为什么在访问Python对象属性时使用getattr()而不是__dict__? [英] Why use getattr() instead of __dict__ when accessing Python object attributes?

问题描述

在源示例和具有某种程度的Python对象自省的SO答案中，常见的模式是:

In source examples and SO answers that have some degree of Python object introspection, a common pattern is:

getattr(some_object, attribute_name_string)

是否有理由偏爱此模式?

Is there a reason why this pattern is preferred to:

some_object.__dict__[attribute_name_string]

哪个似乎更直接地显示出了什么情况?是因为后者太接近CPython中可能会更改的特定实现了吗?

which seems to be more directly showing what's going on? Is it because the latter is too close to a particular implementation in CPython that may be subject to change?

注意事项:原始问题错误地将流行用语标识为:

NB Original question incorrectly identified the popular idiom as:

some_object.__getattr__(attribute_name_string)

推荐答案

some_object.__getattr__ 不是是常见的模式. getattr(some_object, attribute_name_string)是动态访问属性的正确方法.

some_object.__getattr__ is not a common pattern. getattr(some_object, attribute_name_string) is the proper way of accessing attributes dynamically.

并非所有实例具有一个__dict__属性；例如，使用__slots__的类将没有该属性.接下来，不一定在实例上 找到属性，而是类属性. getattr()会找到那些，查看__dict__不会找到它们.该类的属性也可能取决于描述符协议.

Not all instances have a __dict__ attribute; a class that uses __slots__ for example won't have that attribute. Next, attributes are not necessarily found on the instance, but are class attributes instead. getattr() will find those, looking at __dict__ will not find them. Attributes on the class may also depend on the descriptor protocol.

可能有直接访问__getattr__挂钩或__dict__属性的用途，但这些只是专门用途.例如，仅当未在其他地方首次找到属性时才使用__getattr__(因此，对于在类或instance.__dict__中出现的不属性).

There may be uses for direct access to the __getattr__ hook or the __dict__ attribute, but those are specialised uses only. __getattr__ is only used if the attribute was not first found elsewhere, for example (so for attributes not present on the class or in instance.__dict__).

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