How can I limit iterations of a loop in Python? - TagMerge
3How can I limit iterations of a loop in Python?How can I limit iterations of a loop in Python?

How can I limit iterations of a loop in Python?

Asked 1 years ago
3 answers

How can I limit iterations of a loop in Python?

for index, item in enumerate(items):
    if index == limit:

Is there a shorter, idiomatic way to write the above? How?

Including the index

zip stops on the shortest iterable of its arguments. (In contrast with the behavior of zip_longest, which uses the longest iterable.)

range can provide a limited iterable that we can pass to zip along with our primary iterable.

So we can pass a range object (with its stop argument) to zip and use it like a limited enumerate.

zip(range(limit), items)

Using Python 3, zip and range return iterables, which pipeline the data instead of materializing the data in lists for intermediate steps.

for index, item in zip(range(limit), items):
    print(index, item)

To get the same behavior in Python 2, just substitute xrange for range and itertools.izip for zip.

from itertools import izip
for index, item in izip(xrange(limit), items):

If not requiring the index, itertools.islice

You can use itertools.islice:

for item in itertools.islice(items, 0, stop):

which doesn't require assigning to the index.

Composing enumerate(islice(items, stop)) to get the index

As Pablo Ruiz Ruiz points out, we can also compose islice with enumerate.

for index, item in enumerate(islice(items, limit)):
    print(index, item)

Why isn't this built into enumerate?

Here's enumerate implemented in pure Python (with possible modifications to get the desired behavior in comments):

def enumerate(collection, start=0):  # could add stop=None
    i = start
    it = iter(collection)
    while 1:                         # could modify to `while i != stop:`
        yield (i, next(it))
        i += 1

The above would be less performant for those using enumerate already, because it would have to check whether it is time to stop every iteration. We can just check and use the old enumerate if don't get a stop argument:

_enumerate = enumerate

def enumerate(collection, start=0, stop=None):
    if stop is not None:
        return zip(range(start, stop), collection)
    return _enumerate(collection, start)

This extra check would have a slight negligible performance impact.

As to why enumerate does not have a stop argument, this was originally proposed (see PEP 279):

This function was originally proposed with optional start and stop arguments. GvR [Guido van Rossum] pointed out that the function call enumerate(seqn, 4, 6) had an alternate, plausible interpretation as a slice that would return the fourth and fifth elements of the sequence. To avoid the ambiguity, the optional arguments were dropped even though it meant losing flexibility as a loop counter. That flexibility was most important for the common case of counting from one, as in:

for linenum, line in enumerate(source,1):  print linenum, line

So apparently start was kept because it was very valuable, and stop was dropped because it had fewer use-cases and contributed to confusion on the usage of the new function.

Avoid slicing with subscript notation

Another answer says:

Why not simply use

for item in items[:limit]: # or limit+1, depends

Here's a few downsides:

  • It only works for iterables that accept slicing, thus it is more limited.
  • If they do accept slicing, it usually creates a new data structure in memory, instead of iterating over the reference data structure, thus it wastes memory (All builtin objects make copies when sliced, but, for example, numpy arrays make a view when sliced).
  • Unsliceable iterables would require the other kind of handling. If you switch to a lazy evaluation model, you'll have to change the code with slicing as well.

You should only use slicing with subscript notation when you understand the limitations and whether it makes a copy or a view.


I would presume that now the Python community knows the usage of enumerate, the confusion costs would be outweighed by the value of the argument.

Until that time, you can use:

for index, element in zip(range(limit), items):


for index, item in enumerate(islice(items, limit)):

or, if you don't need the index at all:

for element in islice(items, 0, limit):

And avoid slicing with subscript notation, unless you understand the limitations.

Source: link


You can use itertools.islice for this. It accepts start, stop and step arguments, if you're passing only one argument then it is considered as stop. And it will work with any iterable.

itertools.islice(iterable, stop)
itertools.islice(iterable, start, stop[, step])


>>> from itertools import islice
>>> items = list(range(10))
>>> limit = 5
>>> for item in islice(items, limit):
    print item,
0 1 2 3 4

Example from docs:

islice('ABCDEFG', 2) --> A B
islice('ABCDEFG', 2, 4) --> C D
islice('ABCDEFG', 2, None) --> C D E F G
islice('ABCDEFG', 0, None, 2) --> A C E G

Source: link


Why not simply use

for item in items[:limit]: # or limit+1, depends
    print(item)    # or whatever function of that item.

This will only work for some iterables, but since you specified Lists, it works.

It doesn't work if you use Sets or dicts etc.

Source: link

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