This patch makes this work:
>>> range(4) == range(4)
True

This is in similar fashion to dict.keys
>>> {1:2}.keys() == {1:2}.keys()
True

Tests included.

Your patch does not check the return values of PyObject_RichCompare
calls for NULL.  This is probably ok given the current restrictions on
the type of start/step/stop, but adding the null checks will make the
code more robust.

I have no opinion whether the patch should go in, but in any case:

* You should use PyObject_RichCompareBool.
* You should error-check the returns of those.

Ah yes, and variable declarations must be at the start of a block, else
MSVC (and possible other compilers) will complain.

Actually, the patch contains an exploitable bug:

$ cat x.py
class X(int):
    def __eq__(self, other):
        raise ValueError

x = range(X(1),X(10),X(1))
x == x

$ ./python x.py
Segmentation fault (core dumped)

On Wed, Apr 9, 2008 at 6:08 PM, Georg Brandl <report@bugs.python.org> wrote:

>  Ah yes, and variable declarations must be at the start of a block

You can avoid the need for extra local variables by declaring
range_richcompare arguments as rangeobject* and casting the function
to richcmpfunc in type initialization.  See for example
weakrefobject.c implementation.

Thanks for the help.

And, a __hash__ method should be added.

On Thu, Apr 10, 2008 at 3:16 AM, Amaury Forgeot d'Arc
<report@bugs.python.org> wrote:
>
>  And, a __hash__ method should be added.
>

See issue2268 for a slice.__hash__ implementation which should work
for range as well.

Including __hash__ in this patch. Alexander, thanks; that could worked
perfectly.

Once the review for this is completed I have no objection to it going in.

The patch does not add unit tests for hash(range(..)).  I would actually 
test that hash(range(a,b,c)) == hash((a,b,c)) for various values of a, 
b,  and c.

A nit-pick: while I personally like the coding style with line breaks 
before binary ops, python C style appears to be the opposite.

Code review:

Objects/rangeobject.c:

line 259-260:
AFAIK register is completely useless in this day and age; drop it.

line 296 and further:
Please move the || and && operators to the end of the previous line.

line 313 etc:
This all fits on one line.

Line 319-320:
Ditto.

Line 361:
Please make the comment line up.

Lib/test/test_builtin.py:
I'd like to see another test indicating which of the following is true:

range(0, 11, 2) == range(0, 10, 2)

or

range(0, 11, 2) != range(0, 10, 2)

(since they produce the same sequence of values).

Also, add a test for __hash__() of a range().

Thanks for the review! How does this look?

Close.  Just a few nits:

line 298:
These two fit on one line. Possibly the whole thing fits. If it doesn't,
the '{' should be on a line by itself.

(Are you aware of PEP 7, C code guidelines? I realize it's incomplete,
but you should still study it -- if you find things it's missing,
propose a patch to update it...!)

line 356:
Still not aligned.

Are you sure you're viewing this with a fixed-width font and tabs set to
8 spaces?

The patch uses Py_SIZE(v) on a rangeobject?
I guess you borrowed too much from the tuple implementation ;-)

Attaching a new patch. I'm not really sure what to do about the hash. I
just removed the Py_SIZE parts. I hope that's OK.

On Tue, Apr 15, 2008 at 10:21 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>  I'm not really sure what to do about the hash. I
>  just removed the Py_SIZE parts. I hope that's OK.

If you want to maintain hash(range(a,b,c)) == hash((a,b,c)) invariant,
you need to replace len with 3, not 0.  While I think your latest
implementation is ok, reproducing tuple hash will make correctness
more obvious.

Thanks Alexander. Trying again.

With range_eq5.patch applied on an x86-64 Linux box:

$ ./python 
Python 3.0a4+ (py3k:62359M, Apr 16 2008, 13:36:31) 
[GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> hash(range(1,2,3))
-4094131973719604815
>>> hash(((1,2,3)))
2528502973977326415


Also, please fix indentation at Objects/rangeobject.c:271.

range_eq5.patch fails to reproduce tuple hash because in the tuple hash
implementation the len variable varies between iterations over items. I
would just use literals for the three different values of mult rather
than copying mult += (long)(82520L + 3 + 3).

Why would you want to have hash(range(a,b,c)) == hash((a,b,c)) ?
It'd be more logical to have hash(range(a,b,c)) ==
hash(tuple(range(a,b,c))) ... which is not the same thing at all :)

On Wed, Apr 16, 2008 at 3:40 PM, Antoine Pitrou <report@bugs.python.org> wrote:
..
>  Why would you want to have hash(range(a,b,c)) == hash((a,b,c)) ?

No particular reason other than that this is easy to test and gives
some assurance that hash values are reasonable.

>  It'd be more logical to have hash(range(a,b,c)) ==
>  hash(tuple(range(a,b,c))) ... which is not the same thing at all :)

No, this is not correct because range(..) == range(..) is not the same
as tuple(range(..)) == tuple(range(..)) in the proposed
implementation.  See Guido's example above and Benjamin's test case in
the eq5 patch.

When I was trying to get the hash to resemble tuple's, it occurred to
me: Why not just hash a tuple?

On Wed, Apr 16, 2008 at 9:24 PM, Benjamin Peterson wrote:
..
> Why not just hash a tuple?

There are a few reasons, but neither is good enough to have another
round of code review :-)

1. It is strange to have the hash function allocate new objects.  If
that was a type frequently used as a dict key, I would be concerned
about a possibility that dictionary lookup may trigger gc.

2. While reproducing hash(tuple) is a good starting point, there may
be a reason to choose different values for the magic constants.

3. If you don't want to mess with hash(tuple) complexity, a simple xor
of start/stop/step hashes (maybe with a check to prevent accidental -1
return) should be good enough.

