LoaderChain: Add should_cache() method.

oxrlib/loaders.py

@@ -20,6 +20,9 @@ class FileCache(cache.CacheBase):
     def open(self, path):
         return self.CacheFile(path)
 
+    def is_cache(self):
+        return True
+
 
 class OXRAPIRequest:
     DEFAULT_API_ROOT = 'https://openexchangerates.org/api/'
@@ -30,6 +33,9 @@ class OXRAPIRequest:
         self.app_id = app_id
         self.open_url = open_func
 
+    def is_cache(self):
+        return False
+
     def _get_response_encoding(self, response, default=None):
         try:
             content_type = response.getheader('Content-Type', 'application/json')
@@ -69,9 +75,11 @@ class OXRAPIRequest:
 class LoaderChain:
     def __init__(self):
         self.loaders = []
+        self.can_cache = False
 
     def add_loader(self, loader):
         self.loaders.append(loader)
+        self.can_cache = self.can_cache or loader.is_cache()
 
     def _wrap_load_method(orig_func):
         @functools.wraps(orig_func)
@@ -93,3 +101,6 @@ class LoaderChain:
     @_wrap_load_method
     def historical(self, date, base):
         pass
+
+    def should_cache(self):
+        return self.can_cache and self.used_loader and not self.used_loader.is_cache()

tests/test_LoaderChain.py

@@ -11,8 +11,9 @@ SUCCESS_S = '"success"\n'
 ERROR = oxrlib.errors.LoaderNoDataError("test")
 
 class FakeLoader:
-    def __init__(self, result):
+    def __init__(self, result, *, is_cache=False):
         self.result = result
+        self._is_cache = is_cache
 
     def _respond(self, *args, **kwargs):
         return io.StringIO(self.result)
@@ -20,13 +21,13 @@ class FakeLoader:
     def __getattr__(self, name):
         return self._respond
 
+    def is_cache(self):
+        return self._is_cache
+
 
 class FakeErrorLoader(FakeLoader):
-    def __init__(self, error):
-        self.error = error
-
     def _respond(self, *args, **kwargs):
-        raise self.error
+        raise self.result
 
 
 @pytest.fixture
@@ -70,3 +71,21 @@ def test_no_success(lchain, any_date, error_loader, count):
         assert error is ERROR
     else:
         assert False, "{} not raised".format(type(ERROR).__name__)
+
+def test_should_cache(lchain, any_date, good_loader):
+    cache_loader = FakeErrorLoader(ERROR, is_cache=True)
+    lchain.add_loader(cache_loader)
+    lchain.add_loader(good_loader)
+    lchain.historical(any_date, 'USD')
+    assert lchain.should_cache()
+
+def test_should_cache_unable(lchain, any_date, good_loader):
+    lchain.add_loader(good_loader)
+    lchain.historical(any_date, 'USD')
+    assert not lchain.should_cache(), "suggested using unavailable cache"
+
+def test_should_cache_unneeded(lchain, any_date):
+    loader = FakeLoader(SUCCESS_S, is_cache=True)
+    lchain.add_loader(loader)
+    lchain.historical(any_date, 'USD')
+    assert not lchain.should_cache(), "suggested rewriting cache"