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When is it necessary to define __getattr__() and __setattr()__ methods for a class?

The official Python Programming FAQ advises the programmer to define the methods object.__getattr__(self, name) and object.__setattr__(self, name, value) under certain circumstances when implementing the OOP concept of delegation. However, I can't find good general advice on when defining these two methods is necessary, customary, or useful.

The Python Programming FAQ's example is the following code

class UpperOut:

    def __init__(self, outfile):
        self._outfile = outfile

    def write(self, s):
        self._outfile.write(s.upper())

    def __getattr__(self, name):
        return getattr(self._outfile, name)

"to implement[] a class that behaves like a file but converts all written data to uppercase". The accompanying text also contains a vague note telling the programmer to define __setattr__ in a careful way whenever attributes need to be set/modified (as opposed to just being read).

One question about this code is why their example doesn't use inheritance, which would presumably take care of what __getattr__ does. However, as one commenter kindly pointed out, file doesn't exist as a distinct type in Python 3, which may point us to an answer: Maybe they wanted to illustrate pure delegation (that is: delegation without inheritance) as one use case of overwriting __getattr__ and __setattr__. (If one uses plain inheritance, attributes are inherited by default and hence don't need to be accessed explicitly via calls to __getattr__ or __setattr__.)

For reference, a definition of "delegation" from Wikipedia is the following:

In object-oriented programming, delegation refers to evaluating a member [...] of one object [...] in the context of another [...] object [...]. Delegation can be done explicitly [...]; or implicitly, by the member lookup rules of the language [...]. Implicit delegation is the fundamental method for behavior reuse in prototype-based programming, corresponding to inheritance in class-based programming.

Even though the two concepts are often discussed together, delegation is not the same as inheritance:

The term "inheritance" is loosely used for both class-based and prototype-based programming, but in narrow use the term is reserved for class-based programming (one class inherits from another), with the corresponding technique in prototype-based programming being instead called delegation (one object delegates to another).

Note that delegation is uncommon in Python.

Note that this question on this site is about use cases of vanilla getattr(object, name) and setattr(object, name, value) and is not directly related.



source https://stackoverflow.com/questions/75531642/when-is-it-necessary-to-define-getattr-and-setattr-methods-for-a-cla

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