Does Python have a cleaner way to express "if x contains a|b|c|d..."?

Question

The Pythonic way to check if a string x is a substring of y is:

if x in y:

Finding if x is equivalent to a, b, c, d, e, f or g is also Pythonic:

if x in [a,b,c,d,e,f,g]:

But checking if some string x contains either a, b, c, d, e, f or g seems clunky:

if a in x or b in x or c in x or d in x or e in x or f in x or g in x

Is there a more Pythonic method of checking if a string x contains an element of a list?

I know it is trivial to write this myself using a loop or using a regex:

re.search('(dog|cat|bird|mouse|elephant|pig|cow)', x)

but I was wondering if there was a cleaner way that does not involve regex.

score 124 · Accepted Answer · edited May 23 '17 at 12:24

124

The Pythonic approach would be to use any():

if any(s in x for s in (a,b,c,d,e,f,g)):

From the linked documentation:

any(iterable)

Return True if any element of the iterable is true. If the iterable is empty, return False. Equivalent to:
def any(iterable):
    for element in iterable:
        if element:
            return True
    return False

Also, notice that I've used a tuple instead of a list here. If your a-g values are pre-defined, then a tuple would indeed be preferred. See: Are tuples more efficient than lists in Python?

edited May 23 '17 at 12:24

Community

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answered Oct 31 '13 at 18:32

arshajii

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Inner parenthesis aren't nessesary `any(s in x for s in a, b, c, d, e, f, g)` is fine. – Igonato Oct 31 '13 at 18:40
@Igonato At least on Python 3 this is a syntax error. – Oct 31 '13 at 18:42
19

@Igonato I find the inner parenthesis add readability. – arshajii Oct 31 '13 at 18:43
You can also use `if any(s in x for s in "abcdefg")`, since a string is also iterable. – chepner Oct 31 '13 at 18:57
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@chepner: You can't. a,b,c,.. are variables, not chars! – Markon Oct 31 '13 at 18:58
Yeah, I just realized that :) – chepner Oct 31 '13 at 18:59
This would be more efficient as `any(s in frozenset((a,b,c,d,e,f,g)) for s in x)` without changing readability much at all. – Ceasar Nov 05 '13 at 21:04
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@CeasarBautista No, that's completely different. What you have checks if any of the characters of `x` are in the set `{a,...,g}`, which isn't what we want. – arshajii Nov 05 '13 at 23:24
The Are tuples more efficient than lists in Python? link is great! – laike9m Nov 08 '13 at 09:28

score 27 · Answer 2 · answered Oct 31 '13 at 18:32

27

if any(q in x for q in [a,b,c,d,e,f,g]):

I think that's about as short & Pythonic as you can get it.

answered Oct 31 '13 at 18:32

jwodder

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score 10 · Answer 3 · answered Nov 01 '13 at 05:25

10

A bit late to the party, but

not frozenset(x).isdisjoint(frozenset(y))

would work, and may be faster (algorithmically, but maybe not for smaller test cases).

answered Nov 01 '13 at 05:25

Snakes and Coffee

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I find this one much harder to read. – gerrit Nov 01 '13 at 13:19
this is rather obscure – siledh Nov 06 '13 at 07:27
@siledh not really, it's referenced in some other answers, and the fact that using `set` and similar types will reduce the lookup cost isn't too esoteric either. – Snakes and Coffee Nov 07 '13 at 03:03

score 3 · Answer 4 · answered Nov 01 '13 at 04:05

3

without using any but simply max

def is_in(symbol, lst):
    return max([symbol in x for x in lst]) 

print is_in('a',['ae','br','tl'])
print is_in('c',['ae','br','tl'])

gives

>>> 
True
False

answered Nov 01 '13 at 04:05

kiriloff

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It's faster as `max(...)` instead of `max([...])`. – pts Nov 01 '13 at 09:21
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It's much faster as `any(...)` instead of `max(...)`... – shx2 Nov 01 '13 at 13:08
yes, `any` is preferable as it will short-circuit when a match is found, `max` will check every string every time. – Tom Nov 05 '13 at 22:32

Does Python have a cleaner way to express "if x contains a|b|c|d..."?

4 Answers4