):
+    def show_source(self, error):  # type: (Violation) -> Optional[str]
         """Show the physical line generating the error.
 
         This also adds an indicator for the particular part of the line that
@@ -170,7 +176,7 @@ class BaseFormatter(object):
         # one
         return error.physical_line + pointer
 
-    def _write(self, output):
+    def _write(self, output):  # type: (str) -> None
         """Handle logic of whether to use an output file or print()."""
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)
@@ -178,6 +184,7 @@ class BaseFormatter(object):
             print(output, end=self.newline)
 
     def write(self, line, source):
+        # type: (Optional[str], Optional[str]) -> None
         """Write the line either to the output file or stdout.
 
         This handles deciding whether to write to a file or print to standard
@@ -195,7 +202,7 @@ class BaseFormatter(object):
         if source:
             self._write(source)
 
-    def stop(self):
+    def stop(self):  # type: () -> None
         """Clean up after reporting is finished."""
         if self.output_fd is not None:
             self.output_fd.close()
diff --git a/src/flake8/formatting/default.py b/src/flake8/formatting/default.py
index e1061f3..55a5d01 100644
--- a/src/flake8/formatting/default.py
+++ b/src/flake8/formatting/default.py
@@ -1,6 +1,11 @@
 """Default formatting class for Flake8."""
+from typing import Optional, Set
+
 from flake8.formatting import base
 
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from flake8.style_guide import Violation
+
 
 class SimpleFormatter(base.BaseFormatter):
     """Simple abstraction for Default and Pylint formatter commonality.
@@ -18,9 +23,9 @@ class SimpleFormatter(base.BaseFormatter):
 
     """
 
-    error_format = None
+    error_format = None  # type: str
 
-    def format(self, error):
+    def format(self, error):  # type: (Violation) -> Optional[str]
         """Format and write error out.
 
         If an output filename is specified, write formatted errors to that
@@ -44,7 +49,7 @@ class Default(SimpleFormatter):
 
     error_format = "%(path)s:%(row)d:%(col)d: %(code)s %(text)s"
 
-    def after_init(self):
+    def after_init(self):  # type: () -> None
         """Check for a custom format string."""
         if self.options.format.lower() != "default":
             self.error_format = self.options.format
@@ -61,28 +66,27 @@ class FilenameOnly(SimpleFormatter):
 
     error_format = "%(path)s"
 
-    def after_init(self):
+    def after_init(self):  # type: () -> None
         """Initialize our set of filenames."""
-        self.filenames_already_printed = set()
+        self.filenames_already_printed = set()  # type: Set[str]
 
-    def show_source(self, error):
+    def show_source(self, error):  # type: (Violation) -> Optional[str]
         """Do not include the source code."""
-        pass
 
-    def format(self, error):
+    def format(self, error):  # type: (Violation) -> Optional[str]
         """Ensure we only print each error once."""
         if error.filename not in self.filenames_already_printed:
             self.filenames_already_printed.add(error.filename)
             return super(FilenameOnly, self).format(error)
+        else:
+            return None
 
 
 class Nothing(base.BaseFormatter):
     """Print absolutely nothing."""
 
-    def format(self, error):
+    def format(self, error):  # type: (Violation) -> Optional[str]
         """Do nothing."""
-        pass
 
-    def show_source(self, error):
+    def show_source(self, error):  # type: (Violation) -> Optional[str]
         """Do not print the source."""
-        pass
diff --git a/src/flake8/main/application.py b/src/flake8/main/application.py
index c7859b9..f3c5382 100644
--- a/src/flake8/main/application.py
+++ b/src/flake8/main/application.py
@@ -2,9 +2,10 @@
 from __future__ import print_function
 
 import logging
+import optparse
 import sys
 import time
-from typing import List, Optional
+from typing import Dict, List, Optional, Set
 
 import flake8
 from flake8 import checker
@@ -18,11 +19,8 @@ from flake8.options import manager
 from flake8.plugins import manager as plugin_manager
 
 if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
-    # fmt: off
-    # `typing.Type` as introduced in 3.5.2
-    from typing import Type  # noqa: F401 (until flake8 3.7)
-    from flake8.formatting.base import BaseFormatter  # noqa: F401, E501 (until flake8 3.7)
-    # fmt: on
+    from typing import Type  # `typing.Type` was introduced in 3.5.2
+    from flake8.formatting.base import BaseFormatter
 
 
 LOG = logging.getLogger(__name__)
@@ -32,7 +30,6 @@ class Application(object):
     """Abstract our application into a class."""
 
     def __init__(self, program="flake8", version=flake8.__version__):
-        # type: (str, str) -> None
         """Initialize our application.
 
