Python >=3.5:在运行时检查类型注释 [英] Python >=3.5: Checking type annotation at runtime

问题描述

typing 模块(或任何其他模块)是否展示了一个 API 来在运行时对变量进行类型检查，类似于 isinstance() 但了解isinstance() 中定义的类型类代码>打字?

Does the typing module (or any other module) exhibit an API to typecheck a variable at runtime, similar to isinstance() but understanding the type classes defined in typing?

我想运行类似于:

from typing import List
assert isinstance([1, 'bob'], List[int]), 'Wrong type'

推荐答案

typing 模块中没有这样的功能，而且很可能永远不会有.

There is no such function in the typing module, and most likely there won't ever be.

检查一个对象是否是一个类的实例——这仅仅意味着这个对象是由类的构造函数创建的"——是测试一些标记的简单问题.

Checking whether an object is an instance of a class - which only means "this object was created by the class' constructor" - is a simple matter of testing some tagging.

然而，检查一个对象是否是一个类型的实例"并不一定是可判定的:

However, checking whether an object is an "instance" of a type is not necessarily decidable:

assert isinstance(foo, Callable[[int], str]), 'Wrong type'

尽管检查 foo 的类型注释很容易(假设它不是 lambda)，但根据莱斯定理，检查它是否符合它们通常是不可判定的.

Although it is easy to inspect the typing annotations of foo (assuming it's not a lambda), checking whether it complies to them is generally undecidable, by Rice's theorem.

即使使用更简单的类型，例如 List[int]，测试也很容易变得效率太低，无法用于除最小的玩具示例之外的任何其他内容.

Even with simpler types, such as List[int] the test will easily become far too inefficient to be used for anything but the smallest toy examples.

xs = set(range(10000))
xs.add("a")
xs.pop()
assert isinstance(xs, Set[int]), 'Wrong type'

<小时>

允许类型检查器以相对有效的方式执行此操作的技巧是保守的:类型检查器试图证明 foo 总是返回 int.如果失败，它会拒绝该程序，即使该程序可能是有效的，即这个函数很可能会被拒绝，尽管它是完全安全的:

---

The trick that allows type checker to perform this operation in a relatively efficient way, is to be conservative: the type checker tries to prove that foo always return int. If it fails, it rejects the program, even though the program may be valid, i.e. this function is likely to be rejected, although it is perfectly safe:

def foo() -> int:
    if "a".startswith("a"):
        return 1
    return "x"

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