__getattr__ 在模块上 [英] __getattr__ on a module

问题描述

如何在类、模块上实现与 __getattr__ 等效的代码?

How can implement the equivalent of a __getattr__ on a class, on a module?

当调用模块的静态定义属性中不存在的函数时，我希望在该模块中创建一个类的实例，并使用与模块的属性查找失败相同的名称调用该类的方法.

When calling a function that does not exist in a module's statically defined attributes, I wish to create an instance of a class in that module, and invoke the method on it with the same name as failed in the attribute lookup on the module.

class A(object):
    def salutation(self, accusative):
        print "hello", accusative

# note this function is intentionally on the module, and not the class above
def __getattr__(mod, name):
    return getattr(A(), name)

if __name__ == "__main__":
    # i hope here to have my __getattr__ function above invoked, since
    # salutation does not exist in the current namespace
    salutation("world")

给出:

matt@stanley:~/Desktop$ python getattrmod.py 
Traceback (most recent call last):
  File "getattrmod.py", line 9, in <module>
    salutation("world")
NameError: name 'salutation' is not defined

推荐答案

不久前，Guido 声明所有特殊方法查找都在新式类绕过 __getattr__ 和 __getattribute__.Dunder 方法以前适用于模块 - 例如，在使用这些技巧之前，您可以简单地通过定义 __enter__ 和 __exit__ 将模块用作上下文管理器坏了.

A while ago, Guido declared that all special method lookups on new-style classes bypass __getattr__ and __getattribute__. Dunder methods had previously worked on modules - you could, for example, use a module as a context manager simply by defining __enter__ and __exit__, before those tricks broke.

最近一些历史特性卷土重来，其中包括 __getattr__ 模块，以及现有的 hack(一个模块用 sys.modules 中的类替换自身导入时间)应该不再需要.

Recently some historical features have made a comeback, the module __getattr__ among them, and so the existing hack (a module replacing itself with a class in sys.modules at import time) should be no longer necessary.

在 Python 3.7+ 中，您只需使用一种显而易见的方法.要自定义模块上的属性访问，请在模块级别定义一个 __getattr__ 函数，该函数应接受一个参数(属性名称)，并返回计算值或引发 AttributeError:

In Python 3.7+, you just use the one obvious way. To customize attribute access on a module, define a __getattr__ function at the module level which should accept one argument (name of attribute), and return the computed value or raise an AttributeError:

# my_module.py

def __getattr__(name: str) -> Any:
    ...

这也将允许挂钩到from"导入，即您可以为诸如 from my_module import nothing 之类的语句返回动态生成的对象.

This will also allow hooks into "from" imports, i.e. you can return dynamically generated objects for statements such as from my_module import whatever.

在相关说明中，除了模块 getattr，您还可以在模块级别定义一个 __dir__ 函数来响应 dir(my_module).有关详细信息，请参阅 PEP 562.

On a related note, along with the module getattr you may also define a __dir__ function at module level to respond to dir(my_module). See PEP 562 for details.

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