#TIL : Do not use mutable objects as default parameters

Jul 24, 2018 4 mins read

Do not use mutable objects as default parameters

I learned this from learn-python3

Example :

def append_if_multiple_of_five(number, magical_list=[]):
    if number % 5 == 0:
        magical_list.append(number)
    return magical_list

print(append_if_multiple_of_five(100))
print(append_if_multiple_of_five(105))
print(append_if_multiple_of_five(123))
print(append_if_multiple_of_five(123, []))
print(append_if_multiple_of_five(123))

Result :

[100]
[100, 105]
[100, 105]
[]
[100, 105]

So default parameters in Python are shared between function calls if it isn’t passed from caller. So be careful because shared mutable object can affect your logic between function calls, where MAGIC was born !

One safe way to achieve the goal, use None as replacement like this

def append_if_multiple_of_five(number, magical_list=None):
    if not magical_list:
        magical_list = []
    if number % 5 == 0:
        magical_list.append(number)
    return magical_list

print(append_if_multiple_of_five(100))
print(append_if_multiple_of_five(105))
print(append_if_multiple_of_five(123))
print(append_if_multiple_of_five(123, []))
print(append_if_multiple_of_five(123))

Result :

[100]
[105]
[]
[]
[]
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