feat: add option to run with cache support

fixes #601

src/flake8/cacher.py

@@ -0,0 +1,29 @@
+"""Cache files manager"""
+import hashlib
+import os
+import pickle
+
+
+class Cacher(object):
+    def __init__(self, file_name, cache_path):
+        # type: (str, str) -> None
+        self.file_name = file_name
+        enc = hashlib.md5(file_name.encode())
+        self.cache_path = os.path.join(cache_path, str(enc.hexdigest()))
+
+    def get(self):
+        if not os.path.exists(self.cache_path):
+            return
+        with open(self.cache_path, "rb") as fr:
+            mtime, results = pickle.load(fr)
+            # if the mtime doesn't change then return cache.
+            # otherwise it is invalid
+            current_mtime = os.path.getmtime(self.file_name)
+            if mtime == current_mtime:
+                return results
+
+    def save(self, results):
+        """saves the given result to the cache file"""
+        with open(self.cache_path, "wb") as fw:
+            current_mtime = os.path.getmtime(self.file_name)
+            pickle.dump((current_mtime, results), fw)

src/flake8/checker.py

@@ -14,6 +14,7 @@ except ImportError:
     multiprocessing = None  # type: ignore
 
 from flake8 import defaults
+from flake8 import cacher
 from flake8 import exceptions
 from flake8 import processor
 from flake8 import utils
@@ -360,6 +361,7 @@ class FileChecker(object):
         self.options = options
         self.filename = filename
         self.checks = checks
+        self.cacher = cacher.Cacher(filename, self.options.cache_location)
         # fmt: off
         self.results = []  # type: List[Tuple[str, int, int, str, Optional[str]]]  # noqa: E501
         # fmt: on
@@ -583,6 +585,22 @@ class FileChecker(object):
             self.run_logical_checks()
 
     def run_checks(self):
+        """cached wrapper of self.run_checks"""
+        # handle both cases where cache is enabled/disabled or invalid
+        cache_available = False
+        if self.options.cache:
+            saved = self.cacher.get()
+            if saved is not None:
+                self.filename, self.results, self.statistics = saved
+                cache_available = True
+
+        if cache_available is False:
+            result = self._run_checks()
+            if self.options.cache:
+                self.cacher.save(result)
+        return self.filename, self.results, self.statistics
+
+    def _run_checks(self):
         """Run checks against the file."""
         try:
             self.process_tokens()

src/flake8/main/application.py

@@ -3,6 +3,7 @@ from __future__ import print_function
 
 import argparse
 import logging
+import os
 import sys
 import time
 from typing import Dict, List, Optional, Set, Tuple
@@ -336,6 +337,9 @@ class Application(object):
         self.make_formatter()
         self.make_guide()
         self.make_file_checker_manager()
+        if self.options.cache:
+            if not os.path.exists(self.options.cache_location):
+                os.makedirs(self.options.cache_location)
 
     def report(self):
         """Report errors, statistics, and benchmarks."""

src/flake8/main/options.py

@@ -298,6 +298,21 @@ def register_default_options(option_manager):
     )
 
     # Flake8 options
+    add_option(
+        "--cache",
+        default=False,
+        parse_from_config=False,
+        action="store_true",
+        help="Cache flake8 results on consecutive runs.",
+    )
+
+    add_option(
+        "--cache-location",
+        default=".cache/flake8",
+        parse_from_config=True,
+        help="Location to store cached results.",
+    )
+
     add_option(
         "--enable-extensions",
         default="",