python - Why is there no xrange function in Python3?

Question

Recently I started using Python3 and it's lack of xrange hurts.

Simple example:

1. Python2:

   from time import time as t
   def count():
     st = t()
     [x for x in xrange(10000000) if x%4 == 0]
     et = t()
     print et-st
   count()
   

2. Python3:

   from time import time as t
   
   def xrange(x):
   
       return iter(range(x))
   
   def count():
       st = t()
       [x for x in xrange(10000000) if x%4 == 0]
       et = t()
       print (et-st)
   count()
   

The results are, respectively:

1. 1.53888392448
2. 3.215819835662842

Why is that? I mean, why xrange has been removed? It's such a great tool to learn. For the beginners, just like myself, like we all were at some point. Why remove it? Can somebody point me to the proper PEP, I can't find it.

Answer 1

Some performance measurements, using timeit instead of trying to do it manually with time.

First, Apple 2.7.2 64-bit:

In [37]: %timeit collections.deque((x for x in xrange(10000000) if x%4 == 0), maxlen=0)
1 loops, best of 3: 1.05 s per loop

Now, python.org 3.3.0 64-bit:

In [83]: %timeit collections.deque((x for x in range(10000000) if x%4 == 0), maxlen=0)
1 loops, best of 3: 1.32 s per loop

In [84]: %timeit collections.deque((x for x in xrange(10000000) if x%4 == 0), maxlen=0)
1 loops, best of 3: 1.31 s per loop

In [85]: %timeit collections.deque((x for x in iter(range(10000000)) if x%4 == 0), maxlen=0) 
1 loops, best of 3: 1.33 s per loop

Apparently, 3.x range really is a bit slower than 2.x xrange. And the OP's xrange function has nothing to do with it. (Not surprising, as a one-time call to the __iter__ slot isn't likely to be visible among 10000000 calls to whatever happens in the loop, but someone brought it up as a possibility.)

But it's only 30% slower. How did the OP get 2x as slow? Well, if I repeat the same tests with 32-bit Python, I get 1.58 vs. 3.12. So my guess is that this is yet another of those cases where 3.x has been optimized for 64-bit performance in ways that hurt 32-bit.

But does it really matter? Check this out, with 3.3.0 64-bit again:

In [86]: %timeit [x for x in range(10000000) if x%4 == 0]
1 loops, best of 3: 3.65 s per loop

So, building the list takes more than twice as long than the entire iteration.

And as for "consumes much more resources than Python 2.6+", from my tests, it looks like a 3.x range is exactly the same size as a 2.x xrange—and, even if it were 10x as big, building the unnecessary list is still about 10000000x more of a problem than anything the range iteration could possibly do.

And what about an explicit for loop instead of the C loop inside deque?

In [87]: def consume(x):
   ....:     for i in x:
   ....:         pass
In [88]: %timeit consume(x for x in range(10000000) if x%4 == 0)
1 loops, best of 3: 1.85 s per loop

So, almost as much time wasted in the for statement as in the actual work of iterating the range.

If you're worried about optimizing the iteration of a range object, you're probably looking in the wrong place.

---

Meanwhile, you keep asking why xrange was removed, no matter how many times people tell you the same thing, but I'll repeat it again: It was not removed: it was renamed to range, and the 2.x range is what was removed.

Here's some proof that the 3.3 range object is a direct descendant of the 2.x xrange object (and not of the 2.x range function): the source to 3.3 range and 2.7 xrange. You can even see the change history (linked to, I believe, the change that replaced the last instance of the string "xrange" anywhere in the file).

So, why is it slower?

Well, for one, they've added a lot of new features. For another, they've done all kinds of changes all over the place (especially inside iteration) that have minor side effects. And there'd been a lot of work to dramatically optimize various important cases, even if it sometimes slightly pessimizes less important cases. Add this all up, and I'm not surprised that iterating a range as fast as possible is now a bit slower. It's one of those less-important cases that nobody would ever care enough to focus on. No one is likely to ever have a real-life use case where this performance difference is the hotspot in their code.