CODEWITHSUNDEEP
pythongenericsmypy
Joe Lapp
Joe Lapp

Reputation: 2965

Generic superclass "not defined" despite mypy being happy

I've been struggling with strange situations where mypy is happy, but python crashes when I run the code. Here's an issue I'm having with generics. I have two files in the same directory. When I run python3 in sample_parser.py, I get the following error:

Traceback (most recent call last):
  File "/Users/joe/repos/ut-norm-james/so_generics/sample_parser.py", line 8, in <module>
    class SampleParser(LookaheadParser[str]):
NameError: name 'LookaheadParser' is not defined

Here's lookahead_parser.py:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import TypeVar, Generic, Optional, Callable


_T = TypeVar("_T")
StateFunc = Callable[[_T], Callable]


class LookaheadParser(Generic[_T]):
    def __init__(self):
        self._token_index: int = 0
        self._tokens: list[_T] = []
        self._state: StateFunc

    def _parse(self) -> None:
        while self._token_index < len(self._tokens):
            self._state = self._state(self._tokens[self._token_index])
            self._token_index += 1

    def _lookahead(self, offset: int) -> Optional[_T]:
        prospective_index = self._token_index + offset
        if prospective_index < len(self._tokens):
            return self._tokens[prospective_index]
        return None

    def _advance(self, offset: int) -> None:
        self._token_index += offset

Here's sample_parser.py:

from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from .lookahead_parser import LookaheadParser, StateFunc


class SampleParser(LookaheadParser[str]):
    def __init__(self, raw_text: str):
        self._raw_text = raw_text
        self._state = self._state_first

    def parse(self) -> None:
        self._parse()

    def _state_first(self, token: str) -> StateFunc:
        pass  # stub


if __name__ == "__main__":
    pass  # stub

I'm running Python 3.9.6, but it was also happening in 3.9.5. The problem occurs whether I use _T or T, in case you're looking askance at that. mypy itself is not reporting any problems.

Upvotes: 0

Views: 573

Answers (1)

Silvio Mayolo
Silvio Mayolo

Reputation: 70267

You only import lookahead_parser if TYPE_CHECKING is true. That variable is false at runtime, so the module never gets loaded. Just remove the if TYPE_CHECKING: part and do the import unconditionally.

from .lookahead_parser import LookaheadParser, StateFunc

Upvotes: 2

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