bisect_left should be similar to bisect_right. However, the current implement didn't reflect it. A little bit update for the bisect_left function could make the user easy to understand their relation.

def bisect_left(a, x, lo=0, hi=None):
    if lo < 0:
        raise ValueError('lo must be non-negative')
    if hi is None:
        hi = len(a)
    while lo < hi:
        mid = (lo+hi)//2
        # <= should be the only difference between bisect_left and bisect_right
        if x <= a[mid]: hi = mid
        else: lo = mid+1
    return lo

So as far as possible, CPython only uses __lt__ ("<") element comparisons for its order-sensitive algorithms.  This is documented for list.sort(), but the bisect and heapq modules strive to do the same.

The point is to minimize the number of comparison methods a new type needs to implement to participate (and "just one:  __lt__" is ideal).

Your change would require they implement "__le__" too, so is unlikely to be accepted for CPython.

I concur with Tim.

Thank you for the suggestion though.

New changeset 3c88199e0be352c0813f145d7c4c83af044268aa by Miss Skeleton (bot) (Raymond Hettinger) in branch 'master':
bpo-38626:  Add comment explaining why __lt__ is used. (GH-16978)
https://github.com/python/cpython/commit/3c88199e0be352c0813f145d7c4c83af044268aa