From 0e73124949d364003ded3b4966fa11dd404288b9 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Fri, 3 May 2024 11:37:32 -0500
Subject: [PATCH 1/4] gh-118476: Fix corner cases in islice() rough equivalent.

---
 Doc/library/itertools.rst | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 9a5cb8be37d3496..9ffa431706b6724 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -496,7 +496,8 @@ loops that truncate the stream.
    data where the internal structure has been flattened (for example, a
    multi-line report may list a name field on every third line).
 
-   Roughly equivalent to::
+   Rough equivalent without error checking for negative indices or *step*
+   size of zero::
 
       def islice(iterable, *args):
           # islice('ABCDEFG', 2) → A B
@@ -504,24 +505,13 @@ loops that truncate the stream.
           # islice('ABCDEFG', 2, None) → C D E F G
           # islice('ABCDEFG', 0, None, 2) → A C E G
           s = slice(*args)
-          start, stop, step = s.start or 0, s.stop or sys.maxsize, s.step or 1
-          it = iter(range(start, stop, step))
-          try:
-              nexti = next(it)
-          except StopIteration:
-              # Consume *iterable* up to the *start* position.
-              for i, element in zip(range(start), iterable):
-                  pass
-              return
-          try:
-              for i, element in enumerate(iterable):
-                  if i == nexti:
-                      yield element
-                      nexti = next(it)
-          except StopIteration:
-              # Consume to *stop*.
-              for i, element in zip(range(i + 1, stop), iterable):
-                  pass
+          start, stop, step = s.start or 0, s.stop, s.step or 1
+          indices = count() if stop is None else range(max(stop, start))
+          nexti = start
+          for i, element in zip(indices, iterable):
+              if i == nexti:
+                  yield element
+                  nexti += step
 
 
 .. function:: pairwise(iterable)

From 09f779d3bc999c3bb421c574144e0f0e038e9013 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Fri, 3 May 2024 11:38:40 -0500
Subject: [PATCH 2/4] Grammar fix

---
 Doc/library/itertools.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 9ffa431706b6724..54bda06b340a539 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -496,7 +496,7 @@ loops that truncate the stream.
    data where the internal structure has been flattened (for example, a
    multi-line report may list a name field on every third line).
 
-   Rough equivalent without error checking for negative indices or *step*
+   Rough equivalent without error checking for negative indices or a *step*
    size of zero::
 
       def islice(iterable, *args):

From 74e03c91c8cc255953fb3d1017e0b80418c5aad2 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Fri, 3 May 2024 12:55:50 -0500
Subject: [PATCH 3/4] Improve argument checking

---
 Doc/library/itertools.rst | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 54bda06b340a539..25859d7197ea378 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -496,8 +496,7 @@ loops that truncate the stream.
    data where the internal structure has been flattened (for example, a
    multi-line report may list a name field on every third line).
 
-   Rough equivalent without error checking for negative indices or a *step*
-   size of zero::
+   Roughly equivalent to::
 
       def islice(iterable, *args):
           # islice('ABCDEFG', 2) → A B
@@ -505,7 +504,11 @@ loops that truncate the stream.
           # islice('ABCDEFG', 2, None) → C D E F G
           # islice('ABCDEFG', 0, None, 2) → A C E G
           s = slice(*args)
-          start, stop, step = s.start or 0, s.stop, s.step or 1
+          start = 0 if s.start is None else s.start
+          stop = s.stop
+          step = 1 if s.step is None else s.step
+          if start < 0 or (stop is not None and stop < 0) or step <= 0:
+              raise ValueError
           indices = count() if stop is None else range(max(stop, start))
           nexti = start
           for i, element in zip(indices, iterable):

From 7bc98b8026fbc127ebcc850e6a7e198d01ffe74c Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Fri, 3 May 2024 15:31:33 -0500
Subject: [PATCH 4/4] Improve variable name

---
 Doc/library/itertools.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 25859d7197ea378..8d946d01db379f3 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -510,11 +510,11 @@ loops that truncate the stream.
           if start < 0 or (stop is not None and stop < 0) or step <= 0:
               raise ValueError
           indices = count() if stop is None else range(max(stop, start))
-          nexti = start
+          next_i = start
           for i, element in zip(indices, iterable):
-              if i == nexti:
+              if i == next_i:
                   yield element
-                  nexti += step
+                  next_i += step
 
 
 .. function:: pairwise(iterable)