         :param str program:
@@ -43,7 +40,7 @@ class Application(object):
         #: The timestamp when the Application instance was instantiated.
         self.start_time = time.time()
         #: The timestamp when the Application finished reported errors.
-        self.end_time = None
+        self.end_time = None  # type: float
         #: The name of the program being run
         self.program = program
         #: The version of the program being run
@@ -56,33 +53,35 @@ class Application(object):
         options.register_default_options(self.option_manager)
         #: The preliminary options parsed from CLI before plugins are loaded,
         #: into a :class:`optparse.Values` instance
-        self.prelim_opts = None
+        self.prelim_opts = None  # type: optparse.Values
         #: The preliminary arguments parsed from CLI before plugins are loaded
-        self.prelim_args = None
+        self.prelim_args = None  # type: List[str]
         #: The instance of :class:`flake8.options.config.ConfigFileFinder`
         self.config_finder = None
 
         #: The :class:`flake8.options.config.LocalPlugins` found in config
-        self.local_plugins = None
+        self.local_plugins = None  # type: config.LocalPlugins
         #: The instance of :class:`flake8.plugins.manager.Checkers`
-        self.check_plugins = None
+        self.check_plugins = None  # type: plugin_manager.Checkers
+        # fmt: off
         #: The instance of :class:`flake8.plugins.manager.ReportFormatters`
-        self.formatting_plugins = None
+        self.formatting_plugins = None  # type: plugin_manager.ReportFormatters
+        # fmt: on
         #: The user-selected formatter from :attr:`formatting_plugins`
-        self.formatter = None
+        self.formatter = None  # type: BaseFormatter
         #: The :class:`flake8.style_guide.StyleGuideManager` built from the
         #: user's options
-        self.guide = None
+        self.guide = None  # type: style_guide.StyleGuideManager
         #: The :class:`flake8.checker.Manager` that will handle running all of
         #: the checks selected by the user.
-        self.file_checker_manager = None
+        self.file_checker_manager = None  # type: checker.Manager
 
         #: The user-supplied options parsed into an instance of
         #: :class:`optparse.Values`
-        self.options = None
+        self.options = None  # type: optparse.Values
         #: The left over arguments that were not parsed by
         #: :attr:`option_manager`
-        self.args = None
+        self.args = None  # type: List[str]
         #: The number of errors, warnings, and other messages after running
         #: flake8 and taking into account ignored errors and lines.
         self.result_count = 0
@@ -96,7 +95,7 @@ class Application(object):
         #: Whether the program is processing a diff or not
         self.running_against_diff = False
         #: The parsed diff information
-        self.parsed_diff = {}
+        self.parsed_diff = {}  # type: Dict[str, Set[int]]
 
     def parse_preliminary_options_and_args(self, argv=None):
         # type: (Optional[List[str]]) -> None
diff --git a/src/flake8/main/git.py b/src/flake8/main/git.py
index eed5c09..683396a 100644
--- a/src/flake8/main/git.py
+++ b/src/flake8/main/git.py
@@ -90,9 +90,7 @@ def install():
         os.path.join(hooks_directory, "pre-commit")
     )
     if os.path.exists(pre_commit_file):
-        raise exceptions.GitHookAlreadyExists(
-            "File already exists", path=pre_commit_file
-        )
+        raise exceptions.GitHookAlreadyExists(path=pre_commit_file)
 
     executable = get_executable()
 
diff --git a/src/flake8/main/mercurial.py b/src/flake8/main/mercurial.py
index 65ef8ce..4ea410a 100644
--- a/src/flake8/main/mercurial.py
+++ b/src/flake8/main/mercurial.py
@@ -7,6 +7,7 @@
 import configparser
 import os
 import subprocess
+from typing import Set
 
 from flake8 import exceptions as exc
 
@@ -101,7 +102,7 @@ def install():
 
 
 def get_filenames_from(repository, kwargs):
-    seen_filenames = set()
+    seen_filenames = set()  # type: Set[str]
     node = kwargs["node"]
     for revision in range(repository[node], len(repository)):
         for filename in repository[revision].files():
diff --git a/src/flake8/main/setuptools_command.py b/src/flake8/main/setuptools_command.py
index 1b57c0d..55dc378 100644
--- a/src/flake8/main/setuptools_command.py
+++ b/src/flake8/main/setuptools_command.py
@@ -1,6 +1,6 @@
 """The logic for Flake8's integration with setuptools."""
 import os
-from typing import List
+from typing import List, Tuple
 
 import setuptools
 
@@ -48,7 +48,7 @@ class Flake8(setuptools.Command):
 
     def package_files(self):
         """Collect the files/dirs included in the registered modules."""
-        seen_package_directories = ()
+        seen_package_directories = ()  # type: Tuple[str, ...]
         directories = self.distribution.package_dir or {}
         empty_directory_exists = "" in directories
         packages = self.distribution.packages or []
diff --git a/src/flake8/main/vcs.py b/src/flake8/main/vcs.py
index f7f139a..398643c 100644
--- a/src/flake8/main/vcs.py
+++ b/src/flake8/main/vcs.py
@@ -17,7 +17,7 @@ def install(option, option_string, value, parser):
     For more information about the callback signature, see:
     https://docs.python.org/3/library/optparse.html#optparse-option-callbacks
     """
-    installer = _INSTALLERS.get(value)
+    installer = _INSTALLERS[value]
     errored = False
     successful = False
     try:
diff --git a/src/flake8/options/config.py b/src/flake8/options/config.py
index 468341b..026618c 100644
--- a/src/flake8/options/config.py
+++ b/src/flake8/options/config.py
@@ -4,6 +4,7 @@ import configparser
 import logging
 import os.path
 import sys
+from typing import Dict, List, Sequence, Tuple, Union
 
 from flake8 import utils
 
@@ -54,12 +55,15 @@ class ConfigFileFinder(object):
 
