python versions of bisect_right and bisect_left can fail guarantees

Bug report

Bug description:

The documentation for bisect.bisect_right and bisect.bisect_left states in both cases that

The returned insertion point ip partitions the array a into two slices such that
all(elem <= x for elem in a[lo : ip]) is true for the left slice and
all(elem > x for elem in a[ip : hi]) is true for the right slice.

But in the python version of bisect, that guarantee can fail:

import sys
import _bisect as c_bisect
sys.modules['_bisect'] = None
import bisect as py_bisect

def check_guarantees(bisect_func, a, x, lo, hi):
    ip = bisect_func(a, x, lo, hi)
    check_pass = True 
    if not all(elem <= x for elem in a[lo : ip]): 
        print("\tFAIL. Elements of",a[lo:ip]," are not <=",x)
        check_pass = False
    if not all(elem > x for elem in a[ip : hi]):
        print("\tFAIL. Elements of",a[ip:hi]," are not >",x)
        check_pass = False
    if  check_pass: print("\tOk")

#print(c_bisect.bisect_right) # <built-in function bisect_right>
#print(py_bisect.bisect_right) # <function bisect_right at 0x0000023164AE7C40>

a = [0,1,2,3,4]
x = 8
lo = 3
hi = -1

print("bisect_left from C")
check_guarantees(c_bisect.bisect_left, a, x, lo, hi)

print("bisect_left from python")
check_guarantees(py_bisect.bisect_left, a, x, lo, hi)

print("bisect_right from C")
check_guarantees(c_bisect.bisect_right, a, x, lo, hi)

print("bisect_right from python")
check_guarantees(py_bisect.bisect_right, a, x, lo, hi)

Attempt this online
Outputs :

bisect_left from C
	Ok
bisect_left from python
	FAIL. Elements of [3]  are not > 8
bisect_right from C
	Ok
bisect_right from python
	FAIL. Elements of [3]  are not > 8

I think there is an extra check in the C version for hi < 0.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-125915