python versions of bisect_right and bisect_left can fail guarantees

[mike-neergaard]

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

[tomasr8]

I think neither the Python nor the C implementation are actually meant to work with a negative hi argument. The C implementation does special case hi == -1 but that's only because it is the default value produced by argument clinic when you omit hi (same for the Python case when hi is None).

We could either make it an error to pass a negative value as is the case with lo currently, or fix it to work with negative values.

cc @rhettinger since you last worked on this module

[mike-neergaard]

Bounds checking sounds great to me.