在 troubleshooting a semi-related problem in the Python chat 时,我在 mypy 中发现了一些我不明白的行为。
from typing import Union, List, Dict
def f(x: Union[
Dict[str, float],
Dict[str, str],
Dict[str, int],
]):
pass
f({"a": 1}) #passes
f({"a": "b"}) #passes
f({"a": 1.0}) #passes
def g(x: Union[
Dict[str, float],
Dict[str, Union[str, int]],
]):
pass
g({"a": 1}) #fails
g({"a": "b"}) #fails
g({"a": 1.0}) #passes
def h(x: Dict[str, Union[float, str, int]]):
pass
h({"a": 1}) #passes
h({"a": "b"}) #passes
h({"a": 1.0}) #passes
当我在这个脚本上执行 mypy 时,它只提示中间函数 g
:C:\Users\Kevin\Desktop>mypy test.py
test.py:20: error: Argument 1 to "g" has incompatible type "Dict[str, int]"; expected "Union[Dict[str, float], Dict[str, Union[str, int]]]"
test.py:20: note: "Dict" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
test.py:20: note: Consider using "Mapping" instead, which is covariant in the value type
test.py:21: error: Argument 1 to "g" has incompatible type "Dict[str, str]"; expected "Union[Dict[str, float], Dict[str, Union[str, int]]]"
test.py:21: note: "Dict" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
test.py:21: note: Consider using "Mapping" instead, which is covariant in the value type
Found 2 errors in 1 file (checked 1 source file)
(正如注释所暗示的那样,用 Dict
替换 Mapping
可以消除错误,但为了这个问题,我必须使用 Dict。)这些错误令我感到惊讶。据我所知,每个函数的类型注释应该简化为同一组类型:一个字典,其键是字符串,值是浮点数/字符串/整数。那么为什么只有
g
有不兼容的类型呢? mypy 是否对两个联盟的存在感到困惑?
最佳答案
这是因为 Dict
是不变的。它应该是不变的,因为它是可变的。Dict[str, int]
不是 Dict[str, Union[str, int]]
的子类型(即使 int
是 Union[int, str]
的子类型)
如果你要做这样的事情怎么办:
d: Dict[str, Union[str, int]]
u: Dict[str, int]
d = u # Mypy error: Incompatible type
d["Key"] = "value"
Mypy 假设字典是同质的:它们永远只包含一种类型。与此相反,例如,
Tuples
旨在包含异构数据:允许每个项目具有不同的类型。如果您需要异构
Dict
,您可以使用 TypedDict
,但只需要一组固定的字符串键:from typing import List, TypedDict
Mytype = TypedDict('Mytype', {'x': str, 'a': List[str]})
s: Mytype = {"x": "y", "a": ["b"]}
s['a'].append('c')
笔记:Unless you are on Python 3.8 or newer (where
TypedDict
is available in standard library typing module) you need to install typing_extensions using pip to use TypedDict
关于python - 为什么 mypy 拒绝我的 "mixed union"类型声明?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/62900750/