Python Class Initialization and Memory Model Behavior

Question

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?

The words you're looking for are "Mutable Default Argument". Google it, or read, for example, ["Least Astonishment" in Python: The Mutable Default Argument](http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument). — agf, Sep 01 '11 at 13:15
Perfect! Those were the words and link I needed. Thanks @agf! — Dan, Sep 01 '11 at 13:29

score 0 · Accepted Answer · edited May 23 '17 at 12:29

0

@agf came up with the link that answers the question: "Least Astonishment" and the Mutable Default Argument

The accepted response for that link points to the following useful explanation: http://effbot.org/zone/default-values.htm

edited May 23 '17 at 12:29

Community

1
1

answered Sep 01 '11 at 13:35

Dan

1,258
1
10
22

Python Class Initialization and Memory Model Behavior

1 Answers1