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"Expected TypedDict key to be string literal" false positive? #4122
Description
Shouldn't something like this work?
from mypy_extensions import TypedDict
Details = TypedDict('Details', {'first_name': str, 'last_name': str})
DetailsSubset = TypedDict('DetailsSubset', {'first_name': str, 'last_name': str}, total=False)
defaults = {'first_name': 'John', 'last_name': 'Luther'} # type: Details
def generate(data: DetailsSubset) -> Details:
return {**defaults, **data}I'm getting:
% mypy code.py
code.py: error: Expected TypedDict key to be string literal
Oh, and the error message is missing the line number, that could be handy to have.
I'm using mypy 0.530 and Python 3.6.2.
Activity
gvanrossum commentedon Oct 16, 2017
Hm, the missing line number is a bug. The error is clearly about the final line of your example.
Unfortunately mypy isn't smart enough to understand that in this particular case the result will have exactly the desired set of keys. It basically gives up as soon as it sees
{**expr}. Maybe we could teach it to calculate the minimal and maximal sets of keys for such usage (if**exprreferences a TypedDict) and give better diagnostics based on that.I believe there are many other ways of constructing a TypedDict that won't be accepted; I think the best way is to just use a cast or put
# type: ignoreon it.ilevkivskyi commentedon Oct 16, 2017
The missing line number should be easy to fix, but the
{**expr}type detection might be harder.killthekitten commentedon Jul 9, 2020
This is still an issue in
0.782. Has anything changed since0.530that could make the type detection for this case feasible? I.e. the discussion in #4441 seems relevant.My current workaround is to use
cast. A bit better than# type: ignore.Support flexible TypedDict creation/update (#15425)