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