How to interpret Python syntax for if else block

Question

Below is an easy level coding problem, based on string manipulation:-

I was able to solve it using a for loop by defining conditions for positive and negative numbers.

But while going through other solutions, I found this interesting solution in Python for the same question.

I have been working in Python for the past year but this is my first time coming across the below syntax for if else block

if [str(x) > A[i], str(x) < A[i]][A[0] == '-']:

Below is the complete code :

def maxValue(self, A, x):
        for i in xrange(len(A)):
            if [str(x) > A[i], str(x) < A[i]][A[0] == '-']:
                return A[:i] + str(x) + A[i:]
        return A + str(x)

How can I interpret this block? Where can I find documentation for it?

It's not any kind of new syntax, it's just unconventional use of something you already know. What does [] in term position do? What does [] in postfix position do? What do booleans do when coerced to int? — hobbs
– hobbs, Commented May 30, 2021 at 6:03
That's an old style: X if C else Y which is written as [Y, X][C]. Note that in this form both X and Y are evaluated which does not occur in the if expression. — Dan D.
– Dan D., Commented May 30, 2021 at 6:09
So basically X if C else Y and [Y, X][C]` are functionally different when it comes to execution? First expression is much more safer then, right? — Pratap Alok Raj
– Pratap Alok Raj, Commented May 30, 2021 at 6:14
Thanks DanD and hobbs for your insights. I completely understood the scenarios and even got some additional info on how can this be different for python 2.x and python 3.x — Pratap Alok Raj
– Pratap Alok Raj, Commented May 30, 2021 at 6:56

Alexander Slesarenko · Accepted Answer · 2021-05-30 06:25:40Z

2

The less hacky way to write it:

def maxValue(self, A, x):
    for i in xrange(len(A)):        
        if (A[0] == '-' and str(x) < A[i]) or (A[0] != '-' and str(x) > A[i]):
            return A[:i] + str(x) + A[i:]
    return A + str(x)

Explanation: if checks one of [str(x) > A[i], str(x) < A[i]] array values depending on condition A[0] == '-'.

If A[0] == '-' is False, it turns into 0, thus gets [str(x) > A[i], str(x) < A[i]][0] and checks str(x) > A[i] condition.

If (A[0] == '-') == True, True turns to 1 and checks [str(x) > A[i], str(x) < A[i]][1] or str(x) < A[i].

answered May 30, 2021 at 6:25

Alexander Slesarenko

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3 Comments

Pratap Alok Raj Over a year ago

Will it be same for python 2.x and python 3.x

Alexander Slesarenko Over a year ago

@AlokRaj Yes, you can test with print([2, 3][False] == 2 and [2, 3][True] == 3)

Pratap Alok Raj Over a year ago

Got the solution, but thanks for insights :) In python 2.x, True and False can be reassigned but In Python 3.x True and False are keywords and will always be equal to 1 and 0. It's a really small but awesome concept :D

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