python / python/mypy

TypedDict totality ignored when unpacked for function call

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#12,754 2 comments 0 reactions 0 assignees View on GitHub

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bug topic-typed-dict
Dominant language
Python
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Description

Bug Report

When unpacking a typeddict for a function call, mypy doesn't seem to check the totality of the typeddict, and instead assumes the keys are all present.

from typing import TypedDict

def my_function(foo: int, bar: int, baz: str) -> None:
    pass

class MyTypedDict(TypedDict, total=False):
    foo: int
    bar: int
    baz: str
    
    
a: MyTypedDict = {'bar': 1}

my_function(**a) # Should be error here, as a doesn't contain foo or baz

https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=5444e1100000a8f1678fe9cbcfc23921

Expected Behavior

Mypy should not assume that keys are present when the TypedDict has total=False, and treat it as potentially having any combination of they keys present.

Actual Behavior

Mypy allows this, but when executed it gives the following error:

Traceback (most recent call last):
  File "<path>\typeddict1.py", line 17, in <module>
    my_function(**a)  # Should be error here, as a doesn't contain foo or baz
TypeError: my_function() missing 2 required positional arguments: 'foo' and 'baz'

#11753

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.

Research direction

Start by reproducing the TypedDict total=False unpacking example from the issue or its mypy-play link, then trace the function-call argument checking for TypedDict unpacking. Done means mypy reports an error when optional keys may be missing, while preserving valid behavior for required keys.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
48/100

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