Mypy：我应该如何输入一个以字符串为键且值可以是字符串或字符串列表的字典？

Question

I am using Python 3.8.1 and mypy 0.782. I don't understand why mypy complains about the following code:

from typing import Union, List, Dict
Mytype = Union[Dict[str, str], Dict[str, List[str]]]
s: Mytype = {"x": "y", "a": ["b"]}

Mypy gives the following error on line 3:

Incompatible types in assignment (expression has type "Dict[str, Sequence[str]]", variable has type "Union[Dict[str, str], Dict[str, List[str]]]")

If I change the last line to s: Mytype = {"a": ["b"]} mypy doesn't complain. However, when adding yet one more line s["a"].append("c") leads to an error:

error: Item "str" of "Union[str, List[str]]" has no attribute "append"

How can the above mentioned be explained? How should I type a dict that has strings as keys and the values can be either strings or lists of strings?

Found this: https://github.com/python/mypy/issues/2984#issuecomment-285716826 but still not completely sure why the above mentioned happens and how should I fix it.

EDIT: Although it's still not clear why the suggested modification Mytype = Dict[str, Union[str, List[str]]] does not resolve the error with s['a'].append('c') I think the TypeDict approach suggested in the comments as well as in https://stackoverflow.com/a/62862029/692695 is the way to go, so marking that approach as a solution.

See similar question at: Indicating multiple value in a Dict[] for type hints , suggested in the comments by Georgy.

Answer 1

Because s: Mytype cannot have type Dict[str, str] and type Dict[str, List[str]] at the same time. You could do what you want like this:

Mytype = Dict[str, Union[str, List[str]]]

But maybe problems, because Dict is invariant

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Also you could useTypedDict , but only a fixed set of string keys is expected:

from typing import List, TypedDict

MyType = TypedDict('MyType', {'x': str, 'a': List[str]})
s: MyType = {"x": "y", "a": ["b"]}

s['a'].append('c')

NOTE:

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

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And, of course, you can use Any :

Mytype = Dict[str, Any]