Fixed #20599 -- Changed wording of ValueError raised by _load_library

The _load_library method on BasePasswordHasher turns ImportErrors
into ValueErrors, this masks ImportErrors in the algorithm library.
Changed it to a clearer worded error message that includes
the ImportError string.

django/contrib/auth/hashers.py

@@ -171,12 +171,12 @@ class BasePasswordHasher(object):
                 name = mod_path = self.library
             try:
                 module = importlib.import_module(mod_path)
-            except ImportError:
-                raise ValueError("Couldn't load %s password algorithm "
-                                 "library" % name)
+            except ImportError as e:
+                raise ValueError("Couldn't load %r algorithm library: %s" %
+                                 (self.__class__.__name__, e))
             return module
-        raise ValueError("Hasher '%s' doesn't specify a library attribute" %
-                         self.__class__)
+        raise ValueError("Hasher %r doesn't specify a library attribute" %
+                         self.__class__.__name__)
 
     def salt(self):
         """

django/contrib/auth/tests/test_hashers.py

@@ -2,7 +2,7 @@
 from __future__ import unicode_literals
 
 from django.conf.global_settings import PASSWORD_HASHERS as default_hashers
-from django.contrib.auth.hashers import (is_password_usable,
+from django.contrib.auth.hashers import (is_password_usable, BasePasswordHasher,
     check_password, make_password, PBKDF2PasswordHasher, load_hashers,
     PBKDF2SHA1PasswordHasher, get_hasher, identify_hasher, UNUSABLE_PASSWORD)
 from django.utils import unittest
@@ -128,9 +128,8 @@ class TestUtilsHashPass(unittest.TestCase):
         self.assertRaises(ValueError, identify_hasher, encoded)
 
     def test_bad_algorithm(self):
-        def doit():
+        with self.assertRaises(ValueError):
             make_password('lètmein', hasher='lolcat')
-        self.assertRaises(ValueError, doit)
         self.assertRaises(ValueError, identify_hasher, "lolcat$salt$hash")
 
     def test_bad_encoded(self):
@@ -178,3 +177,17 @@ class TestUtilsHashPass(unittest.TestCase):
                 state['upgraded'] = True
             self.assertFalse(check_password('WRONG', encoded, setter))
             self.assertFalse(state['upgraded'])
+
+    def test_load_library_no_algorithm(self):
+        with self.assertRaises(ValueError) as e:
+            BasePasswordHasher()._load_library()
+        self.assertEqual("Hasher 'BasePasswordHasher' doesn't specify a "
+                         "library attribute", str(e.exception))
+
+    def test_load_library_importerror(self):
+        PlainHasher = type(str('PlainHasher'), (BasePasswordHasher,),
+                           {'algorithm': 'plain', 'library': 'plain'})
+        with self.assertRaises(ValueError) as e:
+            PlainHasher()._load_library()
+        self.assertEqual("Couldn't load 'PlainHasher' algorithm library: "
+                         "No module named plain", str(e.exception))