NamedTuple with dependent default values causes unexpected 'Name is not defined' errors

[sndrtj]

Hi there,

When there is a NamedTuple with a default value that depends on one or more other default values, followed by a default value that should resolve into a NameError, all preceding default values get the Name <variable> is not defined error as well.

E.g.

from typing import NamedTuple

class Example(NamedTuple):
    a: int = 1
    b: int = 2
    c: int = a*b
    d: int = does_not_exist*5

Causes:

scratch.py:6: error: Name 'a' is not defined
scratch.py:6: error: Name 'b' is not defined
scratch.py:7: error: Name 'does_not_exist' is not defined

If I comment out line 6, I do, however, get the expected result.

from typing import NamedTuple

class Example(NamedTuple):
    a: int = 1
    b: int = 2
    # c: int = a*b
    d: int = does_not_exist*5

scratch.py:7: error: Name 'does_not_exist' is not defined

This was tried on version 0.760.

$ mypy --version
mypy 0.760

[ilevkivskyi]

Named tuple class bodies are special-cased in mypy. In particular, the assignment r.h.s. are default values in constructors, not more than that. I think it may make sense to support this, since one would naturally expect this to work (and the error message is unclear).

A possible downside is that if we allow this, people might expect that such assignment will cause default value for c to be dynamically calculated, i.e., that Example(3, 5).c is 15, while in fact it is 2.