List comprehension with duplicated function call [duplicate]

Question

I want to transform a string such as following:

'   1   ,   2  ,    ,   ,   3   '

into a list of non-empty elements:

['1', '2', '3']

My solution is this list comprehension:

print [el.strip() for el in mystring.split(",") if el.strip()]

Just wonder, is there a nice, pythonic way to write this comprehension without calling el.strip() twice?

Answer 1

You can use a generator inside the list comprehension:

  [x for x in (el.strip() for el in mylist.split(",")) if x]
#             \__________________ ___________________/
#                                v
#                        internal generator

The generator thus will provide stripped elements, and we iterate over the generator, and only check the truthiness. We thus save on el.strip() calls.

You can also use map(..) for this (making it more functional):

  [x for x in map(str.strip, mylist.split(",")) if x]
#             \______________ ________________/
#                            v
#                           map

But this is basically the same (although the logic of the generator is - in my opinion - better encapsulated).

Answer 2

As a simple alternative to get a list of non-empty elements (in addition to previous good answers):

import re

s = '   1   ,   2  ,    ,   ,   3   '
print(re.findall(r'[^\s,]+', s))

The output:

['1', '2', '3']