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gh-95337: update TypeVarTuple example (#95338)

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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
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Adrian Garcia Badaracco 2022-08-30 09:57:03 -05:00 committed by GitHub
parent 22ed5233b7
commit 07f12b5c15
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1 changed files with 13 additions and 8 deletions
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21
Doc/library/typing.rst
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@ -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