Does Python implement short-circuiting in built-in functions such as min()?

Question

Does Python 3 implement short-circuiting in built-in functions whenever possible, just like it does for boolean statements?

A specific example, take the below code snippet:

min((20,11), key = lambda x : x % 10) # 20

Does Python evaluate beforehand that the minimum value possible of the function passed as the key argument is 0, and therefore stops right after evaluating the first integer in the iterable passed (20) as 20 % 10 is equal to 0? Or does it have to evaluate all the elements in the iterable before returning the answer?

I guess short-circuiting isn't even always possible especially for more complex functions, but what about for well-known, built-in functions or operators like %?

I couldn't find the answer in the official docs.

Thanks,

You can check it yourself by looking at how its implemented. Check min_max function at - github.com/python/cpython/blob/main/Python/bltinmodule.c — Pankaj
– Pankaj, Commented May 13, 2022 at 14:12

Germán Martinez · Accepted Answer · 2022-05-13 14:19:20Z

2

python has to evaluate all values inside the iterable because the languaje evaluate element by element, if you have in your tuple something that is not a number it will trigger an exception when try to perform the % operation. Python can not guess what is inside your list. You can test this by defining a function instead of a lambda and set debug point inside.

def my_mod(x): 
    import ipdb; ipdb.set_trace() 
    return x % 20

then call the function

min((20,11), key = my_mod)

you can do a quick error test case with

min((20,11, "s"), key = my_mod)

It will trigger an exception but first had to evaluate all the previous element in the list.

answered May 13, 2022 at 14:19

Germán Martinez

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1 Comment

qwr Apr 11 at 20:07

Min has to evaluate everything. OTOH any and all can short-circuit.

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