From 8ecf3ff0b61356161e1e0c930acc44257cb78ef1 Mon Sep 17 00:00:00 2001
From: Rebecca Chen <rechen@google.com>
Date: Mon, 3 May 2021 16:34:03 -0700
Subject: [PATCH] Make the `start` argument to itertools.count optional.
 (#5332)

start should be optional: https://docs.python.org/3.8/library/itertools.html#itertools.count
Directly changing the second overload to mark start as optional
generated a mypy error about overlapping overloads, but adding a third
overload seems to work.

I also noticed that `_NStep` was defined as a TypeVar when I think it
makes more sense for it to be a Union, so I went ahead and changed that
as well.
---
 stdlib/itertools.pyi | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/stdlib/itertools.pyi b/stdlib/itertools.pyi
index 3a77edafe..7fe08ca80 100644
--- a/stdlib/itertools.pyi
+++ b/stdlib/itertools.pyi
@@ -13,6 +13,7 @@ from typing import (
     Tuple,
     Type,
     TypeVar,
+    Union,
     overload,
 )
 from typing_extensions import Literal, SupportsIndex
@@ -20,7 +21,7 @@ from typing_extensions import Literal, SupportsIndex
 _T = TypeVar("_T")
 _S = TypeVar("_S")
 _N = TypeVar("_N", int, float, SupportsFloat, SupportsInt, SupportsIndex, SupportsComplex)
-_NStep = TypeVar("_NStep", int, float, SupportsFloat, SupportsInt, SupportsIndex, SupportsComplex)
+_Step = Union[int, float, SupportsFloat, SupportsInt, SupportsIndex, SupportsComplex]
 
 Predicate = Callable[[_T], object]
 
@@ -30,7 +31,9 @@ class count(Iterator[_N], Generic[_N]):
     @overload
     def __new__(cls) -> count[int]: ...
     @overload
-    def __new__(cls, start: _N, step: _NStep = ...) -> count[_N]: ...
+    def __new__(cls, start: _N, step: _Step = ...) -> count[_N]: ...
+    @overload
+    def __new__(cls, *, step: _N) -> count[_N]: ...
     def __next__(self) -> _N: ...
     def __iter__(self) -> Iterator[_N]: ...