[Python-checkins] gh-95337: update TypeVarTuple example (#95338)

[JelleZijlstra]

https://github.com/python/cpython/commit/07f12b5c1567581aa77d523e462b0e7f75c1f05c
commit: 07f12b5c1567581aa77d523e462b0e7f75c1f05c
branch: main
author: Adrian Garcia Badaracco <1755071+adriangb at users.noreply.github.com>
committer: JelleZijlstra <jelle.zijlstra at gmail.com>
date: 2022-08-30T07:57:03-07:00
summary:

gh-95337: update TypeVarTuple example (#95338)

Co-authored-by: Alex Waygood <Alex.Waygood at Gmail.com>

files:
M Doc/library/typing.rst

diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index c6dd6976f23..0939973cf24 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -1305,20 +1305,25 @@ These are not used in annotations. They are building blocks for creating generic
         T = TypeVar('T')
         Ts = TypeVarTuple('Ts')
 
-        def remove_first_element(tup: tuple[T, *Ts]) -> tuple[*Ts]:
-            return tup[1:]
+        def move_first_element_to_last(tup: tuple[T, *Ts]) -> tuple[*Ts, T]:
+            return (*tup[1:], tup[0])
 
         # T is bound to int, Ts is bound to ()
-        # Return value is (), which has type tuple[()]
-        remove_first_element(tup=(1,))
+        # Return value is (1,), which has type tuple[int]
+        move_first_element_to_last(tup=(1,))
 
         # T is bound to int, Ts is bound to (str,)
-        # Return value is ('spam',), which has type tuple[str]
-        remove_first_element(tup=(1, 'spam'))
+        # Return value is ('spam', 1), which has type tuple[str, int]
+        move_first_element_to_last(tup=(1, 'spam'))
 
         # T is bound to int, Ts is bound to (str, float)
-        # Return value is ('spam', 3.0), which has type tuple[str, float]
-        remove_first_element(tup=(1, 'spam', 3.0))
+        # Return value is ('spam', 3.0, 1), which has type tuple[str, float, int]
+        move_first_element_to_last(tup=(1, 'spam', 3.0))
+
+        # This fails to type check (and fails at runtime)
+        # because tuple[()] is not compatible with tuple[T, *Ts]
+        # (at least one element is required)
+        move_first_element_to_last(tup=())
 
     Note the use of the unpacking operator ``*`` in ``tuple[T, *Ts]``.
     Conceptually, you can think of ``Ts`` as a tuple of type variables