使用 enumerate() 和 takewhile() 在 python 中返回一个列表

Question

I'm trying to define a method to return a list of the first n elements of a Fibonacci generator using enumerate() and takewhile(). My code goes into an infinite loop and never checks the condition in takewhile() to stop iterating. I know islice() is much easier, but I need to define the method using only enumerate and takewhile.

from itertools import takewhile


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


def take_e(n, gen):
    fib_list = []
    for (i, elem) in enumerate(gen):
        if takewhile(i < n, gen):
            fib_list.append(elem)
        else:
            return fib_list


fibonacci = fibonacci_unbounded
n = 8

print(take_e(n, fibonacci()))

Answer 1

zip works much like enumerate when you use a range iterator. The nice part is that zip stops on the shortest iteration. So ditch takewhile and use an enumerator that stops at the right place.

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

def take_e(n, gen):
    return list((elem) for _,elem in zip(range(n), gen))

fibonacci = fibonacci_unbounded
n = 8
print(take_e(n, fibonacci()))

If using takewhile and enumerate is a requirement, you should take values from the enumeration until the condition is met.

from itertools import takewhile

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

def take_e(n, gen):
    return list(val for _,val in takewhile(
        lambda i_val: i_val[0] < n, enumerate(gen)))

fibonacci = fibonacci_unbounded
n = 8
print(take_e(n, fibonacci()))

Answer 2

Here's a way that works. I was hoping to find a way to not need to add the index into the logic so that you can test for the "nth" item returned:

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

def take_e(n, gen):
    return [x[1] for x in takewhile(lambda x: x[0] < n, enumerate(gen()))]

print(take_e(9, fibonacci_unbounded))

Result:

[0, 1, 1, 2, 3, 5, 8, 13, 21]

It would be simpler if I didn't have to use the takewhile . I've never used it before, so I'm not aware if there are special tricks to using it. It would be easier to just pass the count you desire into the base fibonacci generator and have it stop at the appropriate moment.