Do not use a mutable like list or dictionary as a default value to an argument. Python’s default arguments are evaluated once when the function is defined. Using a mutable default argument and mutating it will mutate that object for all future calls to the function as well.

Bad practice

def my_function(elem, l=[]):
    l.append(elem)
    return l

print(my_function(2)) # [2]
print(my_function(5)) # [2, 5]

Recommended:

def my_function(elem, l=None):
    if l is None:
        l = []
    l.append(elem)
    return l

print(my_function(2)) # [2]
print(my_function(5)) # [5]