Best practice for setting the default value of a parameter that's supposed to be a list in Python?

Question

I have a Python function that takes a list as a parameter. If I set the parameter\'s default value to an empty list like this:

def func(items=[]):
    print          


        

Answer 1

For mutable object as a default parameter in function- and method-declarations the problem is, that the evaluation and creation takes place at exactly the same moment. The python-parser reads the function-head and evaluates it at the same moment.

Most beginers asume that a new object is created at every call, but that's not correct! ONE object (in your example a list) is created at the moment of DECLARATION and not on demand when you are calling the method.

For imutable objects that's not a problem, because even if all calls share the same object, it's imutable and therefore it's properties remain the same.

As a convention you use the None object for defaults to indicate the use of a default initialization, which now can take place in the function-body, which naturally is evaluated at call-time.