         # caches to avoid double-reading config files
         self._local_configs = None
-        self._local_found_files = []
+        self._local_found_files = []  # type: List[str]
         self._user_config = None
-        self._cli_configs = {}
+        # fmt: off
+        self._cli_configs = {}  # type: Dict[str, configparser.RawConfigParser]
+        # fmt: on
 
     @staticmethod
     def _read_config(files):
+        # type: (Union[Sequence[str], str]) -> Tuple[configparser.RawConfigParser, List[str]]  # noqa: E501
         config = configparser.RawConfigParser()
         if isinstance(files, (str, type(u""))):
             files = [files]
@@ -83,6 +87,7 @@ class ConfigFileFinder(object):
         return (config, found_files)
 
     def cli_config(self, files):
+        # type: (str) -> configparser.RawConfigParser
         """Read and parse the config file specified on the command-line."""
         if files not in self._cli_configs:
             config, found_files = self._read_config(files)
@@ -379,7 +384,7 @@ def get_local_plugins(config_finder, cli_config=None, isolated=False):
         raw_paths = utils.parse_comma_separated_list(
             config.get(section, "paths").strip()
         )
-        norm_paths = []
+        norm_paths = []  # type: List[str]
         for base_dir in base_dirs:
             norm_paths.extend(
                 path
diff --git a/src/flake8/options/manager.py b/src/flake8/options/manager.py
index 0ded13a..5d21127 100644
--- a/src/flake8/options/manager.py
+++ b/src/flake8/options/manager.py
@@ -2,6 +2,7 @@
 import collections
 import logging
 import optparse  # pylint: disable=deprecated-module
+from typing import Any, Callable, Dict, List, Optional, Set
 
 from flake8 import utils
 
@@ -108,7 +109,7 @@ class Option(object):
         self.comma_separated_list = comma_separated_list
         self.normalize_paths = normalize_paths
 
-        self.config_name = None
+        self.config_name = None  # type: Optional[str]
         if parse_from_config:
             if not long_option_name:
                 raise ValueError(
@@ -143,7 +144,7 @@ class Option(object):
         """Normalize the value based on the option configuration."""
         if self.normalize_paths:
             # Decide whether to parse a list of paths or a single path
-            normalize = utils.normalize_path
+            normalize = utils.normalize_path  # type: Callable[..., Any]
             if self.comma_separated_list:
                 normalize = utils.normalize_paths
             return normalize(value, *normalize_args)
@@ -200,13 +201,13 @@ class OptionManager(object):
         self.parser = optparse.OptionParser(
             prog=prog, version=version, usage=usage
         )
-        self.config_options_dict = {}
-        self.options = []
+        self.config_options_dict = {}  # type: Dict[str, Option]
+        self.options = []  # type: List[Option]
         self.program_name = prog
         self.version = version
-        self.registered_plugins = set()
-        self.extended_default_ignore = set()
-        self.extended_default_select = set()
+        self.registered_plugins = set()  # type: Set[PluginVersion]
+        self.extended_default_ignore = set()  # type: Set[str]
+        self.extended_default_select = set()  # type: Set[str]
 
     @staticmethod
     def format_plugin(plugin):
@@ -231,6 +232,7 @@ class OptionManager(object):
         self.options.append(option)
         if option.parse_from_config:
             name = option.config_name
+            assert name is not None  # nosec (for mypy)
             self.config_options_dict[name] = option
             self.config_options_dict[name.replace("_", "-")] = option
         LOG.debug('Registered option "%s".', option)
@@ -326,7 +328,7 @@ class OptionManager(object):
             values = self.parser.get_default_values()
 
         self.parser.rargs = rargs
-        self.parser.largs = largs = []
+        largs = []  # type: List[str]
         self.parser.values = values
 
         while rargs:
@@ -340,7 +342,8 @@ class OptionManager(object):
                 optparse.BadOptionError,
                 optparse.OptionValueError,
             ) as err:
-                self.parser.largs.append(err.opt_str)
+                # TODO: https://gitlab.com/pycqa/flake8/issues/541
+                largs.append(err.opt_str)  # type: ignore
 
         args = largs + rargs
         options, xargs = self.parser.check_values(values, args)
diff --git a/src/flake8/plugins/manager.py b/src/flake8/plugins/manager.py
index 303c0f9..1554aa2 100644
--- a/src/flake8/plugins/manager.py
+++ b/src/flake8/plugins/manager.py
@@ -1,17 +1,12 @@
 """Plugin loading and management logic and classes."""
 import logging
-import sys
+from typing import Any, Dict, List, Set
 
 import entrypoints
 
 from flake8 import exceptions
 from flake8 import utils
 
-if sys.version_info >= (3, 3):
-    import collections.abc as collections_abc
-else:
-    import collections as collections_abc
-
 LOG = logging.getLogger(__name__)
 
