Bisect, is it possible to work with descending sorted lists?

Probably the easiest thing is to borrow the code from the library and make your own version

def reverse_insort(a, x, lo=0, hi=None):
    """Insert item x in list a, and keep it reverse-sorted assuming a
    is reverse-sorted.

    If x is already in a, insert it to the right of the rightmost x.

    Optional args lo (default 0) and hi (default len(a)) bound the
    slice of a to be searched.
    """
    if lo < 0:
        raise ValueError('lo must be non-negative')
    if hi is None:
        hi = len(a)
    while lo < hi:
        mid = (lo+hi)//2
        if x > a[mid]: hi = mid
        else: lo = mid+1
    a.insert(lo, x)

In Python 3.10, insort gain a key parameter, from the documentation:

key specifies a key function of one argument that is used to extract a comparison key from each input element. The default value is None (compare the elements directly).

So, to insert in a descending sorted list do:

import bisect

x = [1.0,2.0,3.0,4.0]
x.reverse()           
bisect.insort(x, 2.5, key=lambda x: -1 * x)
print(x) 

Output

[4.0, 3.0, 2.5, 2.0, 1.0]

It is better to stick to the built-in python library, per @reve_etrange's comment unless you are working on a platform that allows you to use alternative C-extension implementations which might be faster. The following would leverage the built-in Python bisect module.

ASC = 'asc'
DESC = 'desc'

def bisect_left(l, e, lo=None, hi=None, order=ASC):
    """Find first index, starting from left, to insert e."""
    lo = lo or 0
    hi = hi or len(l)
    if order not in (ASC, DESC):
        raise ValueError('order must either be asc or desc.')
    if order == ASC:
        return bisect.bisect_left(l, e, lo, hi)
    return len(l) - bisect.bisect_right(l[::-1], e, lo, hi)


def bisect_right(l, e, lo=None, hi=None, order=ASC):
    """Find first index, starting from right, to insert e."""
    lo = lo or 0
    hi = hi or len(l)
    if order not in (ASC, DESC):
        raise ValueError('order must either be asc or desc.')
    if order == ASC:
        return bisect.bisect_right(l, e, lo, hi)
    return len(l) - bisect.bisect_left(l[::-1], e, lo, hi)