Potential bug with dataclass, default_factory and Tuple[str]

[chaoflow]

With mypy version: 0.710, Python 3.7.3, inspired by https://docs.python.org/3/library/dataclasses.html#dataclasses.field

from typing import List, Tuple
from dataclasses import dataclass, field

@dataclass
class Data:
    foo: List[str] = field(default_factory=list)
    bar: Tuple[str] = field(default_factory=tuple)

results in:

$ mypy mwe.py
mwe.py:7: error: Incompatible types in assignment (expression has type "Tuple[_T_co, ...]", variable has type "Tuple[str]")

[JelleZijlstra]

This seems correct to me. Recall that Tuple[str] means specifically a one-element tuple, but if you construct a Data() object without arguments, Data.bar will be a zero-element tuple, which is a different type. You probably want to use bar: Tuple[str, ...].

[chaoflow]

@JelleZijlstra Thank you very much for pointing that out!