Question

Is filter/map equivalent to list comprehension?
Suppose I have the following function

def fib_gen():
    a,b = 0,1
    yield 0
    yield 1
    while True:
        a,b = b,a+b
        yield b

Now I can use list comprehension to list fib numbers:

a = fib_gen()
print [a.next() for i in range(int(sys.argv[1]))]

Suppose I want to list only even fib numbers. I would do the following with filter/map:

a = fib_gen()
print filter(even, map(lambda x: a.next(), range(int(sys.argv[1]))))

How can I get the same result with list comprehension?

Answer 1

You could use a generator to store the intermediate result, and “filter” on it.

fibs = (a.next() for i in whatever)
even_fibs = [num for num in fibs if num % 2 == 0]

or in one line:

even_fibs = [num for num in (a.next() for i in whatever) if num % 2 == 0]

Note that, if you want to take a definite number of elements from an iterator, you could use itertools.islice instead:

from itertools import islice
fibs_max_count = int(sys.argv[1])
even_fibs = [num for num in islice(fib_gen(), fibs_max_count) if num%2 == 0]