The extendList function defaults myList to an empty list. However, myList is only created when the function is first defined. Therefore subsequent calls to extendList will mutate and return the original list.
def extendList(val, myList=[]):
myList.append(val)
return myList
list1 = extendList(10)
list2 = extendList(123,[])
list3 = extendList('a')
print "list1 = %s" % list1 # list1 = [10, 'a']
print "list2 = %s" % list2 # list2 = [123]
print "list3 = %s" % list3 # list3 = [10, 'a']The desired implementation would default the myList argument with the value None in order to prevent the myList variable from being initiated as a list.
def extendListUpdated(val, myList=None):
# if no list is provided initialize an empty list
if myList is None:
myList = []
myList.append(val)
return myList
list1 = extendListUpdated(10)
list2 = extendListUpdated(123,[])
list3 = extendListUpdated('a')
print "list1 = %s" % list1 # [10]
print "list2 = %s" % list2 # [123]
print "list3 = %s" % list3 # ['a']Note that the None value is a way to signify empty or no value here. It’s commonly defined as
Assigning a value of None to a variable is one way to reset it to its original, empty state.