Add get_or_fetch cache helper and tests (#72)

disagreement/cache.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import time
-from typing import TYPE_CHECKING, Dict, Generic, Optional, TypeVar
+from typing import TYPE_CHECKING, Callable, Dict, Generic, Optional, TypeVar
 from collections import OrderedDict
 
 if TYPE_CHECKING:
@@ -40,6 +40,14 @@ class Cache(Generic[T]):
         self._data.move_to_end(key)
         return value
 
+    def get_or_fetch(self, key: str, fetch_fn: Callable[[], T]) -> T:
+        """Return a cached item or fetch and store it if missing."""
+        value = self.get(key)
+        if value is None:
+            value = fetch_fn()
+            self.set(key, value)
+        return value
+
     def invalidate(self, key: str) -> None:
         self._data.pop(key, None)
 

tests/test_cache.py

@@ -26,3 +26,42 @@ def test_cache_lru_eviction():
     assert cache.get("b") is None
     assert cache.get("a") == 1
     assert cache.get("c") == 3
+
+
+def test_get_or_fetch_uses_cache():
+    cache = Cache()
+    cache.set("a", 1)
+
+    def fetch():
+        raise AssertionError("fetch should not be called")
+
+    assert cache.get_or_fetch("a", fetch) == 1
+
+
+def test_get_or_fetch_fetches_and_stores():
+    cache = Cache()
+    called = False
+
+    def fetch():
+        nonlocal called
+        called = True
+        return 2
+
+    assert cache.get_or_fetch("b", fetch) == 2
+    assert called
+    assert cache.get("b") == 2
+
+
+def test_get_or_fetch_fetches_expired_item():
+    cache = Cache(ttl=0.01)
+    cache.set("c", 1)
+    time.sleep(0.02)
+    called = False
+
+    def fetch():
+        nonlocal called
+        called = True
+        return 3
+
+    assert cache.get_or_fetch("c", fetch) == 3
+    assert called