Unions with invalid arguments are still allowed

Open
#3,258 1 comment 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Reproduce the issue with the shown Union[123] example and trace how mypy validates Union arguments. Add a regression test showing that the invalid argument is rejected, then run the relevant type-checker tests and confirm valid Union arguments still pass.

Written by the indexing model from the issue text.

Description

bug topic-runtime-semantics 
from typing import Union
Union[123]

Doesn't give any errors, but fails at runtime:

Traceback (most recent call last):
  File "tst.py", line 2, in <module>
    Union[123]
  File "/media/ryan/stuff/anaconda/lib/python3.6/typing.py", line 626, in inner
    return func(*args, **kwds)
  File "/media/ryan/stuff/anaconda/lib/python3.6/typing.py", line 744, in __getitem__
    parameters = tuple(_type_check(p, msg) for p in parameters)
  File "/media/ryan/stuff/anaconda/lib/python3.6/typing.py", line 744, in <genexpr>
    parameters = tuple(_type_check(p, msg) for p in parameters)
  File "/media/ryan/stuff/anaconda/lib/python3.6/typing.py", line 356, in _type_check
    raise TypeError(msg + " Got %.100r." % (arg,))
TypeError: Union[arg, ...]: each arg must be a type. Got 123.
Dominant language
Python
Stars
20.6k
Forks
3.3k
Avg merge
1d 18h
Merged PRs (30d)
54

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from python/mypy

All issues in python/mypy

Similar issues

More Python issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.