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Why doesn't len ​​() support iterators?

Many of the built-in Python functions ( any(), all(), sum()to indicate some) take iteration, but why len()not?

You can always use it sum(1 for i in iterable)as an equivalent, but why len()doesn't it take iterations in the first place?

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2 answers

Many iterations are determined by expressions of generators that do not have a well-defined len. Take the following, which is repeated forever:

def sequence(i=0):
    while True:
        i+=1
        yield i

, , . sum. , - , .

, sum(1 for i in iterable), , . , i- , . , , . , ( list tuple ) - , len.

+10

:

def forever():
    while True:
        yield 1

. , , , (-, , , ) , , :

len(list(the_iterable))

, : ? , .

, , :

num_elements = len(the_iterable)
for element in the_iterable:
    ...

:

num_elements = 0
for element in the_iterable:
    num_elements += 1
    ...

, , :

num_relevant = len(x for x in xrange(100000) if x%14==0)

( ):

num_relevant = len([x for x in xrange(100000) if x%14==0])

sum, , , , , :

num_relevant = sum(1 for _ in (x for x in xrange(100000) if x%14==0))

, , , :

def exhaustive_len(iterable):
    length = 0
    for _ in iterable: length += 1
    return length

exhaustive_len(x for x in xrange(100000) if x%14==0)

, , , , :

def yield_numbers():
    yield 1; yield 2; yield 3; yield 5; yield 7

the_nums = yield_numbers()
total_nums = exhaustive_len(the_nums)
for num in the_nums:
    print num

exhaustive_len .

EDIT: Ah exhaustive_len(open("file.txt")), , , , list.

+6

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