MyPy treats initialization of a variable as an assignment for variance checks for complex dictionary.

The problem is that when initializing a variable from a dictionary expression it treats the type as invariant, but in the case of initialialization, it is covariant.

Example:

CODES = {
    "a": "b",
    "c": "d",
    "e": "f",
}

# MyPy rejects this

PATTERNS: dict[str, tuple[str | None, str | None]] = {
    "1" + k: ((v + "1" if v else None), "x")
    for (k, v) in CODES.items()
} | {
    "2" + k: ((v + "2" if v else None), "x")
    for (k, v) in CODES.items()
}

# But is OK with this:

X: str | None = "x"

PATTERNS2: dict[str, tuple[str | None, str | None]] = {
    "1" + k: ((v + "1" if v else None), X)
    for (k, v) in CODES.items()
} | {
    "1" + k: ((v + "1" if v else None), X)
    for (k, v) in CODES.items()
}

Both patterns should be OK.

首先运行提供的 MyPy 示例，并比较 PATTERNS 和 PATTERNS2 的诊断结果。跟踪与带注解初始化和方差检查结合的字典表达式的类型检查路径。添加一个覆盖该示例的回归测试；完成的标准是两个模式都被接受，同时不削弱无关的方差检查。