Temporary variable within list comprehension

Question

It happens to me quite often to have a piece of code that looks like this.

raw_data  = [(s.split(\',\')[0], s.split(\',\')[1]) for s in all_lines if s.split(\',         


        

Answer 1

If you have two actions for processing, you may embed another list comprehension:

raw_data  = [(lhs, rhs) 
            for lhs, rhs 
            in [s.split(',')[:2] for s in all_lines]
            if rhs != '"NaN"']

You can use generator inside (it gives a small performance gain too):

            in (s.split(',')[:2] for s in all_lines)

It will even be faster than your implementation:

import timeit

setup = '''import random, string;
all_lines = [','.join((random.choice(string.letters),
                    str(random.random() if random.random() > 0.3 else '"NaN"')))
                    for i in range(10000)]'''
oneloop = '''[(s.split(',')[0], s.split(',')[1]) 
              for s in all_lines if s.split(',')[1] != '"NaN"']'''
twoloops = '''raw_data  = [(lhs, rhs) 
                for lhs, rhs 
                in [s.split(',') for s in all_lines]
                if rhs != '"NaN"']'''

timeit.timeit(oneloop, setup, number=1000)  # 7.77 secs
timeit.timeit(twoloops, setup, number=1000) # 4.68 secs