No need to get hung-up on the hash function.  I can fix that up after a 
checkin and use something simple like: hashvalue = (start*prime1+seqlen)
*prime2+step.  That doesn't involve object creation and it produces the 
same hash value for range(5,10,2) and range(5,9,2) which are equivalent.

> It produces the same hash value for range(5,10,2) and range(5,9,2) 
> which are equivalent.
If "equivalent" means "__eq__", they are not.

This does not invalidate your formula, of course: different objects may
have the same hash.
It is also simple to understand: just mix the numbers together.

That was a typo.  The pair was range(5,10,2)-->[5, 7, 9] and range
(5,11,2)-->[5, 7, 9].  The formula works because the it uses seqlen 
instead of a stop value.

Attaching a new patch with a few improvements. I tried to implemented
Raymond's hash function (I think this is how the math should be done.).
The rich compare function now short-circuits when a value isn't equal.
Also, I made ranges with the same set of integers equal even if the stop
is different.

Comments?

about range_eq7.patch:

- Did you change your mind about range equality?
  range(0,10,2) == range(0,9,2)
seems True now; it was not with range_eq6.patch

- The hash function will fail with big values (and wrongly returns a
value even when an exception is set). I suggest to call PyObject_Hash
instead of PyNumber_AsSsize_t.

- Now that you short-circuit the comparison, it is enough to have only
one boolean variable (is_equal), which may replace all uses of
start_same, stop_same and step_same.

On Fri, Apr 25, 2008 at 5:31 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>  Comments?

In the range_hash function, len, start, step locals should be declared
Py_ssize_t, not long. Also, you can use range_length() instead of
PyObject_Size() and you need to clear error if you get len == -1.

See issue2690. With your patch,

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: Python int too large to convert to C ssize_t

but

True

You can avoid this problem by using range_length_obj instead of
PyObject_Size in range_richcompare.

It looks like e-mail processor eats '>>>' examples.  My examples were

>>> range(2**100) == range(2**100+1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: Python int too large to convert to C ssize_t

and

>>> range(2**100) == range(2**100)
True

On Fri, Apr 25, 2008 at 5:57 PM, Amaury Forgeot d'Arc
<report@bugs.python.org> wrote:
..
>  - Did you change your mind about range equality?
>   range(0,10,2) == range(0,9,2)
>  seems True now; it was not with range_eq6.patch
>

This makes me think: what would you say to an idea to normalize ranges
in constructor so that range(5,10,2) returns range(5,11,2).

Thanks for the help.

Yes, after thinking for a while, I decided that range equality should
represent the set of integers and not the values in the constructor.
Normalization would be a good idea, but I think that's another issue
I'll tackle after this.

Now you get an error for hashing a huge range.

On Fri, Apr 25, 2008 at 3:36 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>
>  Benjamin Peterson <musiccomposition@gmail.com> added the comment:
>
>  Thanks for the help.
>
>  Yes, after thinking for a while, I decided that range equality should
>  represent the set of integers and not the values in the constructor.
>  Normalization would be a good idea, but I think that's another issue
>  I'll tackle after this.

The two go hand-in-hand; you shouldn't have two range() objects that
compare equal and yet have different stop attribute values.

>  Now you get an error for hashing a huge range.
>
>  Added file: http://bugs.python.org/file10110/range_eq8.patch
>
>
>
>  __________________________________
>  Tracker <report@bugs.python.org>
>  <http://bugs.python.org/issue2603>
>  __________________________________
>

On Fri, Apr 25, 2008 at 5:38 PM, Guido van Rossum
<report@bugs.python.org> wrote:
>
>  Guido van Rossum <guido@python.org> added the comment:
>  The two go hand-in-hand; you shouldn't have two range() objects that
>  compare equal and yet have different stop attribute values.

If it makes any difference, the attributes aren't even available through Python.

On Fri, Apr 25, 2008 at 3:41 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>  If it makes any difference, the attributes aren't even available through Python.

But they are deducible via the str() or repr(). And IMO they *should*
be available.

I think I'd be okay with normalization on creation, so that range(0,
5, 2) returns range(0, 6, 2). Hm, but isn't that odd? Why not the
other way around?

Here's a normalizing patch. It breaks the repr tests because the numbers
change.

On Fri, Apr 25, 2008 at 7:10 PM, Guido van Rossum
<report@bugs.python.org> wrote:
..
>  I think I'd be okay with normalization on creation, so that range(0,
>  5, 2) returns range(0, 6, 2). Hm, but isn't that odd? Why not the
>  other way around?

I find it natural to have start + len*step  = stop invariant rather
than start +(len-1)*step + 1 = stop.  I may be influenced by C++ (STL)
tradition of giving preference to "i != stop" over "i < stop"
condition so that algorithms support iterators that are not ordered.
I also believe some algorithmic simplifications will be possible with
 start + len*step  = stop invariant.

With respect to range_eq8_normalize2.patch, it is unusual to have a 
function that consumes a reference to its argument.  I would combine 
normalize_stop with PyNumber_Index and make it similar to validate_step 
with respect to reference counting.

Note that if you choose stop = start + len*step normaization, you will 
not need to create 'one' in normalize_stop.

With your patch I see

>>> range(0,6,2)
range(0, 6, 2)
>>> range(0,5,2)
range(0, 5, 2)

I would expect one of these ranges normalized.

... and your patch produces wrong results:

>>> list(range(5,0,-2)) # expected [5, 3, 1]
[5, 3]

See my patch in issue2690 for a way to compute length correctly in 
range_new.

I merged your range calculating code. After reading that bug report, I
think we need to start a discussion on python-dev about range size
constraints before moving forward any more. (We have people implementing
different things under different assumptions left and right.)

Methinks this one is also better rejected.  Please reopen if you feel
strongly.