Gotcha!

Default Arguments

• Mutable defaults for function/method arguments

• Default arguments in Python: two easy blunders

Do not use mutable arguments i.e. list, dictionary, or a class instance e.g:

def functionF(argString="abc", argList=[]):

The objects that provide the default values are not created at the time that functionF is called. They are created at the time that the statement that defines the function is executed.

In this example, the string argument will behave as expected because it is immutable.

To solve the problem, use None e.g:

def functionF(argString="abc", argList=None):
    if argList is None: argList = []