fix incorrectly written lazy properties [GH-6]

The lazy properties weren't lazy...on the opposite the way they were written
they were rebuilt the property was called.

Author: Sarcasm

compdb/backend/json.py

@@ -11,6 +11,7 @@
 class JSONCompilationDatabase(CompilationDatabaseInterface):
     def __init__(self, json_db_path):
         self.json_db_path = json_db_path
+        self.__data = None
 
     @classmethod
     def probe_directory(cls, directory):
@@ -48,7 +49,7 @@ def _dict_to_compile_command(d):
 
     @property
     def _data(self):
-        if not hasattr(self, '__data'):
+        if self.__data is None:
             with open(self.json_db_path) as f:
                 self.__data = json.load(f)
         return self.__data

compdb/includedb.py

@@ -141,6 +141,7 @@ class IncludedByDatabase(CompilationDatabaseInterface):
     def __init__(self, graph, database):
         self.graph = graph
         self.database = database
+        self.__db_index = None
 
     def __repr__(self):
         return '<IncludedByGraph: graph = {}, database = {}>'.format(
@@ -177,7 +178,7 @@ def _bfs_walk_from(self, path):
 
     @property
     def _db_index(self):
-        if not hasattr(self, '__db_index'):
+        if self.__db_index is None:
             self.__db_index = frozenset(self.database.get_all_files())
         return self.__db_index