I know this has been documented somewhere on Stack Overflow, but I can’t for

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Asked: May 25, 20262026-05-25T06:01:10+00:00 2026-05-25T06:01:10+00:00

I know this has been documented somewhere on Stack Overflow, but I can’t for

0

I know this has been documented somewhere on Stack Overflow, but I can’t for the life of me find it… I’ll be happy to accept any relevant links.

I have the following code:

class A:
    def __init__(self, x=[]):
        x += [1]
        print id(x), x

print "Try with default:"
a = A()
b = A()
c = A()
d = A()

print "Try with custom:"
a = A([1])
b = A([2])
c = A([3])
d = A([4])

Which generates the following output:

Try with default:
4342272584 [1]
4342272584 [1, 1]
4342272584 [1, 1, 1]
4342272584 [1, 1, 1, 1]
Try with custom:
4342456688 [1, 1]
4342456688 [2, 1]
4342456688 [3, 1]
4342456688 [4, 1]

Why is it when using the default constructor value, the array grows on each subsequent construction?