Question

I’d like to know if it is possible to use a list comprehension with if/ else that need not result in a list of the same length as the length of the list being processed? (ie. without the final else)

>>> L = [0, 1, 2, 3, 4, 5, 6]
>>> [v * 10 if v < 3 else v * 2 if v > 3 else v for v in L] #if/else/if/else
[0, 10, 20, 3, 8, 10, 12]

works fine. But suppose I want to omit the 3, to get:

[0, 10, 20, 8, 10, 12]  # No number 3

I would have thought this would work:

>>> [v * 10 if v < 3 else v * 2 if v > 3 for v in L] #if/else/if

But it is a syntax error..

So I thought ‘maybe’ this would work:

>>> [v * 10 if v < 3 else v * 2 if v > 3 else pass for v in L] #if/else/if/else pass

But it doesn’t..

This is a curiosity question, I realise this is not neccessarily the most readable/appropriate approach to the above processing.

Am I missing something? Can it be done?
(I’m on python 2.6.5)

Answer 1

Yes, that’s possible:

[foo for foo in bar if foo.something]

Or in your case:

[v * 10 if v < 3 else v * 2 for v in L if v != 3]

I’s also mentioned in the docs.