FAQ: Learn Python: Function Arguments - Don't Use Mutable Default Arguments

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This community-built FAQ covers the “Don’t Use Mutable Default Arguments” exercise from the lesson “Learn Python: Function Arguments”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Learn Python 3

FAQs on the exercise Don’t Use Mutable Default Arguments

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1 Like
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Hi, I was still a little confused on the concept so I searched further online and came upon this information I believe may be useful for future learners as they come to this point. I hope it helps, happy coding!

https://docs.python-guide.org/writing/gotchas/

Edit: not advertising, just info I found useful

6 Likes
3

The explanation says: ‘Sets and dictionaries are two other mutable objects in Python.’
what is meant with ‘Sets’?

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A set is a “group” that contains non-repeated objects so lets think about the word

“banana” a set that contains its characters would be (“b”,“a”,“n”)

1 Like
5

Is there any example where this could be actually useful?

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Very useful! This is the relevant text in your link, which really should have been mentioned in the Codecademy lesson:
“Python’s default arguments are evaluated once when the function is defined, not each time the function is called (like it is in say, Ruby). This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.”

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I think it would be worth pointing out in this exercise that

order2 = update_order({'item': 'soda', 'cost': '1.50'})

not only returns unexpected results for order2, but it will also update the value of order1.

I see that this is pointed out in the Hint, but I think that this is a really important point that should be belabored.

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  1. Based on the hint, does that mean doesn’t matter what variable you call the list (order2, order1, or you can call it whatever), it will update for all variables? Did I understand it correctly?
  2. Why is that?
    Thanks in advance!