`elif` in list comprehension conditionals

Question

Consider this example:

l = [1, 2, 3, 4, 5]

for values in l:
    if values == 1:
        print('yes')
    elif values == 2:
        print('no')
    else:
        print('idle')

Rather than printing the results, I want to use a list comprehension to create a list of results, like ['yes', 'no', 'idle', 'idle', 'idle'].

How can we represent the elif logic in a list comprehension? Up until now, I have only used if and else in list comprehension, as in if/else in a list comprehension.

Think about how you would write the explicit loop using only `if` and `else`, if `elif` didn't exist. Then translate that into what you already know about using `if` and `else` (i.e., the ternary operator) in a list comprehension. — Karl Knechtel, Jan 30 '23 at 05:55

score 394 · Accepted Answer · edited Oct 28 '20 at 21:30

394

Python's conditional expressions were designed exactly for this sort of use-case:

>>> l = [1, 2, 3, 4, 5]
>>> ['yes' if v == 1 else 'no' if v == 2 else 'idle' for v in l]
['yes', 'no', 'idle', 'idle', 'idle']

edited Oct 28 '20 at 21:30

Dharman

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answered Apr 03 '12 at 05:23

Raymond Hettinger

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There's some interesting history in the syntax. For many years before their introduction "tertiary expressions" were one of the five most-requested changes in the language. Since Guido van Rossum explicitly designed it as a statement-based language, he firmly resisted for a long time (tertiary expressions, and particularly their abuse, are sources of much obscurity in code). When he finally succumbed, he announced he had deliberately chosen a syntax that discouraged overuse. As usual, he did an elegant design job nevertheless. – holdenweb Jan 23 '18 at 09:42
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Ternary, dammit (he wrote, noticing his dyslexic mistake too late to edit). – holdenweb Jan 23 '18 at 09:48
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While I up-vote this answer, I want to mention this: for 1 pair of if/else is easy to read, 2 pairs: it's getting harder to understand. Don't even mention 3 pairs. If the expression needs 3 or more pairs, a dictionary or a separate function will make things easier to read and understand. – Hai Vu Jan 23 '18 at 13:29
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I would like to add not a solution for this problem, but a reminder of clean code: since this list comprehension has three conditionals, it could probably be refactored into a more descriptive method. My point is this: https://martinfowler.com/bliki/FunctionLength.html :) – Alvaro Cavalcanti Jan 23 '18 at 21:23
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I stumbled upon a case where I needed an elif, but only two values. Using this example, I would've needed just `['yes', 'no']` to be made. To do this, you can do: `['yes' if v == 1 else 'no' for v in l if values in [1,2]]`. I currently can't think of a cleaner way to do this. – dTanMan Nov 07 '19 at 16:05

score 63 · Answer 2 · answered Apr 03 '12 at 05:23

63

>>> d = {1: 'yes', 2: 'no'}
>>> [d.get(x, 'idle') for x in l]
['yes', 'no', 'idle', 'idle', 'idle']

answered Apr 03 '12 at 05:23

Ignacio Vazquez-Abrams

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I think this form is a lot easier to digest than trying to do a really long and complicated if/else logic within the list comp – jdi Apr 03 '12 at 05:26
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@jdi Though conditional-expressions may not be to your taste, they *were* specifically designed to handle if-elif-elif-else chains, just as the OP requested. They aren't hard to learn and can gracefully handle situations that aren't as amenable to dictionary lookup logic: ``'A' if grade>=90 else 'B' if grade>=80 else 'C' if grade>=70 else 'F'``. – Raymond Hettinger Apr 03 '12 at 06:27
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If there an advantage of defining `d` outside the comprehension? – Chris_Rands Jan 23 '18 at 09:21
The reason I like the list comprehension better is that it reads just like English. Even a non-programmer would be able to understand what it does. With this solution you have to understand the dict.get() method. – Tim Skov Jacobsen Nov 25 '18 at 08:03
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This is especially useful if there are even more than three options. – Christian Herenz Apr 19 '21 at 15:44

score 48 · Answer 3 · answered Apr 03 '12 at 05:25

You can, sort of.

Note that when you use sytax like:

['yes' if v == 1 else 'no' for v in l]

You are using the ternary form of the if/else operator (if you're familiar with languages like C, this is like the ?: construct: (v == 1 ? 'yes' : 'no')).

The ternary form of the if/else operator doesn't have an 'elif' built in, but you can simulate it in the 'else' condition:

['yes' if v == 1 else 'no' if v == 2 else 'idle' for v in l]

This is like saying:

for v in l:
    if v == 1 :
        print 'yes'
    else:
        if v == 2:
            print 'no'
        else:
            print 'idle'

So there's no direct 'elif' construct like you asked about, but it can be simulated with nested if/else statements.

Great explanation... +1 for including the equivalent expression using the regular if else syntax. — mortysporty, Apr 13 '21 at 07:44

score 3 · Answer 4 · edited Jan 02 '23 at 15:09

3

Another easy way is to use conditional list comprehension like this:

l=[1,2,3,4,5]
print [[["no","yes"][v==1],"idle"][v!=1 and v!=2] for v in l]

gives you the correct anwer:

['yes', 'no', 'idle', 'idle', 'idle']

edited Jan 02 '23 at 15:09

RF1991

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answered Sep 25 '14 at 17:50

Stefan Gruenwald

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score 3 · Answer 5 · edited Feb 05 '18 at 19:44

3

Maybe you want this:

l = [1, 2, 3, 4, 5] 

print ([['idle','no','yes'][2*(n==1)+(n==2)] for n in l])

edited Feb 05 '18 at 19:44

answered Feb 05 '18 at 19:23

Ratnesh Kushwaha

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I think this is quite clever, but not very legible. – veziop Jun 04 '23 at 10:56

score 3 · Answer 6 · answered Apr 03 '12 at 05:24

3

You can use list comprehension is you are going to create another list from original.

>>> l = [1, 2, 3, 4, 5]
>>> result_map = {1: 'yes', 2: 'no'}
>>> [result_map[x] if x in result_map else 'idle' for x in l]
['yes', 'no', 'idle', 'idle', 'idle']

answered Apr 03 '12 at 05:24

San4ez

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`elif` in list comprehension conditionals

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