 __all__ = ("Checkers", "Plugin", "PluginManager", "ReportFormatters")
@@ -37,7 +32,7 @@ class Plugin(object):
         self.name = name
         self.entry_point = entry_point
         self.local = local
-        self._plugin = None
+        self._plugin = None  # type: Any
         self._parameters = None
         self._parameter_names = None
         self._group = None
@@ -236,8 +231,8 @@ class PluginManager(object):  # pylint: disable=too-few-public-methods
             Plugins from config (as "X = path.to:Plugin" strings).
         """
         self.namespace = namespace
-        self.plugins = {}
-        self.names = []
+        self.plugins = {}  # type: Dict[str, Plugin]
+        self.names = []  # type: List[str]
         self._load_local_plugins(local_plugins or [])
         self._load_entrypoint_plugins()
 
@@ -310,7 +305,7 @@ class PluginManager(object):  # pylint: disable=too-few-public-methods
         :rtype:
             tuple
         """
-        plugins_seen = set()
+        plugins_seen = set()  # type: Set[str]
         for entry_point_name in self.names:
             plugin = self.plugins[entry_point_name]
             plugin_name = plugin.plugin_name
@@ -345,7 +340,7 @@ def version_for(plugin):
 class PluginTypeManager(object):
     """Parent class for most of the specific plugin types."""
 
-    namespace = None
+    namespace = None  # type: str
 
     def __init__(self, local_plugins=None):
         """Initialize the plugin type's manager.
@@ -398,9 +393,7 @@ class PluginTypeManager(object):
     def _generate_call_function(method_name, optmanager, *args, **kwargs):
         def generated_function(plugin):  # noqa: D105
             method = getattr(plugin, method_name, None)
-            if method is not None and isinstance(
-                method, collections_abc.Callable
-            ):
+            if method is not None and callable(method):
                 return method(optmanager, *args, **kwargs)
 
         return generated_function
diff --git a/src/flake8/plugins/pyflakes.py b/src/flake8/plugins/pyflakes.py
index 2f233c4..9be0c81 100644
--- a/src/flake8/plugins/pyflakes.py
+++ b/src/flake8/plugins/pyflakes.py
@@ -10,6 +10,7 @@ except ImportError:
 else:
     demandimport.disable()
 import os
+from typing import List
 
 import pyflakes
 import pyflakes.checker
@@ -59,8 +60,8 @@ class FlakesChecker(pyflakes.checker.Checker):
     name = "pyflakes"
     version = pyflakes.__version__
     with_doctest = False
-    include_in_doctest = []
-    exclude_from_doctest = []
+    include_in_doctest = []  # type: List[str]
+    exclude_from_doctest = []  # type: List[str]
 
     def __init__(self, tree, file_tokens, filename):
         """Initialize the PyFlakes plugin with an AST tree and filename."""
diff --git a/src/flake8/processor.py b/src/flake8/processor.py
index 09185e5..239f908 100644
--- a/src/flake8/processor.py
+++ b/src/flake8/processor.py
@@ -3,7 +3,7 @@ import contextlib
 import logging
 import sys
 import tokenize
-from typing import List
+from typing import Any, Dict, List, Tuple
 
 import flake8
 from flake8 import defaults
@@ -66,9 +66,9 @@ class FileProcessor(object):
         #: Number of blank lines
         self.blank_lines = 0
         #: Checker states for each plugin?
-        self._checker_states = {}
+        self._checker_states = {}  # type: Dict[str, Dict[Any, Any]]
         #: Current checker state
-        self.checker_state = None
+        self.checker_state = None  # type: Dict[Any, Any]
         #: User provided option for hang closing
         self.hang_closing = options.hang_closing
         #: Character used for indentation
@@ -93,8 +93,10 @@ class FileProcessor(object):
         self.previous_logical = ""
         #: Previous unindented (i.e. top-level) logical line
         self.previous_unindented_logical_line = ""
+        # fmt: off
         #: Current set of tokens
-        self.tokens = []
+        self.tokens = []  # type: List[Tuple[int, str, Tuple[int, int], Tuple[int, int], str]]  # noqa: E501
+        # fmt: on
         #: Total number of lines in the file
         self.total_lines = len(self.lines)
         #: Verbosity level of Flake8
diff --git a/src/flake8/statistics.py b/src/flake8/statistics.py
index f2131b5..100227f 100644
--- a/src/flake8/statistics.py
+++ b/src/flake8/statistics.py
@@ -1,15 +1,19 @@
 """Statistic collection logic for Flake8."""
 import collections
+from typing import Dict, Generator, List, Optional
+
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from flake8.style_guide import Violation
 
 
 class Statistics(object):
     """Manager of aggregated statistics for a run of Flake8."""
 
