python_default_arguments.py

# Example showing how a subtlety in the way Python handles default
# arguments can cause a bug.

class C(object):
    def __init__(self, list1=[], list2=None):
        # Wrong: if the list1 default argument of [] is used, that
        # default will be *shared* among all instances of the class.
        # If it is later modified, other instances can unexpectedly
        # inherit the modifications.
        self.list1 = list1

        # With list2 we do things the right way
        if list2 is None:
            list2 = []
        self.list2 = list2

c1 = C()
c1.list1.append('foo')
c1.list1.append('bar')

c1.list2.append('peachy')
c1.list2.append('keen')

c2 = C()

print c2.list1  # prints ['foo', 'bar']
print c2.list2  # prints []

c2.list1.append('baz')

print c1.list1  # prints ['foo', 'bar', 'baz']