Python - Why these parentheses in if-statement alters the real condition?

Question

I was doing a leetcode question while I had the following python code:

pairs = [(1, 3)]
addend = 3
sum = 4
if ((addend, sum - addend) or (sum - addend, addend)) in pairs:
    print("Pair exists")

The expected output when I ran this code should be

Pair exists

But instead this prints nothing, which I assume means ((addend, sum - addend) or (sum - addend, addend)) evaluates to False.

Then I removed the outer parentheses and made it

if (addend, sum - addend) or (sum - addend, addend) in pairs:
        print("Pair exists")

This gave me the right output.

My second guess was this pair of redundant parentheses actually calculates ((addend, sum - addend) or (sum - addend, addend)), so I put ((1, 3) or (3, 1)) in the Python3.7 console directly and that's the output

>>> ((1, 3) or (3, 1))
(1, 3)

But still this won't make sense since (1, 3) is indeed in pairs.

Could anybody explain why putting these parentheses invalidates the statement?

`if (addend, sum - addend) in pairs or (sum - addend, addend) in pairs:` — Stephen Rauch, Feb 07 '19 at 03:45
The parentheses are *not* redundant`(addend, sum-addend) or (sum-addend,addend)` -> `(3, 1)` which is not in `pairs` — juanpa.arrivillaga, Feb 07 '19 at 03:46
https://docs.python.org/3/reference/expressions.html#operator-precedence — Stephen Rauch, Feb 07 '19 at 03:46
I would really love to answer but someone marked it as duplicate, so here it is in a comment: U9-Forward is right, you should use `any`, or use a double `or`. However if you were curious as to why your first method wasn't working, it was because you had `((addend, sum-addend) or (sum-addend,addend))`, which equates to `((3,1) or (1,3))`, and since, as you saw, only the first in this pair is left after using `or`, you are left with `(3,1)`, which does not appear in `pairs` because `pairs = [(1,3)]`, hence no output. The most curious thing here is how or is behaving in the brackets. Very unusual — Recessive, Feb 07 '19 at 03:55
@Recessive Thanks a lot for your comment. Yea, "how or is behaving in the brackets" makes this question appear. Really like funny magic! — Hang, Feb 07 '19 at 03:58

score 2 · Accepted Answer · answered Feb 07 '19 at 03:55

2

The result of a Python or expression is the first operand that is truthy, or False if neither is.

The first expression you tried is

((addend, sum - addend) or (sum - addend, addend)) in pairs

It can be evaluated as

((3, 1) or (1, 3)) in pairs

Since both operands to the or expression in parentheses are non-empty tuples, the expression evaluates to

(3, 1) in pairs

The result is understandably false.

If you strip off the parentheses, you run into the fact that or has lower precedence than in. So

(addend, sum - addend) or (sum - addend, addend) in pairs

is the same as

(3, 1) or ((1, 3) in pairs)

Again, both operands are truthy, so the expression evaluates to (3, 1) regardless of whether pairs contains anything.

What you were probably trying to express is

(addend, sum - addend) in pairs or (sum - addend, addend) in pairs

answered Feb 07 '19 at 03:55

Mad Physicist

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Thanks Mad Physicist! After reading all the above answers and comments I was still confused about why stripping off the parentheses would make the statement True, until you said "or has lower precedence than in" and '''if (3,1)''' would evaluate to True. That's really helpful! – Hang Feb 07 '19 at 04:20
@Hang. Glad it helped you. Some things Python does are a bit unusual, especially for a beginner, so I try to express what went into the aha moment I had when I first understood the topic. Good luck to you in your endeavors. – Mad Physicist Feb 07 '19 at 04:48
1

Yea definitely, and you got right to the point! Appreciate it again and best wishes for you too in the new year! – Hang Feb 07 '19 at 14:39

U13-Forward · Answer 2 · 2019-02-07T03:46:59.443

1

You should use any:

if any(i in pairs for i in [(addend, sum-addend),(sum-addend,addend)]):

Or use:

if (addend, sum - addend) in pairs or (sum - addend, addend) in pairs:

You can't use or because or just picks an element which is not indeed that that's the element in pairs, so use the above codes.

edited Feb 07 '19 at 03:46

answered Feb 07 '19 at 03:46

U13-Forward

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This doesn't answer the question. And what's wrong with using `or`? – juanpa.arrivillaga Feb 07 '19 at 03:46
@juanpa.arrivillaga Edited mine. – U13-Forward Feb 07 '19 at 03:48
U9-Forward Thanks for your answer! And @juanpa.arrivillaga I just tried putting '''((3,1) or (1,3))''' in the console and it did give me '''(3,1)'''. So seems like the parentheses do calculate these '''(3,1) or (1,3)''' and whatever before '''or''' gets returned? – Hang Feb 07 '19 at 03:54
@Hang not exactly, [see this question](https://stackoverflow.com/questions/47007680/strange-use-of-and-or-operator) – juanpa.arrivillaga Feb 07 '19 at 03:54
@juanpa.arrivillaga Thank you! Will see – Hang Feb 07 '19 at 03:55
U9-Forward thanks for your recommended way of writing or-condition in the right way in python. I will start using if any() in the future – Hang Feb 07 '19 at 04:22
@Hang Happy to help, :-), `any` is very , lol :-) – U13-Forward Feb 07 '19 at 04:31
Yea I see! ```any``` is powerfully !!! XD – Hang Feb 07 '19 at 04:38
@Hang Yes, https://docs.python.org/3/library/functions.html#any – U13-Forward Feb 07 '19 at 04:40

Python - Why these parentheses in if-statement alters the real condition?

2 Answers2