Question

Why does the following:

class A(object):
    def __init__(self, var=[]):
        self._var = var
        print 'var = %s %s' % (var, id(var))

a1 = A()
a1._var.append('one')

a2 = A()

result in:

var = [] 182897439952
var = ['one'] 182897439952

I don’t understand why it is not using a new instance of a list when using optional keyword arguments, can anyone explain this?

Answer 1

The empty list in your function definition is created once, at the time the function itself is created. It isn’t created every time the function is called.

If you want a new one each time, do this:

class A(object):
    def __init__(self, var=None):
        if var is None:
            var = []
        self._var = var