Lazy map function in Python

Question

Is there a way of making map lazy? Or is there another implementation of it built-in in Python?

I want something like this to work:

from itertools import count

for x in map(lambda x: x**2, count()):
    print x

Of course, the above code won't end, but I'd like just to enter any condition (or more complex logic) inside the for and stop at some point.

See here: Know when to be Lazy. In short: either use generator expressions or use the itertools module. — Robert Harvey
– Robert Harvey, Commented Mar 8, 2013 at 1:24
@RobertHarvey: Nice link. In fact, other than doing x*2 instead of x**2, the blog is pretty much perfectly tailored to this question! — abarnert
– abarnert, Commented Mar 8, 2013 at 1:27

abarnert · Accepted Answer · 2013-03-08 01:25:36Z

43

use itertools.imap on Python 2.x or upgrade to Python 3.x

You can also just use a simple generator expression that is far more pythonic:

foo = (x**2 for x in count())

edited Mar 8, 2013 at 1:25

abarnert

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answered Mar 8, 2013 at 1:24

JBernardo

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2 Comments

Gareth Latty Over a year ago

+1 for recommending the generator expression. Wherever you need lambda, map() isn't a good choice.

Oscar Mederos Over a year ago

Thanks for your reply. I just tried to make a simpler code sample for the question (when using map).

Pavel Anossov · Accepted Answer · 2013-03-08 01:24:02Z

4

itetools.imap is lazy.

In [3]: itertools.imap?
Type:       type
String Form:<type 'itertools.imap'>
Docstring:
imap(func, *iterables) --> imap object

Make an iterator that computes the function using arguments from
each of the iterables.  Like map() except that it returns
an iterator instead of a list and that it stops when the shortest
iterable is exhausted instead of filling in None for shorter
iterables.

answered Mar 8, 2013 at 1:24

Pavel Anossov

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