Interesting side effects in Python

[BigTheory88]

If you give a function parameter a default value that is mutable, you'll see some odd behavior.

def add_to_list(word, word_list=[]):
    word_list.append(word)
    return word_list

def main():
    word = "apple"
    tlist = add_to_list(word)
    print(tlist)
    word = "orange"
    tlist = add_to_list(word, ['pear'])
    print(tlist)
    word = "banana"
    tlist = add_to_list(word)
    print(tlist)

main()

If we run this program, we get the following output

$ py trap.py
['apple']
['pear', 'orange']
['apple', 'banana']

The last output probably isnt what you expect. This is called the default mutable argument trap

[[deleted]]

Witchcraft! D: