auth: Allow Python classes as request validators (see #4)

In addition to setuptools entry point names, the authentication subsystem now
accepts Python classes directly as the value of the `milla.request_validator`
configuration setting. This removes the dependency on setuptools for
authentication, and allows more flexibility in application configuration.

src/milla/auth/decorators.py

@@ -18,9 +18,12 @@
 '''
 
 from functools import wraps
-from milla.auth import RequestValidator, NotAuthorized, permissions
-import milla
+from milla.auth import permissions
+import milla.auth
+try:
     import pkg_resources
+except ImportError:
+    pkg_resources = None
 
 
 __all__ = ['auth_required', 'require_perms']
@@ -40,20 +43,32 @@ def _find_request(*args, **kwargs):
 
 def _validate_request(func, requirement, *args, **kwargs):
     request = _find_request(*args, **kwargs)
-    ep_name = request.config.get('request_validator', 'default')
 
-    # Override the RequestVariable name with a class from the specified
-    # entry point, if one is available. Otherwise, the default is used.
-    for ep in pkg_resources.iter_entry_points(VALIDATOR_EP_GROUP, ep_name):
+    rv = request.config.get('request_validator', 'default')
+    if hasattr(rv, 'validate'):
+        # Config specifies a request validator class explicitly instead
+        # of an entry point name, so use it directly
+        validator = rv()
+    elif pkg_resources:
+        for ep in pkg_resources.iter_entry_points(VALIDATOR_EP_GROUP, rv):
             try:
-            RequestValidator = ep.load()
+                validator = ep.load()()
+                break
             except:
+                # Ignore errors loading entry points or creating instances
                 continue
+        else:
+            # No entry point loaded or request validator instance
+            # created, use the default
+            validator = milla.auth.RequestValidator()
+    else:
+        # config does not specify a request validator class, and
+        # setuptools is not available, use the default
+        validator = milla.auth.RequestValidator()
 
     try:
-        validator = RequestValidator()
         validator.validate(request, requirement)
-    except NotAuthorized as e:
+    except milla.auth.NotAuthorized as e:
         return e(request)
     return func(*args, **kwargs)