-    def __init__(self):
+    def __init__(self):  # type: () -> None
         """Initialize the underlying dictionary for our statistics."""
-        self._store = {}
+        self._store = {}  # type: Dict[Key, Statistic]
 
-    def error_codes(self):
+    def error_codes(self):  # type: () -> List[str]
         """Return all unique error codes stored.
 
         :returns:
@@ -19,7 +23,7 @@ class Statistics(object):
         """
         return sorted({key.code for key in self._store})
 
-    def record(self, error):
+    def record(self, error):  # type: (Violation) -> None
         """Add the fact that the error was seen in the file.
 
         :param error:
@@ -34,6 +38,7 @@ class Statistics(object):
         self._store[key].increment()
 
     def statistics_for(self, prefix, filename=None):
+        # type: (str, Optional[str]) -> Generator[Statistic, None, None]
         """Generate statistics for the prefix and filename.
 
         If you have a :class:`Statistics` object that has recorded errors,
@@ -74,11 +79,11 @@ class Key(collections.namedtuple("Key", ["filename", "code"])):
     __slots__ = ()
 
     @classmethod
-    def create_from(cls, error):
+    def create_from(cls, error):  # type: (Violation) -> Key
         """Create a Key from :class:`flake8.style_guide.Violation`."""
         return cls(filename=error.filename, code=error.code)
 
-    def matches(self, prefix, filename):
+    def matches(self, prefix, filename):  # type: (str, Optional[str]) -> bool
         """Determine if this key matches some constraints.
 
         :param str prefix:
@@ -106,6 +111,7 @@ class Statistic(object):
     """
 
     def __init__(self, error_code, filename, message, count):
+        # type: (str, str, str, int) -> None
         """Initialize our Statistic."""
         self.error_code = error_code
         self.filename = filename
@@ -113,7 +119,7 @@ class Statistic(object):
         self.count = count
 
     @classmethod
-    def create_from(cls, error):
+    def create_from(cls, error):  # type: (Violation) -> Statistic
         """Create a Statistic from a :class:`flake8.style_guide.Violation`."""
         return cls(
             error_code=error.code,
@@ -122,6 +128,6 @@ class Statistic(object):
             count=0,
         )
 
-    def increment(self):
+    def increment(self):  # type: () -> None
         """Increment the number of times we've seen this error in this file."""
         self.count += 1
diff --git a/src/flake8/style_guide.py b/src/flake8/style_guide.py
index c617f5a..b644f8b 100644
--- a/src/flake8/style_guide.py
+++ b/src/flake8/style_guide.py
@@ -7,7 +7,7 @@ import itertools
 import linecache
 import logging
 import sys
-from typing import Optional, Union
+from typing import Dict, List, Optional, Set, Union
 
 from flake8 import defaults
 from flake8 import statistics
@@ -68,7 +68,7 @@ class Violation(_Violation):
     """Class representing a violation reported by Flake8."""
 
     def is_inline_ignored(self, disable_noqa):
-        # type: (Violation) -> bool
+        # type: (bool) -> bool
         """Determine if a comment has been added to ignore this line.
 
         :param bool disable_noqa:
@@ -154,7 +154,7 @@ class DecisionEngine(object):
 
     def __init__(self, options):
         """Initialize the engine."""
-        self.cache = {}
+        self.cache = {}  # type: Dict[str, Decision]
         self.selected = tuple(options.select)
         self.extended_selected = tuple(
             sorted(options.extended_default_select, reverse=True)
@@ -332,7 +332,7 @@ class StyleGuideManager(object):
         self.formatter = formatter
         self.stats = statistics.Statistics()
         self.decider = decider or DecisionEngine(options)
-        self.style_guides = []
+        self.style_guides = []  # type: List[StyleGuide]
         self.default_style_guide = StyleGuide(
             options, formatter, self.stats, decider=decider
         )
@@ -390,7 +390,7 @@ class StyleGuideManager(object):
         text,
         physical_line=None,
     ):
-        # type: (str, str, int, int, str, Optional[str]) -> int
+        # type: (str, str, int, Optional[int], str, Optional[str]) -> int
         """Handle an error reported by a check.
 
         :param str code:
@@ -419,6 +419,7 @@ class StyleGuideManager(object):
         )
 
     def add_diff_ranges(self, diffinfo):
+        # type: (Dict[str, Set[int]]) -> None
         """Update the StyleGuides to filter out information not in the diff.
 
