Fixed #26462 -- Fixed Python 2 UnicodeEncodeError when warning about long cache keys.

django/core/cache/backends/base.py

@@ -235,7 +235,7 @@ class BaseCache(object):
         """
         if len(key) > MEMCACHE_MAX_KEY_LENGTH:
             warnings.warn('Cache key will cause errors if used with memcached: '
-                    '%s (longer than %s)' % (key, MEMCACHE_MAX_KEY_LENGTH),
+                    '%r (longer than %s)' % (key, MEMCACHE_MAX_KEY_LENGTH),
                     CacheKeyWarning)
         for char in key:
             if ord(char) < 33 or ord(char) == 127:

tests/cache/tests.py

@@ -585,15 +585,31 @@ class BaseCacheTests(object):
             with warnings.catch_warnings(record=True) as w:
                 warnings.simplefilter("always")
                 # memcached does not allow whitespace or control characters in keys
-                cache.set('key with spaces', 'value')
+                key = 'key with spaces and 清'
+                cache.set(key, 'value')
                 self.assertEqual(len(w), 1)
                 self.assertIsInstance(w[0].message, CacheKeyWarning)
+                self.assertEqual(
+                    # warnings.warn() crashes on Python 2 if message isn't
+                    # coercible to str.
+                    str(w[0].message.args[0]),
+                    "Cache key contains characters that will cause errors if used "
+                    "with memcached: %r" % key,
+                )
             with warnings.catch_warnings(record=True) as w:
                 warnings.simplefilter("always")
                 # memcached limits key length to 250
-                cache.set('a' * 251, 'value')
+                key = ('a' * 250) + '清'
+                cache.set(key, 'value')
                 self.assertEqual(len(w), 1)
                 self.assertIsInstance(w[0].message, CacheKeyWarning)
+                self.assertEqual(
+                    # warnings.warn() crashes on Python 2 if message isn't
+                    # coercible to str.
+                    str(w[0].message.args[0]),
+                    'Cache key will cause errors if used with memcached: '
+                    '%r (longer than %s)' % (key, 250),
+                )
         finally:
             cache.key_func = old_func