Finish up cache and fix tests.

Still TODO: Merge with ignore functionality.

django_migration_linter/cache.py

@@ -0,0 +1,53 @@
+# Copyright 2018 3YOURMIND GmbH
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+# http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import hashlib
+import os
+import pickle
+
+from django_migration_linter.utils import split_path, compose_migration_path
+
+
+class Cache(dict):
+    def __init__(self, django_folder, cache_path):
+        self.django_folder = django_folder
+        project_name = split_path(django_folder)[-2]
+        self.filename = os.path.join(cache_path, '{0}.pickle'.format(project_name))
+
+        if not os.path.exists(os.path.dirname(self.filename)):
+            os.makedirs(os.path.dirname(self.filename))
+
+        super(Cache, self).__init__()
+
+    def load(self):
+        try:
+            f = open(self.filename, 'rb')
+            tmp_dict = pickle.load(f)
+            self.update(tmp_dict)
+            f.close()
+        except IOError:
+            pass
+
+    def save(self):
+        f = open(self.filename, 'wb')
+        pickle.dump(self, f, 2)
+        f.close()
+
+    def md5(self, app_name, migration):
+        path = compose_migration_path(self.django_folder, app_name, migration)
+        hash_md5 = hashlib.md5()
+        with open(path, "rb") as f:
+            for chunk in iter(lambda: f.read(4096), b""):
+                hash_md5.update(chunk)
+        return hash_md5.hexdigest()