Refactored and added tests for backend choosing. Functionally still w…

…orks the same, but mostly rewritten.

goless/backends.py

@@ -92,22 +92,25 @@ def propogate_exc(self, errtype, *args):
 
     return GeventBackend()
 
-_backends = {
-    "stackless": _make_stackless,
-    "gevent": _make_gevent
+
+_default_backends = {
+    'stackless': _make_stackless,
+    'gevent': _make_gevent
 }
 
-current = None
-
-GOLESS_BACKEND = os.getenv("GOLESS_BACKEND", '')
-if GOLESS_BACKEND:
-    if GOLESS_BACKEND not in _backends:
-        raise RuntimeError(
-            "Invalid backend %r specified. Valid backends are: %s"
-            % (GOLESS_BACKEND, _backends.keys()))
-    current = _backends[GOLESS_BACKEND]()
-else:
-    try:
-        current = _make_stackless()
-    except ImportError:
-        current = _make_gevent()
+
+def calculate_backend(name_from_env, backends=None):
+    if backends is None:
+        backends = _default_backends
+    if name_from_env:
+        if name_from_env not in backends:
+            raise RuntimeError(
+                'Invalid backend %r specified. Valid backends are: %s'
+                % (name_from_env, _default_backends.keys()))
+        return backends[name_from_env]()
+    for maker in backends.values():
+        return maker()
+    raise RuntimeError('No backend could be created.')
+
+
+current = calculate_backend(os.getenv('GOLESS_BACKEND', ''))

tests/test_backends.py

@@ -0,0 +1,28 @@
+from . import BaseTests
+from goless import backends
+
+test_backends = dict(
+    a=lambda: 'be_A',
+    b=lambda: 'be_B',
+)
+
+
+class CalcBackendTests(BaseTests):
+    def calc(self, name, testbackends=test_backends):
+        return backends.calculate_backend(name, testbackends)
+
+    def test_valid_envvar_name(self):
+        be = self.calc('a')
+        self.assertEqual(be, 'be_A')
+
+    def test_invalid_envvar_name(self):
+        with self.assertRaises(RuntimeError):
+            self.calc('invalid')
+
+    def test_default(self):
+        be = self.calc('')
+        self.assertIn(be, [v() for v in test_backends.values()])
+
+    def test_all_invalid(self):
+        with self.assertRaises(RuntimeError):
+            self.calc('', {})