Doesgetattr allow modifying passed value?

Question

This is my code:

class myclass:
    def __init__(self):
        self.value = 5

    def __getattr__(self, attrname):
        if attrname == 'value':
            return 10

X = myclass()
print X.value

I suppose the output value should be 10 (__getattr__ modifies returned value). However output is 5.

If I remove initialization of value:

class myclass:

    def __getattr__(self, attrname):
        if attrname == 'value':
            return 10

X = myclass()
print X.value

It works as expected (returns 10). Why?

Martijn Pieters · Accepted Answer · 2017-07-31 09:11:29Z

2

__getattr__ is only used for missing attributes. See the documentation:

Called when an attribute lookup has not found the attribute in the usual places (i.e. it is not an instance attribute nor is it found in the class tree for self).

In your first example, 'value' is not a missing attribute; it was set in __init__. Note that it doesn't matter where it was set, only that it exists.

If you must intercept all attribute access, you need to implement the __getattribute__ hook instead.

answered Jul 31, 2017 at 9:11

Martijn Pieters

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5 Comments

Hash Doe Over a year ago

But setattr works for missing and not-missing attributes, right?

Martijn Pieters Over a year ago

@HashDoe: yes, because setting an attribute would add that attribute if it was missing.

GIZ Over a year ago

@HashDoe Also in 3.X for builtin accesses only __getattr__ and __getattribute__ are not intercepted this matters for proxy classes when they proxy other objects and expect their operator overloading methods to be called.

Martijn Pieters Over a year ago

@direprobs: you mean special methods perhaps? Yes, __getattribute__ will be bypassed for some of those, but that's not specific to Python 3, that's the same in Python 2. See Special method lookup.

GIZ Over a year ago

@MartijnPieters Yes, that's what I meant. Yup, you're right it's not just in 3.X but for new-style classes in both lines 2.X & 3.X.

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