返回生成器表达式时Mypy报类型不兼容错误，是否为Bug？

Great question! This isn't a Mypy bug—it's actually a deliberate distinction in how Mypy types generator expressions versus explicit generator functions. Let's break it down:

The Core Issue #

Your function returns a generator expression ((element for element in some_list)), and while at runtime this is indeed a generator object (which implements the Generator interface), Mypy infers its type as Iterator[Any] rather than Generator[Any, None, None].

Mypy draws this line because:

• A Generator[T, S, R] type implies support for methods like send(), throw(), and close()—even if you set the send type (S) to None (meaning you shouldn't send values to it).
• Generator expressions, however, don't actually handle sent values (calling send() with a non-None value on one will throw a TypeError at runtime). Mypy errs on the side of type safety here by treating them as the more general Iterator[T], which only requires __next__() and __iter__().

Since Generator is a subtype of Iterator, you can't assign an Iterator to a variable/return type expecting a Generator—that would violate the Liskov substitution principle (the parent type doesn't have all the methods of the child type).

Fixes You Can Use #

You have a few solid options here:

1. Use Iterator[Any] as the Return Type #

This is the most accurate annotation for a generator expression, since it matches Mypy's inference and reflects the actual safe operations you can perform on the result:

from typing import List, Any, Iterator

def foo(some_list: List[Any]) -> Iterator[Any]:
    return (element for element in some_list)

2. Rewrite to an Explicit Generator Function #

If you specifically need to annotate as Generator[Any, None, None], replace the generator expression with a function using yield—Mypy will correctly infer this as a Generator type:

from typing import List, Any, Generator

def foo(some_list: List[Any]) -> Generator[Any, None, None]:
    for element in some_list:
        yield element

3. (Not Recommended) Use a Type Ignore #

If you really want to keep the generator expression and Generator annotation, you can suppress the error with a comment—but this bypasses Mypy's type safety checks, so it's not ideal:

from typing import List, Any, Generator

def foo(some_list: List[Any]) -> Generator[Any, None, None]:
    return (element for element in some_list)  # type: ignore

Why the Official Docs Mention Generator #

The typing.Generator docs reference it as a valid return type for generator functions (those using yield), which is different from generator expressions. The runtime overlap (both produce generator objects) doesn't translate perfectly to static typing in Mypy.

内容的提问来源于stack exchange，提问作者gmolau