Why is getattrcatching also existing attributes?

Question

I created this class to test some of the __getattr__ features:

class Trace:
    def __init__(self, val):
        self.val = val

    def __setattr__(self, attr, value):
        print('set ' + attr)

    def __getattr__(self, attr):
        print('get ' + attr)

I then created an instance

a = Trace(10)
print(a.val)
a.val = 5
print(a.val)

But, even if I fetched only existing attributes, this was the output:

set val
get val
None
set val
get val
None

I'm using Python 3.7.

That’s what getattr and setattr do. What were you expecting, and why? — DisappointedByUnaccountableMod, Sep 05 '19 at 21:10
@barny well, I suppose since `self.val` was already defined you would [expect `__getattr__` to never be invoked](https://stackoverflow.com/questions/3278077/difference-between-getattr-vs-getattribute). — Salem, Sep 05 '19 at 21:12
Except `self.val` never *was* defined; `__setattr__` has to actually make the assignment. — chepner, Sep 05 '19 at 21:19

score 4 · Accepted Answer · answered Sep 05 '19 at 21:13

__getattr__ is only called when an attribute isn’t found normally, but __setattr__ has no such restriction (that’s why there’s no equivalent of __getattribute__ for it). You overrode __setattr__ to not actually set an instance attribute, so the attribute is always missing and __getattr__ is called anyway. (Yes, it applies in __init__ too.)

Remove __setattr__ to see your expected behaviour quickly, or add its default behaviour back:

def __setattr__(self, attr, value):
    print('set ' + attr)
    super().__setattr__(attr, value)

Thank you so much, this explanation was really useful! – PMM Sep 05 '19 at 21:18 — PMM, Sep 05 '19 at 21:18

Why is __getattr__catching also existing attributes?

1 Answers1

Why is getattrcatching also existing attributes?