Python: optimisation of AND evaluation

Question

In Python 3, I have to check that 3 conditions are not happening in a function parameter, so, I created 3 functions:

def check_lenght(string_id):
    # if length is right, return True, otherwise, False

def check_format(string_id):
    # return True if some internal format has to be used, in another case, False

def check_case_num(string_id):
    # manual iteration to check if certain characters are not contained, return True, otherwise, False


def valid_product_code(id_str):
    return check_lenght(id_str) and check_format(id_str) and check_case_num(id_str)

So, there are 3 functions, one of them iterating a string, an operation that is very heavy potentially. But, if one function returns already False, it is not necessary to check the remaining ones, we know the logical AND will return False, being able to make this less computationally expensive.

So, I am wondering if Python (CPython or other implementations) is optimizing this, therefore, the usage of return check_lenght(id_str) and check_format(id_str) and check_case_num(id_str) is right, or if it would be better to check these functions one by one and returning False as soon as the first of them is False, having a more optimal but, maybe, less readable solution.

How this expression is being evaluated in Python?

I tried to find the answer to this question googling for it and also in this site

score 2 · Accepted Answer · answered Feb 03 '19 at 16:14

2

It's called short-circuiting and, Yes, Python does support it:

https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not

answered Feb 03 '19 at 16:14

sam46

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Inevitably similar to https://stackoverflow.com/a/2580142/2988730 – Mad Physicist Feb 03 '19 at 16:16
1

hmm. Of course it's been asked and answered before. welcome to SO – sam46 Feb 03 '19 at 16:18
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The trick is knowing where to look for the duplicates. – Mad Physicist Feb 03 '19 at 16:20
Thanks and sorry for the duplicate, I searched in Google and here, but I could not find the answer – iishchenko Feb 03 '19 at 16:42

Markus Unterwaditzer · Answer 2 · 2019-02-03T16:46:56.023

2

a and b and c will not evaluate b or c if a already returns False.

x or y will not evaluate y if x already returns True.

You will find that this is how bool operators work in almost all C-like languages.

edited Feb 03 '19 at 16:46

answered Feb 03 '19 at 16:14

Markus Unterwaditzer

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That doesn't sound right. Perhaps I'm misunderstanding your meaning though. – Mad Physicist Feb 03 '19 at 16:16
@MadPhysicist it's part of the definition of short-circuiting. `b` will not be evaluated if `a` is already `False`. – Markus Unterwaditzer Feb 03 '19 at 16:46
Ah I think I understand the confusion, edited for clarity. – Markus Unterwaditzer Feb 03 '19 at 16:47
Thanks. Vote flipped. – Mad Physicist Feb 03 '19 at 16:50

Python: optimisation of AND evaluation

2 Answers2