Python class 装饰器 "self" 似乎不对

Question

我正在尝试弄清楚如何使用 class 上的装饰器更改 class 的 __setattr__ 的功能，但我运行遇到了问题当试图在替换 __setattr__ 的函数中访问 self 时。如果我将问题行更改为不访问 self，例如用 val = str(val) 替换它，我得到了预期的行为。

我在这里的其他问题中看到类似的问题，但他们使用不同的方法，其中 class 用作装饰器。我下面的方法感觉不那么复杂，所以如果可能的话，我很乐意保持这样。

为什么 a 没有在我期望的 self/foo 上定义？

# Define the function to be used as decorator
# The decorator function accepts the relevant fieldname as argument
# and returns the function that wraps the class
def field_proxied(field_name):

    # wrapped accepts the class (type) and modifies the functionality of
    # __setattr__ before returning the modified class (type)
    def wrapped(wrapped_class):
        super_setattr = wrapped_class.__setattr__

        # The new __setattr__ implementation makes sure that given an int,
        # the fieldname becomes a string of that int plus the int in the
        # `a` attribute
        def setattr(self, attrname, val):
            if attrname == field_name and isinstance(val, int):
                val = str(self.a + val)  # <-- Crash. No attribute `a`
                super_setattr(self, attrname, val)
        wrapped_class.__setattr__ = setattr
        return wrapped_class
    return wrapped

@field_proxied("b")
class Foo(object):
    def __init__(self):
        self.a = 2
        self.b = None

foo = Foo()
# <-- At this point, `foo` has no attribute `a`
foo.b = 4
assert foo.b == "6"  # Became a string

Answer 1

问题很简单，改一行就可以了

def setattr(self, attrname, val):
    if attrname == field_name and isinstance(val, int):
        val = str(self.a + val)
    super_setattr(self, attrname, val)  # changed line

原因是，在你原来的方法中，你只会在attrname == field_name时调用super_setattr。所以 __init__ 中的 self.a = 2 根本不起作用，因为 "a" != "b".

Python class 装饰器 "self" 似乎不对

Python class decorator "self" seems wrong

python

python-decorators

class-decorator