Cache len in directory fetcher

fimfarchive/fetchers/directory.py

@@ -62,6 +62,7 @@ class DirectoryFetcher(Fetcher):
         """
         self.meta_path = meta_path
         self.data_path = data_path
+        self.length: Optional[int] = None
         self.flavors = frozenset(flavors)
 
     def iter_path_keys(self, path: Optional[Path]) -> Iterator[int]:
@@ -111,7 +112,10 @@ class DirectoryFetcher(Fetcher):
         """
         Returns the total number of stories in the directories.
         """
-        return len(self.list_keys())
+        if self.length is None:
+            self.length = len(self.list_keys())
+
+        return self.length
 
     def __iter__(self) -> Iterator[Story]:
         """

tests/fetchers/test_directory.py

@@ -26,6 +26,7 @@ import json
 import pytest
 from pathlib import Path
 from typing import Any, Dict
+from unittest.mock import patch
 
 from fimfarchive.exceptions import InvalidStoryError
 from fimfarchive.fetchers import DirectoryFetcher
@@ -155,6 +156,15 @@ class TestDirectoryFetcher:
         """
         assert 3 == len(fetcher)
 
+    def test_len_caching(test, fetcher):
+        """
+        Tests len is only calculated once.
+        """
+        with patch.object(fetcher, 'list_keys', wraps=fetcher.list_keys) as m:
+            assert 3 == len(fetcher)
+            assert 3 == len(fetcher)
+            assert 1 == len(m.mock_calls)
+
     def test_iter(self, fetcher):
         """
         Tests iter yields all available stories, ordered by key.