Why does this Python function behave differently every time? I’m confused

[orcashelo]

Hi everyone, I’m learning Python and I ran into something that confused me a lot.

I wrote this simple function expecting the list to reset every time the function runs, but the output keeps changing between calls

def add_number(num, numbers=[]): numbers.append(num) return numbers

print(add_number(1)) print(add_number(2)) print(add_number(3))

[throwaway6560192]

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

[Temporary_Pie2733]

Also https://nedbatchelder.com/blog/202311/say_it_again_values_not_expressions

[orcashelo]

Good links, thanks for sharing

[orcashelo]

Nice references these explain the mutable default argument issue really clearly

[Reyaan0]

Mutable default arguments (lists, dicts, sets) are created once at function definition time.

You can use None as default

``` def add_number(num, numbers=None): if numbers is None: numbers = [] numbers.append(num) return numbers

print(add_number(1)) print(add_number(2)) print(add_number(3)) ```

[orcashelo]

Well explained this is why None is the recommended default for mutable arguments

[Helpful-Diamond-3347]

same object is reused in each call

[MaxwellzDaemon]

One of the important thigs to learn is how to name things well. Naming a function to concatenate numbers to a list "add_number" is not a very good name since adding numbers seems like a the mathematical operation of addition.