Mypy: "Name already defined" error is confusing

[sid-kap]

Mypy complains if an if-else statement redefines a global variable with a class definition.

Failing code:

if 5 % 2 == 0:
  X = 5
else:
  class X:
    pass

Error:

test.py:4: error: Name 'X' already defined on line 2

mypy version: 0.570

[gvanrossum]

That's intentional, because after the if-else X has a useless type.

[sid-kap]

1. What do you mean by "has a useless type"?

2. Here's the code I'm depending on that's causing this error: https://github.com/ipython/ipython/blob/dc46d30ae2318456b5f3fb3cef37066ef2cc305e/IPython/core/history.py#L66. What do you suggest I do to fix this error?

[gvanrossum]

I was referring to your example. Apparently you didn't capture the essence of the real code you were having trouble with. There are a number of special cases implemented when two branches define different but equivalent (to the user) types, and your example wasn't one of them.

It seems defining two different classes isn't one of them either. So I recommend putting # type: ignore on the exact line where you get the error.

[sid-kap]

Ok, thanks!

[sid-kap]

In any case, "Name already defined" seems like a very bad explanation for the error. The problem is not that X is already defined—mutating variables is something you do in Python all the time. The problem is that you're trying to reassign something to X that's of a different type.

[gvanrossum]

OK, reopening as a usability issue -- long-term maintainers get blind for those little usability problems, so thanks for bringing that up!

[glenjamin]

I've just run into the same error message, but in a different scenario.

I'm trying to generate some usable stubs for the GitPython project.

This is what __init__.pyi looks like:

# Stubs for git (Python 3.6)
#
# NOTE: This dynamically typed stub was automatically generated by stubgen.

from git.exc import *
from git.objects import *
from git.refs import *
from git.diff import *
from git.db import *
from git.remote import *
from git.index import *
from git.cmd import Git as Git
from git.config import GitConfigParser as GitConfigParser
from git.repo import Repo as Repo
from git.util import Actor as Actor, BlockingLockFile as BlockingLockFile, LockFile as LockFile, Stats as Stats, rmtree as rmtree

Which produces the following error:

type-stubs/git/__init__.pyi:6: error: Name 'util' already defined
type-stubs/git/__init__.pyi:7: error: Name 'tag' already defined
type-stubs/git/__init__.pyi:11: error: Name 'base' already defined

It would be helpful if the error message could tell me where the original definition came from, and possible also the type of each so I can check whether the incompatibility is a type error or a naming/import error.

[sid-kap]

I believe @glenjamin's problem would be fixed by passing in existing as the optional argument original_ctx to self.name_already_defined in SemanticAnalyzerPass2.add_symbol at https://github.com/python/mypy/blob/8f8d136235638d8d0c92d98ce5a2a40a36f43862/mypy/semanal.py#L3192
(i.e. changing this line to self.name_already_defined(name, context, existing)).

In my test code, it now prints Name 'util' already defined on line 1, where line 1 is the line that contains the other wildcard import that imported something called util.
Does this seem like a reasonable change to make? Or is there a reason that the authors chose not to print this information?

[JelleZijlstra]

Yes, that sounds like it would be an improvement!

[Strum355]

Can this issue be closed as the PR was merged, or is this unresolved?

[gvanrossum]

Let's close it.