Explain is_package AttributeError in find_package

When a PEP 302 import hook is used that doesn't implement .is_package()
an AttributeError is raised. This looks like a bug in Flask. This change
fixes that problem in the sense that it explains, that the
AttributeError is intentional.

CHANGES

@@ -9,6 +9,9 @@ Version 0.10.2
 (bugfix release, release date to be announced)
 
 - Fixed broken `test_appcontext_signals()` test case.
+- Raise an :exc:`AttributeError` in :func:`flask.helpers.find_package` with a
+  useful message explaining why it is raised when a PEP 302 import hook is used
+  without an `is_package()` method.
 
 Version 0.10.1
 --------------

flask/helpers.py

@@ -679,8 +679,13 @@ def find_package(import_name):
             filename = sys.modules[import_name].__file__
         package_path = os.path.abspath(os.path.dirname(filename))
         # package_path ends with __init__.py for a package
-        if loader.is_package(root_mod_name):
-            package_path = os.path.dirname(package_path)
+        if hasattr(loader, 'is_package'):
+            if loader.is_package(root_mod_name):
+                package_path = os.path.dirname(package_path)
+        else:
+            raise AttributeError(
+                ('%s.is_package() method is missing but is '
+                 'required by Flask of PEP 302 import hooks') % loader.__class__.__name__)
 
     site_parent, site_folder = os.path.split(package_path)
     py_prefix = os.path.abspath(sys.prefix)

flask/testsuite/config.py

@@ -16,6 +16,7 @@ import pkgutil
 import unittest
 from contextlib import contextmanager
 from flask.testsuite import FlaskTestCase
+from flask._compat import PY2
 
 
 # config keys used for the ConfigTestCase
@@ -291,6 +292,18 @@ class InstanceTestCase(FlaskTestCase):
             if 'site_egg' in sys.modules:
                 del sys.modules['site_egg']
 
+    if PY2:
+        def test_meta_path_loader_without_is_package(self):
+            class Loader(object):
+                def find_module(self, name):
+                    return self
+            sys.meta_path.append(Loader())
+            try:
+                with self.assert_raises(AttributeError):
+                    flask.Flask(__name__)
+            finally:
+                sys.meta_path.pop()
+
 
 def suite():
     suite = unittest.TestSuite()