Clean way to disable `__setattr__` until after initialization

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I've written the following wrapper class. I want to define __setattr__ such that it redirects all attributes to the wrapped class. However, this prevents me from initializing the wrapper class. Any elegant way to fix this?

class Wrapper:
    def __init__(self, value):
        # How to use the default '__setattr__' inside '__init__'?
        self.value = value

    def __setattr__(self, name, value):
        setattr(self.value, name, value)

You are catching all assignments, which prevents the constructor from assigning self.value. You can use self.__dict__ to access the instance dictionary. Try:

class Wrapper:
    def __init__(self, value):
        self.__dict__['value'] = value

    def __setattr__(self, name, value):
        setattr(self.value, name, value)

Another way using object.__setattr__:

class Wrapper(object):
    def __init__(self, value):
        object.__setattr__(self, 'value', value)

    def __setattr__(self, name, value):
        setattr(self.value, name, value)

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A way to disable the __setattr__ until after initialization without changing the self.value = value syntax in the __init__ method is covered here. In short, embed knowledge of initialization in the object and use it in the __setattr__ method. For your Wrapper:

class Wrapper:
    __initialized = False
    def __init__(self, value):
        self.value = value
        self.__initialized = True

    def __setattr__(self, name, value):
        if self.__initialized:
            # your __setattr__ implementation here
        else:
            object.__setattr__(self, name, value)

Clean way to disable `__setattr__` until after initialization, 本文由网友转载,转载信息如下. 标题:Clean way to disable `__setattr__` until after initialization. 作者:Paul Manta. 发表时间:2012-10-21 14:52:41. 9 Clean way to disable `__setattr__` until after initialization; 8 What capacitors are available in chip design? 5 How does additive noise change the SVD;

With __getattr__ overridden as well::

class Wrapper:
    def __init__(self,wrapped):
        self.__dict__['wrapped'] = wrapped
    def __setattr__(self,name,value):
        setattr(self.__dict__['wrapped'],name,value)
    def __getattr__(self,name):
        return getattr(self.__dict__['wrapped'],name)


class A:
    def __init__(self,a):
        self.a = a

wa = Wrapper(A(3))
#wa.a == wa.wrapped.a == 3

清除在初始化之前禁用“__setattr__”的方法- Clean way to disable `__ , [英]Clean way to disable `__setattr__` until after initialization class Wrapper: def __init__(self, value): # How to use the default '__setattr__' inside '__init__'? 9 Clean way to disable `__setattr__` until after initialization; 8 What capacitors are available in chip design? 5 How does additive noise change the SVD;

As suggested in other answers, one idea is to directly access the object dictionary to bypass setattr resolution.

For something easy to read, I suggest the following:

  def __init__(self,wrapped1, wrapped2):
       vars(self).update(dict(
          _wrapped1=wrapped1,
          _wrapped2=wrapped2,
        ))

Using vars is optional, but I find it nicer than directly accessing self.__dict__, and the inline dict() notation allows for grouping all instance variable initialization in a visible block with minimum boilerplate code overhead.

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Python setattr() Function, Python setattr() function is used to set the attribute of an object, given its name. While being in Python. Let us look at how we could use this function in our Python programs. def __init__( self , name, value): After constructing the Student object using setattr() , we use getattr() and sort the subject-wise marks of students. I don’t think it is entirely possible except by using either a tuple or a namedtuple. No matter what, if you override __setattr__() the user can always bypass it by calling object.__setattr__() directly. Any solution that depends on __setattr__ is guaranteed not to work. The following is about the nearest you can get without using some sort

Cannot assign to field of Callable type � Issue #708 � python/mypy , We should really teach mypy about all this (after all it now does know about descriptors!) That's not a mypy problem per se -- you can't initialize a class variable to a __name__) def __set__(self, oself, value): setattr(oself, "_" + self. Mypy misinterprets callable attribute as method when annotated in abstract base class. There are many ways to do so, depending on what variables you are protecting (class variables, instant attributes, local variables, etc.). However the most Pythonic way is the simplest.

初期化後まで '__setattr__'を無効にするクリーンな方法, Clean way to disable `__setattr__` until after initialization class Wrapper: def __ init__(self, value): # How to use the default '__setattr__'� Initialization¶. In Python, instance initialization happens in the __init__ method. Generally speaking, you should keep as little logic as possible in it, and you should think about what the class needs and not how it is going to be instantiated.

Comments
  • What does Wrapper wrap? Why doesn't it inherit object?
  • @Tichodroma It wraps anything. I'm using it in a GUI application; the wrapper notifies listeners when the enclosed object is modified. It doesn't inherit from object because I only want to use Python 3.
  • Nice. One question though: why should I use self.__dict__['value'] inside __setattr__ as well?
  • @Paul Sorry, you don't need that. I thought you were overriding __getattr__ as well.