Type refinements are not applied to index expressions

Ran into this issue today, I believe mypy is incorrectly flagging this as an error:

from typing import Any, Dict, Optional


def rename_keys(
    original_dict: Dict[str, Any], key_mapping: Dict[str, Optional[str]]
) -> Dict[str, Any]:
    """Renames keys in the dictionary using the key_mapping. If the value in key_mapping is None then it will remove the key entirely"""
    return {
        key_mapping.get(k, k): v
        for k, v in original_dict.items()
        if k not in key_mapping or key_mapping[k] is not None
    }

output from mypy:

/tmp/mypytest.py:8: error: Key expression in dictionary comprehension has incompatible type "Optional[str]"; expected type "str"

if key_mapping.get(k) is None then it would fail the if-statement at the end of the dictionary comprehension and therefore get filtered out

if k is not in key_mapping then k is the result of the expression key_mapping.get(k,k). k can't be None because k comes from the keys of original_dict which has the type Dict[str, Any]

Versions: mypy 0.701, Python 3.7.4

no flags

示されている rename_keys の辞書内包表記を mypy 0.701 と Python 3.7.4 で使用して、報告を再現してください。if 条件とインデックスによる参照に対して型の絞り込みがどのように処理されるかを追跡し、その後、None のケースを通過させずに、互換性のない Optional[str] の診断が解消されることを確認してください。