使用 IF 条件在 python 中搜索字符串时，哪个默认变量存储子字符串？

Question

if "a" or "b" or "c" in x:
    print (x)
    print ("a" or "b" or "c")

Here I want to print the substring a or b or c which present in the string and want to do some manipulation on it. So which variable stores the substring?

Answer 1

There is no variable which gets populated under the hood when you find a match. If you need the match index or the matched substring, you need to ask for it specifically.

In any event, "a" or "b" or "c" in x doesn't do what you hope or assume. It checks if "a" is a non- False value (which it is, so the remainder of the alternatives are skipped; but for the record, it would then check if "b" is a non- False value, and then finally if "c" in x returns a non- False value).

Depending on what exactly you are looking for, perhaps a regular expression would solve your problem?

import re
regex = re.compile(r'a|b|c')
match = regex.search(x)
if match:
    print('{0} in {1}'.format(match.group(0), x))

The precise behavior might differ from what you desire. This will find the leftmost match on any one of the alternatives in the regular expression. If you want different behavior (perhaps first check if "a" is anywhere in the string?) then perhaps ask a more well-defined question.

Answer 2

Wrong condition

First of all "a" or "b" or "c" in x does not checks if any of those characters are in the string. It checks the truthiness of each expression, immediately stopping at "a" which is truthy.

Instead you want this condition:

if 'a' in x or 'b' in x or 'c' in x:

It can also be written with any .

if any(c in x for c in ('a', 'b', 'c')):

Finding the substring

As for you question, neither of the above will keep track of which substring was found in x for the expression to be True . You need to do that yourself.

chars = ('a', 'b', 'c')
found_char = next((c for c in chars if c in x), False)
if found_char:
    ...

Answer 3

I don't fully understand but, the efficient-est way (at least i think is):

if any(i in x for i in {'a', 'b', 'c']):
    print ("a" or "b" or "c")

But to get it do:

if any(i in x for i in {'a', 'b', 'c'}):
    print ([i for i in {'a', 'b', 'c'} if i in x])

Answer 4

You can't check three substrings like this. Here if condition is becoming if ("a") or ("b") or ("c" in x) , "a" is True hence will always pass the if condition.

You can the if condition to:

if "a" in x or "b" in x or "c" in x

Answer 5

Try using a regex for this kind of thing.

>>> import re
>>> re.findall('[abc]', 'foobaz')
['b', 'a']

Note that the expression "a" or "b" or "c" in x is just 'a' .

Answer 6

Based on the English interpretation of your code, it looks like you want something like this:

for val in ["a", "b", "c"]:
    if val in x:
        print(x)
        print(val)
        break  # stop checking once the first match is found

Olivier Melançon's answer achieves the same thing, so the choice between that and this would just be a matter of style.

If you want all the matches, you could use the set intersection operator ( & ):

matches = {"a", "b", "c"} & set(x)
print(matches)