remove mutable default arguments

[jurasofish]

Mutable default arguments are a common gotcha. They might not be causing issues currently, but they are a time bomb likely to result in insidious issues as the code around them changes.

[h-g-s]

I'm not sure If I understood the purpose of these changes. Wouldn't it make a bit slower an additional check ?

[jurasofish]

Yeah it might make it a tad slower due to additional checks in some cases, though that's not the case with this pull request any longer.

The issue is that

• a lot of people don't understand how Python behaves with mutable default arguments
• mutable default args typically result in unintended behaviour.
• If code relies on mutable default args that's likely extremely poor quality code.

The changes in this PR aren't addressing a pressing problem, just following best practices and improving the robustness of the code.

For example,

>>> def f(a=[]):
...     a.append(1)
...     print(a)
...     
>>> f([1,2,3])
[1, 2, 3, 1]
>>> f()
[1]
>>> f()
[1, 1]

[sebheger]

Please correct if I am wrong, but I believe, that this change wouldn't harm the performance. The code @jurasofish proposed here is start-of-the-art python programming.

[h-g-s]

Hi @jurasofish and @sebheger , thanks for the information, just understood how horrible mutable defaults are:
https://florimond.dev/blog/articles/2018/08/python-mutable-defaults-are-the-source-of-all-evil/#:~:text=In%20Python%2C%20when%20passing%20a,or%20even%20a%20class%20instance.