Bug: list('abc') not inferred to be of type List[str]

[AbhijeetKrishnan]

list('abc') should be inferred to be of type List[str]. However, we still get errors if used as the argument to a function.

MRE

import collections
named_tuple = collections.namedtuple('Test', list('abc'))

Command used

mypy code.py

Version

mypy 0.720
Python 3.7.4

Actual output

error: List or tuple literal expected as the second argument to namedtuple().

Expected behaviour

I would expect it to succeed without error.

Reference: pandas-dev/pandas#29114 (comment)

[msullivan]

list('abc') does get inferred as being of type List[str]. The issue here is that it is not a list "literal" -- that is, something like ["a", "b", "c"]. mypy needs it to be a literal so that it can easily read off and determine the names of the fields.

It would probably be possible to handle this particular case, but it doesn't seem common enough to be worth doing.

[ilevkivskyi]

FYI, there is an idiomatic way of doing this, that is also supported by mypy:

from collections import namedtuple

C = namedtuple('C', 'a b c')
c = C(1, 2, 3)
c.a  # OK
c.x  # Error

[AbhijeetKrishnan]

Thank you! mypy's behaviour is confusing me though. The error when the second argument is list('abc') is error: List or tuple literal expected as the second argument to namedtuple(). 'a b c' is neither a list nor a tuple (I believe it is a str). Why does mypy not complain in the latter case?