How to best dynamically make use of cypari2?

[hermann@stamm-wilbrandt.de]

I want to add a function to a RSA_numbers_factored python lib.

Determining sum of squares from "sqrt(-1) (mod p)" takes 42min with
Python "gcd()" on gaussian integers, but only few milliseconds for
388342-digit prime with Pari "halfgcd()" not available in Python.

So I want to use "halfgcd()" when cpyari2 is available, and
if not, I want to use Python "gcd()" on gaussian integers.

Here is demonstration without cypari2:

Type "help", "copyright", "credits" or "license" for more information.
> > > from dyn_cypari2 import *
> > > to_sum2sqs(12, 29)
(2, 5)
> > >

Here with cypari2, imported before importing dyn_cypari2:

Type "help", "copyright", "credits" or "license" for more information.
> > > import cypari2
> > > from dyn_cypari2 import *
> > > to_sum2sqs(12, 29)
pari
(5, -2)
> > >


I did it this way, is that OK?
Are there alternatives?

$ cat dyn_cypari2.py
from sys import modules

if "cypari2" in modules:
    import cypari2
    pari = cypari2.Pari()

    def to_sum2sqs(sqrtm1, p):
        print("pari")
        [M,V] = pari.halfgcd(sqrtm1, p)
        return V[1], M[1,0]
else:
    from sympy import gcd, I

    def to_sum2sqs(sqrtm1, p):
        return gcd(p, sqrtm1+I).as_real_imag()

def to_sqrtm1(xy, p):
    return xy[0] * pow(xy[1], -1, p) % p
$


Regards,

Hermann.