Class inheritance and sticky attributes

Asked Aug 08 '17 at 23:09

Active Aug 09 '17 at 00:23

Viewed 17 times

Some python class inheritance is having some unanticipated consequences, namely "sticky" attributes that I would have expected to be reset.

A simplified scenario, that still demonstrates what's happening:

class Foo(object):

    def __init__(self, headers={}):
        self.headers = headers

class Bar(Foo):

    def __init__(self, headers={}):
        super().__init__(headers=headers)

```

If I instantiate an instance of Bar, I can see .headers are blank. Then edit bar.headers, and see they are modified. But then instantiate another instance of Bar, and unexpectedly the .headers is still modified:

In [39]: bar = Bar()

In [40]: bar.headers
Out[40]: {}

In [41]: bar.headers['baz'] = True

In [44]: goober = Bar()

In [45]: goober.headers
Out[45]: {'baz': True}

Is there a better pattern for this kind of inheritance? Otherwise, everything else is working nicely and as expected.

edited Aug 09 '17 at 00:23

martineau

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asked Aug 08 '17 at 23:09

ghukill

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2

This has **nothing to do with inheritance**, rather, you are using a mutable default argument. – juanpa.arrivillaga Aug 08 '17 at 23:12
1

In short, the `headers` default argument is always the same dictionary. – kindall Aug 08 '17 at 23:13
Duly noted, thanks for such a quick response. – ghukill Aug 08 '17 at 23:15

Class inheritance and sticky attributes

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