         This provides information to the underlying StyleGuides so that only
@@ -448,7 +449,7 @@ class StyleGuide(object):
         self.filename = filename
         if self.filename:
             self.filename = utils.normalize_path(self.filename)
-        self._parsed_diff = {}
+        self._parsed_diff = {}  # type: Dict[str, Set[int]]
 
     def __repr__(self):
         """Make it easier to debug which StyleGuide we're using."""
@@ -514,7 +515,7 @@ class StyleGuide(object):
         text,
         physical_line=None,
     ):
-        # type: (str, str, int, int, str, Optional[str]) -> int
+        # type: (str, str, int, Optional[int], str, Optional[str]) -> int
         """Handle an error reported by a check.
 
         :param str code:
@@ -567,6 +568,7 @@ class StyleGuide(object):
         return 0
 
     def add_diff_ranges(self, diffinfo):
+        # type: (Dict[str, Set[int]]) -> None
         """Update the StyleGuide to filter out information not in the diff.
 
         This provides information to the StyleGuide so that only the errors
diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index b95d4db..f822388 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -3,13 +3,14 @@ import collections
 import fnmatch as _fnmatch
 import inspect
 import io
+import logging
 import os
 import platform
 import re
 import sys
 import tokenize
-from typing import Callable, Dict, Generator, List, Pattern, Sequence, Set
-from typing import Tuple, Union
+from typing import Callable, Dict, Generator, List, Optional, Pattern
+from typing import Sequence, Set, Tuple, Union
 
 from flake8 import exceptions
 
@@ -19,6 +20,7 @@ if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
 DIFF_HUNK_REGEXP = re.compile(r"^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@.*$")
 COMMA_SEPARATED_LIST_RE = re.compile(r"[,\s]")
 LOCAL_PLUGIN_LIST_RE = re.compile(r"[,\t\n\r\f\v]")
+string_types = (str, type(u""))
 
 
 def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
@@ -40,7 +42,7 @@ def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
     if not value:
         return []
 
-    if not isinstance(value, (list, tuple)):
+    if isinstance(value, string_types):
         value = regexp.split(value)
 
     item_gen = (item.strip() for item in value)
@@ -77,8 +79,8 @@ def _tokenize_files_to_codes_mapping(value):
     return tokens
 
 
-def parse_files_to_codes_mapping(value):  # noqa: C901
-    # type: (Union[Sequence[str], str]) -> List[Tuple[List[str], List[str]]]
+def parse_files_to_codes_mapping(value_):  # noqa: C901
+    # type: (Union[Sequence[str], str]) -> List[Tuple[str, List[str]]]
     """Parse a files-to-codes maping.
 
     A files-to-codes mapping a sequence of values specified as
@@ -88,20 +90,22 @@ def parse_files_to_codes_mapping(value):  # noqa: C901
     :param value: String to be parsed and normalized.
     :type value: str
     """
-    if isinstance(value, (list, tuple)):
-        value = "\n".join(value)
+    if not isinstance(value_, string_types):
+        value = "\n".join(value_)
+    else:
+        value = value_
 
-    ret = []
+    ret = []  # type: List[Tuple[str, List[str]]]
     if not value.strip():
         return ret
 
     class State:
         seen_sep = True
         seen_colon = False
-        filenames = []
-        codes = []
+        filenames = []  # type: List[str]
+        codes = []  # type: List[str]
 
-    def _reset():
+    def _reset():  # type: () -> None
         if State.codes:
             for filename in State.filenames:
                 ret.append((filename, State.codes))
@@ -110,11 +114,8 @@ def parse_files_to_codes_mapping(value):  # noqa: C901
         State.filenames = []
         State.codes = []
 
-    def _unexpected_token():
-        # type: () -> exceptions.ExecutionError
-
-        def _indent(s):
-            # type: (str) -> str
+    def _unexpected_token():  # type: () -> exceptions.ExecutionError
+        def _indent(s):  # type: (str) -> str
             return "    " + s.strip().replace("\n", "\n    ")
 
         return exceptions.ExecutionError(
@@ -192,7 +193,7 @@ def normalize_path(path, parent=os.curdir):
     return path.rstrip(separator + alternate_separator)
 
 
-def _stdin_get_value_py3():
+def _stdin_get_value_py3():  # type: () -> io.StringIO
     stdin_value = sys.stdin.buffer.read()
     fd = io.BytesIO(stdin_value)
     try:
@@ -211,13 +212,13 @@ def stdin_get_value():
             stdin_value = io.BytesIO(sys.stdin.read())
         else:
             stdin_value = _stdin_get_value_py3()
-        stdin_get_value.cached_stdin = stdin_value
-        cached_value = stdin_get_value.cached_stdin
+        stdin_get_value.cached_stdin = stdin_value  # type: ignore
+        cached_value = stdin_get_value.cached_stdin  # type: ignore
     return cached_value.getvalue()
 
 
 def parse_unified_diff(diff=None):
-    # type: (str) -> Dict[str, Set[int]]
+    # type: (Optional[str]) -> Dict[str, Set[int]]
     """Parse the unified diff passed on stdin.
 
