python instance variable initialization

Question

I'm confused with below situation for python instance variable initialization, multiple instances (i1 and i2) get assigned with same "attr" object if I don't specify value for attr in constructor. Got different objects if I explicitly specify values in constructor.

sample code:

class MyClass:
    def __init__(self, x=1, attr=[]):
        print id(attr)
        self.attr = attr
        self.x = x

if __name__ == '__main__':
   
    i1 = MyClass()
    i2 = MyClass()
    i3 = MyClass(attr=[1,2])
    i4 = MyClass(attr=[1,2])

    print "i1:", vars(i1), id(i1.attr)
    print "i2:", vars(i2), id(i2.attr)
    print "i3:", vars(i3), id(i3.attr)
    print "i4:", vars(i4), id(i4.attr)

output:

38648928
38648928
38664672
38688648
i1: {'x': 1, 'attr': []} 38648928
i2: {'x': 1, 'attr': []} 38648928
i3: {'x': 1, 'attr': [1, 2]} 38664672
i4: {'x': 1, 'attr': [1, 2]} 38688648

Why i1 and i2 has the same 'attr' object?

For the record, even though this is a duplicate, it's a well-written question. It's clear, has code, shows example output, demonstrates you figured out the crucial difference on your own, etc. — DSM, Nov 02 '13 at 22:31
There should be like a Python FAQ with a link to that SO question. — Shashank, Nov 02 '13 at 22:35
Thank you all for the help, now I understand after reading the existing post, but this was a surprise to me, Python is supposed to be "Least Astonishment" :) — user2948769, Nov 03 '13 at 03:00

score 3 · Accepted Answer · answered Nov 02 '13 at 22:30

You create a default list attr=[]. The list created only 1 time. Every time you access the method without seeting attr, it uses the default list attr=[] he created earlier. In the second example, You constantly creating a new list every call of MyClass(attr=[1,2])

python instance variable initialization

sample code:

output:

1 Answers1