Python list append causes strange result

Question

I have really strange problem. Here is the sample code:

class SomeClass(object):
    a = []
    b = []
    def __init__(self, *args, **kwargs):
        self.a =          


        

Answer 1

You are assigning the same list to self.b, not a copy.

If you wanted self.b to refer to a copy of the list, create one using either list() or a full slice:

self.b = self.a[:]

or

self.b = list(self.a)

You can test this easily from the interactive interpreter:

>>> a = b = []  # two references to the same list
>>> a
[]
>>> a is b
True
>>> a.append(42)
>>> b
[42]
>>> b = a[:]  # create a copy
>>> a.append(3.14)
>>> a
[42, 3.14]
>>> b
[42]
>>> a is b
False