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