     :returns:
@@ -231,7 +232,7 @@ def parse_unified_diff(diff=None):
 
     number_of_rows = None
     current_path = None
-    parsed_paths = collections.defaultdict(set)
+    parsed_paths = collections.defaultdict(set)  # type: Dict[str, Set[int]]
     for line in diff.splitlines():
         if number_of_rows:
             # NOTE(sigmavirus24): Below we use a slice because stdin may be
@@ -279,6 +280,7 @@ def parse_unified_diff(diff=None):
                 1 if not group else int(group)
                 for group in hunk_match.groups()
             ]
+            assert current_path is not None  # nosec (for mypy)
             parsed_paths[current_path].update(
                 range(row, row + number_of_rows)
             )
@@ -338,12 +340,12 @@ def is_using_stdin(paths):
     return "-" in paths
 
 
-def _default_predicate(*args):
+def _default_predicate(*args):  # type: (*str) -> bool
     return False
 
 
 def filenames_from(arg, predicate=None):
-    # type: (str, Callable[[str], bool]) -> Generator
+    # type: (str, Optional[Callable[[str], bool]]) -> Generator[str, None, None]  # noqa: E501
     """Generate filenames from an argument.
 
     :param str arg:
@@ -384,8 +386,8 @@ def filenames_from(arg, predicate=None):
         yield arg
 
 
-def fnmatch(filename, patterns, default=True):
-    # type: (str, List[str], bool) -> bool
+def fnmatch(filename, patterns):
+    # type: (str, List[str]) -> bool
     """Wrap :func:`fnmatch.fnmatch` to add some functionality.
 
     :param str filename:
@@ -399,7 +401,7 @@ def fnmatch(filename, patterns, default=True):
         ``default`` if patterns is empty.
     """
     if not patterns:
-        return default
+        return True
     return any(_fnmatch.fnmatch(filename, pattern) for pattern in patterns)
 
 
@@ -452,6 +454,7 @@ def parameters_for(plugin):
 
 
 def matches_filename(path, patterns, log_message, logger):
+    # type: (str, List[str], str, logging.Logger) -> bool
     """Use fnmatch to discern if a path exists in patterns.
 
     :param str path:
@@ -483,7 +486,7 @@ def matches_filename(path, patterns, log_message, logger):
     return match
 
 
-def get_python_version():
+def get_python_version():  # type: () -> str
     """Find and format the python implementation and version.
 
     :returns:
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..f7ac891
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+"""This is here because mypy doesn't understand PEP 420."""
diff --git a/tests/integration/test_checker.py b/tests/integration/test_checker.py
index 0a3703c..a17e5cd 100644
--- a/tests/integration/test_checker.py
+++ b/tests/integration/test_checker.py
@@ -216,9 +216,7 @@ def test_report_order(results, expected_order):
 
     # _handle_results is the first place which gets the sorted result
     # Should something non-private be mocked instead?
-    handler = mock.Mock()
-    handler.side_effect = count_side_effect
-    manager._handle_results = handler
-
-    assert manager.report() == (len(results), len(results))
-    handler.assert_called_once_with('placeholder', expected_results)
+    handler = mock.Mock(side_effect=count_side_effect)
+    with mock.patch.object(manager, '_handle_results', handler):
+        assert manager.report() == (len(results), len(results))
+        handler.assert_called_once_with('placeholder', expected_results)
diff --git a/tests/unit/test_base_formatter.py b/tests/unit/test_base_formatter.py
index f28ab80..ec051a8 100644
--- a/tests/unit/test_base_formatter.py
+++ b/tests/unit/test_base_formatter.py
@@ -44,7 +44,9 @@ def test_format_needs_to_be_implemented():
     """Ensure BaseFormatter#format raises a NotImplementedError."""
     formatter = base.BaseFormatter(options())
     with pytest.raises(NotImplementedError):
-        formatter.format('foo')
+        formatter.format(
+            style_guide.Violation('A000', 'file.py', 1, 1, 'error text', None)
+        )
 
 
 def test_show_source_returns_nothing_when_not_showing_source():
@@ -73,6 +75,7 @@ def test_show_source_updates_physical_line_appropriately(line, column):
     formatter = base.BaseFormatter(options(show_source=True))
     error = style_guide.Violation('A000', 'file.py', 1, column, 'error', line)
     output = formatter.show_source(error)
+    assert output
     _, pointer = output.rsplit('\n', 1)
     assert pointer.count(' ') == (column - 1)
 
