Fix reporting of UndefinedLocal pyflakes error

src/flake8/plugins/pyflakes.py

@@ -57,10 +57,7 @@ def patch_pyflakes():
     """Add error codes to Pyflakes messages."""
     for name, obj in vars(pyflakes.messages).items():
         if name[0].isupper() and obj.message:
-            obj.flake8_msg = "%s %s" % (
+            obj.flake8_code = FLAKE8_PYFLAKES_CODES.get(name, "F999")
-                FLAKE8_PYFLAKES_CODES.get(name, "F999"),
-                obj.message,
-            )
 
 
 patch_pyflakes()
@@ -188,6 +185,9 @@ class FlakesChecker(pyflakes.checker.Checker):
             yield (
                 message.lineno,
                 col,
-                (message.flake8_msg % message.message_args),
+                "{} {}".format(
+                    message.flake8_code,
+                    message.message % message.message_args,
+                ),
                 message.__class__,
             )

tests/unit/test_pyflakes_codes.py

@@ -1,4 +1,5 @@
 """Tests of pyflakes monkey patches."""
+import ast
 
 import pyflakes
 
@@ -13,3 +14,20 @@ def test_all_pyflakes_messages_have_flake8_codes_assigned():
         if name[0].isupper() and obj.message
     }
     assert messages == set(pyflakes_shim.FLAKE8_PYFLAKES_CODES)
+
+
+def test_undefined_local_code():
+    """In pyflakes 2.1.0 this code's string formatting was changed."""
+    src = '''\
+import sys
+
+def f():
+    sys = sys
+'''
+    tree = ast.parse(src)
+    checker = pyflakes_shim.FlakesChecker(tree, (), 't.py')
+    message_texts = [s for _, _, s, _ in checker.run()]
+    assert message_texts == [
+        "F823 local variable 'sys' defined in enclosing scope on line 1 referenced before assignment",  # noqa: E501
+        "F841 local variable 'sys' is assigned to but never used",
+    ]