Default Argument Values in Python

The Issue with Mutable Default Arguments

You've encountered a common gotcha in Python: default argument values are evaluated only once at the point of function definition in the defining scope. This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.

Your Example

1def add_item(item, my_list=[]):
2    my_list.append(item)
3    return my_list
4
5print(add_item(1))  # [1]
6print(add_item(2))  # [1, 2]  
7print(add_item(3))  # [1, 2, 3]

1def add_item(item, my_list=[]):
2    my_list.append(item)
3    return my_list
4
5print(add_item(1))  # [1]
6print(add_item(2))  # [1, 2]  
7print(add_item(3))  # [1, 2, 3]

In this case, my_list is a list that's created when the function is defined, and the same list is used for all function calls.

Why This Happens

• Default argument values are evaluated once at the point of function definition.
• The default value for my_list is a mutable object (a list).
• Each time you call add_item, you're appending to the same list object.

The Solution

To avoid this issue, you can use None as the default argument value and create the list inside the function:

1def add_item(item, my_list=None):
2    if my_list is None:
3        my_list = []
4    my_list.append(item)
5    return my_list
6
7print(add_item(1))  # [1]
8print(add_item(2))  # [2]
9print(add_item(3))  # [3]

1def add_item(item, my_list=None):
2    if my_list is None:
3        my_list = []
4    my_list.append(item)
5    return my_list
6
7print(add_item(1))  # [1]
8print(add_item(2))  # [2]
9print(add_item(3))  # [3]

Alternatively, you can also use a more concise version:

1def add_item(item, my_list=None):
2    my_list = my_list or []
3    my_list.append(item)
4    return my_list

1def add_item(item, my_list=None):
2    my_list = my_list or []
3    my_list.append(item)
4    return my_list

By doing this, you ensure that a new list is created for each function call, and you avoid the issue of the function "remembering" previous calls.