loaders: Add LoaderChain.

oxrlib/loaders.py

@@ -1,4 +1,5 @@
 import cgi
+import functools
 import io
 import urllib.request
 import urllib.parse
@@ -19,6 +20,10 @@ class LoaderSourceError(LoaderError):
     pass
 
 
+class NoLoadersError(Exception):
+    pass
+
+
 class FileCache:
     def __init__(self, dir_path, filename_pattern):
         self.dir_path = dir_path
@@ -75,3 +80,32 @@ class OXRAPIRequest:
             'historical/{}.json'.format(date.isoformat()),
             {'app_id': self.app_id, 'base': base},
         )
+
+
+class LoaderChain:
+    def __init__(self):
+        self.loaders = []
+
+    def add_loader(self, loader):
+        self.loaders.append(loader)
+
+    def _wrap_load_method(orig_func):
+        @functools.wraps(orig_func)
+        def load_wrapper(self, *args, **kwargs):
+            self.used_loader = None
+            error = None
+            for loader in self.loaders:
+                try:
+                    response = getattr(loader, orig_func.__name__)(*args, **kwargs)
+                except LoaderError as this_error:
+                    error = this_error
+                else:
+                    self.used_loader = loader
+                    return response
+            else:
+                raise NoLoadersError() if error is None else error
+        return load_wrapper
+
+    @_wrap_load_method
+    def historical(self, date, base):
+        pass

tests/__init__.py

@@ -1,6 +1,14 @@
+import datetime
 import pathlib
+import random
+
+import pytest
 
 TEST_DIR = pathlib.Path(__file__).parent
 
 def relpath(*parts):
     return TEST_DIR / pathlib.Path(*parts)
+
+@pytest.fixture
+def any_date():
+    return datetime.date.today() - datetime.timedelta(days=730 - random.randint(0, 365))

tests/test_LoaderChain.py

@@ -0,0 +1,71 @@
+import io
+
+import pytest
+
+import oxrlib.loaders
+
+from . import any_date
+
+SUCCESS_S = '"success"\n'
+ERROR = oxrlib.loaders.LoaderNoDataError("test")
+
+class FakeLoader:
+    def __init__(self, result):
+        self.result = result
+
+    def _respond(self, *args, **kwargs):
+        return io.StringIO(self.result)
+
+    def __getattr__(self, name):
+        return self._respond
+
+
+class FakeErrorLoader(FakeLoader):
+    def __init__(self, error):
+        self.error = error
+
+    def _respond(self, *args, **kwargs):
+        raise self.error
+
+
+@pytest.fixture
+def lchain():
+    return oxrlib.loaders.LoaderChain()
+
+@pytest.fixture
+def good_loader():
+    return FakeLoader(SUCCESS_S)
+
+@pytest.fixture
+def error_loader():
+    return FakeErrorLoader(ERROR)
+
+def test_no_loaders(lchain, any_date):
+    try:
+        lchain.historical(any_date, 'USD')
+    except oxrlib.loaders.NoLoadersError:
+        pass
+    else:
+        assert False, "expected NoLoadersError not raised"
+
+def test_one_with_success(lchain, any_date, good_loader):
+    lchain.add_loader(good_loader)
+    response = lchain.historical(any_date, 'USD')
+    assert response.read(32) == SUCCESS_S
+
+def test_two_with_success(lchain, any_date, good_loader, error_loader):
+    lchain.add_loader(error_loader)
+    lchain.add_loader(good_loader)
+    response = lchain.historical(any_date, 'USD')
+    assert response.read(32) == SUCCESS_S
+
+@pytest.mark.parametrize('count', [1, 2])
+def test_no_success(lchain, any_date, error_loader, count):
+    for _ in range(count):
+        lchain.add_loader(error_loader)
+    try:
+        lchain.historical(any_date, 'USD')
+    except type(ERROR) as error:
+        assert error is ERROR
+    else:
+        assert False, "{} not raised".format(type(ERROR).__name__)

tests/test_OXRAPIRequest.py

@@ -1,4 +1,3 @@
-import datetime
 import http.client
 import io
 import json
@@ -10,6 +9,8 @@ import urllib.parse
 import pytest
 import oxrlib.loaders
 
+from . import any_date
+
 APPID_CHARS = string.ascii_letters + string.digits
 RANDOM_APPID = ''.join(random.choice(APPID_CHARS) for _ in range(32))
 API_ROOT = 'http://[100::]/oxrlibtest/'
@@ -67,10 +68,6 @@ class FakeOpener:
 def api_client():
     return oxrlib.loaders.OXRAPIRequest(RANDOM_APPID, API_ROOT)
 
-@pytest.fixture
-def any_date():
-    return datetime.date.today() - datetime.timedelta(days=730 - random.randint(0, 365))
-
 @pytest.mark.parametrize('base', ['USD', 'JPY'])
 def test_success(api_client, any_date, base):
     body = "Good Test"