In the Python 2.6.4 documentation "2. Built-in Functions" at
http://docs.python.org/library/functions.html,
the section about the xrange function (paragraph "CPython implementation detail") contains the following code:

islice(count(start, step), (stop-start+step-1)//step)

However, count only accepts one parameter, so that this solution does
not work. Furthermore, islice only accepts positive values for step.

Therefore, the code does not work.

I tested this with Python 2.5.4 and Python 2.6.4 on Debian Linux (AMD64).

Confirmed. The snippet works for 3.1 and 2.7a2.

from itertools import count, islice
irange = lambda start, stop, step: islice(count(start, step), (stop-start+step-1)//step)


The documentation needs update for 2.6 only.

This kind of snippet seems backward compatible for 2.6:
irange = lambda start, stop, step: islice(count(start), 0, stop, step)

The new snippet does not work for me:

>>> list(irange(-12, 20, 4))
[-12, -8, -4, 0, 4]

I have attached code that seems to work.
It is more than a snipped though.

Right. Insufficient test.

This snippet looks better, if we provide a replacement for 2.6.

>>> irange = lambda start, stop, step: islice(count(start), 0, stop-start, step)

The new snippet works better.

>>> list(irange(-12, 20, 4))
[-12, -8, -4, 0, 4, 8, 12, 16]

However, it does not like large or negative slices:

>>> list(irange(-2**65,2**65,2**61))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
OverflowError: long int too large to convert to int

>>> list(irange(32,2,-3))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
ValueError: Indices for islice() must be non-negative integers or None.

Perhaps the documentation can mention that.

You will prefer this one. It is as fast as the 2.7 version.
The restrictions are described in the itertools documentation.


from itertools import count, takewhile
irange = lambda start, stop, step: takewhile(lambda x: x<stop, (start+i*step for i in count()))

>>> list(irange(-2**65,2**65,2**61))
[-36893488147419103232L, ...

If you need also negative steps:

from itertools import count, takewhile
def irange(start, stop, step):
    if step < 0:
        cond = lambda x: x > stop
    else:
        cond = lambda x: x < stop
    return takewhile(cond, (start + i * step for i in count()))

Great solution!

Thank you

So could we replace

"If a larger range is needed, an alternate version can be crafted using the itertools module: islice(count(start, step), (stop-start+step-1)//step)."

by

"If a larger range is needed, an alternate version can be crafted using the itertools module: takewhile(lambda x: x<stop, (start+i*step for i in count()))."

Thanks, fixed in release26-maint r79796.