如何使用静态检查来确保对象具有特定的方法/属性? [英] How can I use static checking to ensure an object has a certain method/attribute?

问题描述

有没有办法可以注释函数以确保传递给函数的对象具有特定的方法或属性，但我不关心它的实际类型?

Is there I way that I can annotate a function to ensure that an object being passed into the function has a certain method or attribute, but where I don't care about its actual type?

Pycharm 在内部使用类似于 {b} 的语法来指示它推断出的对象需要哪些方法/属性，但这似乎不是有效的 Python 语法:

Pycharm internally uses a syntax that looks like {b} to indicate what methods/attributes it's inferred are required for the object, but that doesn't appear to be valid Python syntax:

def func(a: {b}):  # Error
    a.b = 1

有没有办法让类型检查器帮助鸭子打字，我只关心对象有什么方法/属性，而不关心对象的类型是什么，我不能修改我想要的类型检查?

Is there a way to get the type checker to assist in duck typing, where I only care what methods/attributes the object has, not what the type of the objects are, and where I can't modify the types I want to check for?

推荐答案

Protocols 可以使用.我在这里记录它是因为我发现这是一个很难搜索的话题；特别是检查属性是否存在.

Protocols can be used. I'm documenting it here because I found this to be a difficult topic to search for; especially checking for the existence of attributes.

为了确保属性的存在:

from typing import Protocol

class HasFoo(Protocol):     # Define what's required
    foo: int

class Foo:                  # This class fulfills the protocol implicitly
    def __init__(self):
        self.foo = 1

class Bar:
    def __init__(self):     # This class fails to implicitly fulfill the protocol
        self.bar = 2

def foo_func(f: HasFoo):
    pass

foo_func(Foo())             # Type check succeeds
foo_func(Bar())             # Type check fails

注意foo 后面的类型提示.该行必须在语法上有效，并且类型必须与已检查属性的推断类型相匹配.typing.Any 可以用作占位符，如果您关心 foo 的存在，而不是它的类型.

Note the type hint after foo. It's required for that line to be syntactically valid, and the type must match the inferred type of the checked attributes. typing.Any can be used as a placeholder if you care about the existence of foo, but not its type.

同样，检查方法也可以这样做:

Similarly, the same can be done for checking methods:

class HasFoo(Protocol):
    def foo(self):
        pass

class Foo:
    def foo(self):
        pass
    
class Bar:
    def bar(self):
        pass

def func(f: HasFoo):
    pass

func(Foo())             # Succeeds
func(Bar())             # Fails

---

类型检查是通过Pycharm 2020.2.2完成的.

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