diff --git a/tests/unit/test_debug.py b/tests/unit/test_debug.py
index 0d1f903..2b7995d 100644
--- a/tests/unit/test_debug.py
+++ b/tests/unit/test_debug.py
@@ -72,8 +72,7 @@ def test_information(system, pyversion, pyimpl):
 @mock.patch('json.dumps', return_value='{}')
 def test_print_information_no_plugins(dumps, information, print_mock):
     """Verify we print and exit only when we have plugins."""
-    plugins = []
-    option_manager = mock.Mock(registered_plugins=set(plugins))
+    option_manager = mock.Mock(registered_plugins=set())
     assert debug.print_information(
         None, None, None, None, option_manager=option_manager,
     ) is None
diff --git a/tests/unit/test_decision_engine.py b/tests/unit/test_decision_engine.py
index fd87a45..635c9d8 100644
--- a/tests/unit/test_decision_engine.py
+++ b/tests/unit/test_decision_engine.py
@@ -74,11 +74,10 @@ def test_was_selected_selects_errors(select_list, enable_extensions,
 
 def test_was_selected_implicitly_selects_errors():
     """Verify we detect users implicitly selecting an error."""
-    select_list = []
     error_code = 'E121'
     decider = style_guide.DecisionEngine(
         create_options(
-            select=select_list,
+            select=[],
             extended_default_select=['E'],
         ),
     )
diff --git a/tests/unit/test_option.py b/tests/unit/test_option.py
index eb4f95d..e931d13 100644
--- a/tests/unit/test_option.py
+++ b/tests/unit/test_option.py
@@ -75,4 +75,4 @@ def test_config_name_needs_long_option_name():
 def test_dest_is_not_overridden():
     """Show that we do not override custom destinations."""
     opt = manager.Option('-s', '--short', dest='something_not_short')
-    assert opt.dest == 'something_not_short'
+    assert opt.dest == 'something_not_short'  # type: ignore
diff --git a/tests/unit/test_plugin_type_manager.py b/tests/unit/test_plugin_type_manager.py
index e1392a4..4174c58 100644
--- a/tests/unit/test_plugin_type_manager.py
+++ b/tests/unit/test_plugin_type_manager.py
@@ -1,17 +1,10 @@
 """Tests for flake8.plugins.manager.PluginTypeManager."""
-import sys
-
 import mock
 import pytest
 
 from flake8 import exceptions
 from flake8.plugins import manager
 
-if sys.version_info >= (3, 3):
-    import collections.abc as collections_abc
-else:
-    import collections as collections_abc
-
 TEST_NAMESPACE = "testing.plugin-type-manager"
 
 
@@ -91,7 +84,7 @@ def test_generate_call_function():
         'method_name', optmanager,
     )
 
-    assert isinstance(func, collections_abc.Callable)
+    assert callable(func)
     assert func(plugin) is optmanager
 
 
@@ -168,15 +161,14 @@ def test_provide_options(PluginManager):  # noqa: N803
     PluginManager.return_value = create_mapping_manager_mock(plugins)
     optmanager = object()
     options = object()
-    extra_args = []
 
     type_mgr = FakeTestType()
-    type_mgr.provide_options(optmanager, options, extra_args)
+    type_mgr.provide_options(optmanager, options, [])
 
     for plugin in plugins:
         plugin.provide_options.assert_called_with(optmanager,
                                                   options,
-                                                  extra_args)
+                                                  [])
 
 
 @mock.patch('flake8.plugins.manager.PluginManager')
diff --git a/tests/unit/test_statistics.py b/tests/unit/test_statistics.py
index 31d6276..16896f0 100644
--- a/tests/unit/test_statistics.py
+++ b/tests/unit/test_statistics.py
@@ -82,7 +82,7 @@ def test_recording_statistics():
         assert isinstance(key, stats.Key)
         assert isinstance(value, stats.Statistic)
 
-    assert storage[(DEFAULT_FILENAME, DEFAULT_ERROR_CODE)].count == 1
+    assert storage[stats.Key(DEFAULT_FILENAME, DEFAULT_ERROR_CODE)].count == 1
 
 
 def test_statistics_for_single_record():
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index dcbf8b8..9f6925e 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -163,12 +163,6 @@ def test_fnmatch(filename, patterns, expected):
     assert utils.fnmatch(filename, patterns) is expected
 
 
-def test_fnmatch_returns_the_default_with_empty_default():
-    """The default parameter should be returned when no patterns are given."""
-    sentinel = object()
-    assert utils.fnmatch('file.py', [], default=sentinel) is sentinel
-
-
 def test_filenames_from_a_directory():
     """Verify that filenames_from walks a directory."""
     filenames = list(utils.filenames_from('src/flake8/'))
diff --git a/tox.ini b/tox.ini
index f39c687..464f3a4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -70,13 +70,12 @@ deps =
 commands =
     doc8 docs/source/
 
-[testenv:mypy]
+[testenv:pre-commit]
 basepython = python3
 skip_install = true
-deps =
-    mypy
+deps = pre-commit
 commands =
-    mypy src
+    pre-commit run --all-files --show-diff-on-failure
 
 [testenv:bandit]
 basepython = python3