List comprehension: Returning two (or more) items for each item
Double list comprehension:
[f(x) for x in range(5) for f in (f1,f2)]
Demo:
>>> f1 = lambda x: x
>>> f2 = lambda x: 10*x
>>> [f(x) for x in range(5) for f in (f1,f2)]
[0, 0, 1, 10, 2, 20, 3, 30, 4, 40]
>>> from itertools import chain
>>> f = lambda x: x + 2
>>> g = lambda x: x ** 2
>>> list(chain.from_iterable((f(x), g(x)) for x in range(3)))
[2, 0, 3, 1, 4, 4]
Timings:
from timeit import timeit
f = lambda x: x + 2
g = lambda x: x ** 2
def fg(x):
yield f(x)
yield g(x)
print timeit(stmt='list(chain.from_iterable((f(x), g(x)) for x in range(3)))',
setup='gc.enable(); from itertools import chain; f = lambda x: x + 2; g = lambda x: x ** 2')
print timeit(stmt='list(chain.from_iterable(fg(x) for x in range(3)))',
setup='gc.enable(); from itertools import chain; from __main__ import fg; f = lambda x: x + 2; g = lambda x: x ** 2')
print timeit(stmt='[func(x) for x in range(3) for func in (f, g)]',
setup='gc.enable(); f = lambda x: x + 2; g = lambda x: x ** 2')
print timeit(stmt='list(chain.from_iterable((f(x), g(x)) for x in xrange(10**6)))',
setup='gc.enable(); from itertools import chain; f = lambda x: x + 2; g = lambda x: x ** 2',
number=20)
print timeit(stmt='list(chain.from_iterable(fg(x) for x in xrange(10**6)))',
setup='gc.enable(); from itertools import chain; from __main__ import fg; f = lambda x: x + 2; g = lambda x: x ** 2',
number=20)
print timeit(stmt='[func(x) for x in xrange(10**6) for func in (f, g)]',
setup='gc.enable(); f = lambda x: x + 2; g = lambda x: x ** 2',
number=20)
2.69210777094
3.13900787874
1.62461071932
25.5944058287
29.2623711793
25.7211849286