在Python 3.6中运行时根据Union类型检查变量 [英] Check a variable against Union type at runtime in Python 3.6

问题描述

我正在尝试编写一个使用Python 3.6类型提示的函数装饰器，以检查参数字典是否尊重类型提示，并且如果未引发错误并带有对问题的清晰描述，则将其用于HTTP API.

I'm trying to write a function decorator that uses Python 3.6 type hints to check that a dictionary of arguments respects the type hints and if not raise an error with a clear description of the problem, to be used for HTTP APIs.

问题在于，当函数具有使用Union类型的参数时，我无法在运行时针对该变量检查变量.

The problem is that when the function has a parameter using the Union type I can't check a variable against it at runtime.

例如，我有此功能

from typing import Union
def bark(myname: str, descr: Union[int, str], mynum: int = 3) -> str:
    return descr + myname * mynum

我可以做到:

isinstance('Arnold', bark.__annotations__['myname'])

但不是:

isinstance(3, bark.__annotations__['descr'])

因为Union不能与isinstance或issubclass一起使用.

Because Union cannot be used with isinstance or issubclass.

我找不到使用类型对象检查它的方法. 我尝试自己执行检查，但是在REPL中bark.__annotations__['descr']显示为typing.Union[int, str]时，如果不使用检查bark.__annotations__['descr'].__repr__()的丑陋技巧，我将无法在运行时访问类型列表.

I couldn't find a way to check it using the type object. I tried to implement the check by myself but while bark.__annotations__['descr'] is shown as typing.Union[int, str] in the REPL I can't access the list of the types at runtime, if not using the ugly hack of examining bark.__annotations__['descr'].__repr__().

是否有访问此信息的正确方法?还是故意在运行时不容易访问它?

Is there a proper way to access this information? Or is it deliberately intended to not be easily accessible at runtime?

推荐答案

您可以使用Union的__args__属性，该属性包含tuple可能的内容:

You could use the __args__ attribute of Union which holds a tuple of the "possible contents:

>>> from typing import Union

>>> x = Union[int, str]
>>> x.__args__
(int, str)
>>> isinstance(3, x.__args__)
True
>>> isinstance('a', x.__args__)
True

未记录__args__参数，因此可以将其视为与实现详细信息打交道"，但它似乎是比解析repr更好的方法.

The __args__ argument is not documented so it could be considered "messing with implementation details" but it seems like a better way than parsing the repr.

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