Suppose you have something like this:

class intlist:
        def __init__(self,l = []):
                self.l = l
        def add(self,a):
                self.l.append(a)

def appender(a):
        obj = intlist()
        obj.add(a)
        print obj.l

if __name__ == "__main__":
        for i in range(5):
                appender(i)

A function creates an instance of intlist and calls on this fresh instance the method append on the instance attribute l.

How comes the output of this code is:

[0]

[0, 1]

[0, 1, 2]

[0, 1, 2, 3]

[0, 1, 2, 3, 4]

? If i switch

obj = intlist()

with

obj = intlist(l=[])

I get the desired output

[0]

[1]

[2]

[3]

[4]

Why this happens?

Thanks