incorrect inferred type accessing typed dict with static keys
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Description
Bug Report
mypy does not infer the correct type of a dictionary when constructing a subset of a TypedDict using a constant subset of keys.
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=82fc1eb24628e1ba2aed8294b81e2912
from typing import TypedDict, reveal_type
class MyDict(TypedDict):
key_0: bool
key_1: int
key_2: bool
limits: MyDict = {
'key_0': True,
'key_1': 1,
'key_2': False
}
def test_func() -> dict[str, bool]:
result = {
label: limits.get(label, False)
for label in ('key_0', 'key_2')
}
reveal_type(limits.get('key_0', False))
reveal_type(result)
return result
test_func()
Expected Behavior
In the example above, I expect mypy to accept the return type since both key_0 and key_2 are annotated as bool
Actual Behavior
mypy complains about the return type with
Incompatible return value type (got "dict[str, object]", expected "dict[str, bool]") [return-value]
After mentioning it in my team chat @LefterisJP noticed that this version works https://mypy-play.net/?mypy=latest&python=3.12&gist=63bece9a183493b170f6f55527fbd7d3
from typing import TypedDict, reveal_type, Literal
class MyDict(TypedDict):
key_0: bool
key_1: int
key_2: bool
limits: MyDict = {
'key_0': True,
'key_1': 1,
'key_2': False
}
def test_func() -> dict[str, bool]:
keys: tuple[Literal['key_0', 'key_2'], ...] = ('key_0', 'key_2')
result: dict[str, bool] = {
label: limits.get(label, False)
for label in keys
}
reveal_type(limits.get('key_0', False))
reveal_type(result)
return result
test_func()
In this case, because the keys are explicitly typed as Literals, mypy infers the correct type.
This suggests that mypy is not analyzing constant tuples deeply enough to recognize the accessed keys.
Your Environment
- Mypy version used: mypy 1.18.2 (compiled: yes)
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): I can reproduce it in an environment without any extra configuration. - Python version used: 3.11.13
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the reported behavior in the linked mypy-play examples and compare the inferred types for the plain constant tuple and the explicitly Literal-typed tuple. Done means mypy accepts the shown dict[str, bool] return type for the constant-key case without requiring the explicit Literal annotation.
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
- Mostly clear
- Newbie friendliness
- 45/100