Question

I have function declaration like:

def function(list_of_objects = None)

and if *list_of_objects* not passed (is None) I need to define it like empty list. The explicit way is:

def function(list_of_objects = None):
    if not list_of_objects:
        list_of_objects = list()

or

def function(list_of_objects = None):
    list_of_objects = list() if not list_of_objects else list_of_objects

Does above code equals the next one?

def function(list_of_objects = None):
    list_of_objects = list_of_objects or list()

I tested it, but I’m still not sure

>>> def func(my_list = None):
...     my_list = my_list or list()
...     print(type(my_list), my_list)
... 
>>> func()
(<type 'list'>, [])
>>> func(['hello', 'world'])
(<type 'list'>, ['hello', 'world'])
>>> func(None)
(<type 'list'>, [])
>>> 

Answer 1

The idiomatic way is:

def function(list_of_objects=None):
    if list_of_objects is None:
        list_of_objects = []

None is a singleton so you can use is operator for comparison.

Your code tests truthness of list_of_objects (all if and or variants are equivalent in this case). The following values are considered false in Python:

• None

• False

• zero of any numeric type, for example, 0, 0.0, 0j.

• any empty sequence, for example, '', (), [].

• any empty mapping, for example, {}.

• instances of user-defined classes, if the class defines a __bool__() or __len__() method, when that method returns the integer zero or bool value False.

All other values are considered true — so objects of many types are always true.