Python, pycharm changes attribute from list to None, why?

Question

When I'm creating a new class, if one of my attribute is an empty list, pychram suggest to change it to this:

class meet_angle_cond:
    def __init__(self, xy_lat_lon_list=None):
        if xy_lat_lon_list is None:
           xy_lat_lon_list = []
           self.xy_lat_lon_list = xy_lat_lon_list

I'm struggling to understand why this is better than:

class meet_angle_cond:
    def __init__(self, xy_lat_lon_list=[]):
           self.xy_lat_lon_list = xy_lat_lon_list

It seems to me that this way is more effective and readable.

It is because of [this](https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments) — Abdul Niyas P M, Dec 28 '19 at 07:56
Pycharm is actually doing you a favor by preventing you from getting confusing bugs with mutable default arguments. — Gino Mempin, Dec 28 '19 at 08:00

score 1 · Answer 1 · answered Dec 28 '19 at 08:43

Because lists are mutable. Check these snippets.

def append_to(element, to=[]):
    to.append(element)
    return to

What You Might Have Expected to Happen

my_list = append_to(12)
print(my_list)

my_other_list = append_to(42)
print(my_other_list)

Expected output,

[12]
[42]

Actual output,

[12]
[12, 42]

So pycharm suggests you to use None as default argument.

Python, pycharm changes attribute from list to None, why?

1 Answers1