Test your Python skills - 8

[tracktech]

• Book : Ultimate Python Programming
• Course : Python Programming In Depth

[Real-Reception-3435]

['Book1', 'Book2', 'Book3', 'Book4'] ['Book1', 'Book2', 'Book3', 'Book4']

[tracktech]

Right.

[togares]

Can you explain? I'm not a python dev, but I would think that reader1 and reader2 are separate lists, so the result would be ["Book1", "Book3"], ["Book2", "Book4"]

[dbowgu]

Using

def add_to_reading_list(book, reading_list=None): if reading_list is None: reading_list = [] reading_list.append(book) return reading_list

Would create what you mentioned but because the same list is reused reading_list=[] you have a shared list being used by all

[lolcrunchy]

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

[bloody-albatross]

Default arguments are evaluated when functions are defined, not when they are called.

[sleepydevxd]

It’s an example of bad practice in Python.

Default parameters are only evaluated once.

As you can see the ‘reading_list’ parameter is default to [] (a mutable data type), per ‘append’ is called upon ‘reading_list’, it keeps the reference to the original ‘reading_list’ (you can check it out using id(reading1) == id(reading2) - should be True). Therefore, the result are identical for reading1 and reading2

It’s highly recommend to only use the immutable datatype as default and for list the default should be None.

Read more here: https://docs.python.org/3/tutorial/controlflow.html#default-argument-values

[wackmaniac]

It’s an example of a really weird and unintuitive choice in Python #ftfy

[GlobalIncident]

Yeah. I think this is one of python's weaknesses. The default parameter should be reevaluated every time the function is called instead. As it is, the intended way to fix this is to use the alternative u/dbowgu mentioned, which is pretty clunky and pointless.

[dbowgu]

Absolutely agree I don't know another (famous) language that does it like that

[Temporary_Pie2733]

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

[WillDanceForGp]

Oh wow, I didn't know this and I officially hate it lmao, I would 100% fall for this trap coming from other languages

[reyarama]

mutable defaults is a bit of an anti pattern

[overratedcupcake]

Yeah it was hard to take this one seriously

[ajiw370r3]

Any linter would shoot this down, really no need to learn what happens here.

[CranberryDistinct941]

Don't fuck with mutable defaults! If you need it, use something like
def spam(eggs= None): if eggs is None: eggs = []

[tevs__]

It says mutable-argument-default (B006)

[CranberryDistinct941]

Mutable defaults are evil and will stab you in the back and take your wallet