Passing dictionary as an optional argument?

[Georgi Guninski]

I am not python expert, but got hurt by passing dictionary as
optional argument.

Session:

In [1]: def f(n,a,ca={}):
...: for i in range(n):
...: ca[i+a]=i+a
...: print(i,"len(cac)",len(ca))
...:

In [2]: f(2,2)
0 len(cac) 1
1 len(cac) 2

In [3]: f(2,4)
0 len(cac) 3
1 len(cac) 4

What is the scientific way to avoid `0 len(cac) 3`?

[Nico Van Cleemput]

Hi George

The reason is that you are using a mutable object as a default object, but it is only instantiated once and then reused. See e.g. here for a longer explanation: https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422

To briefly give the solution: use ca=None and then create the dictionary in your function if ca is None.

Cheers

Nico

Op do 15 jun 2023 om 07:28 schreef Georgi Guninski <ggun...@gmail.com>:

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