Exporting polynomials about mitm-sage HOT 4 OPEN

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@nthiery I think you misunderstood the question about the canonical function to build a polynomial.

I need a function f such that f(R, d) returns a polynomial in polynomial ring R with coefficient dictionary d.

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Sorry if I have not made my point clear. Up to curyfication, R is that function:

def f(R, d):
     return R(d)

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I've added the code as envisioned, but there are still some issues.

Nicolas recommends hooking directly into the pickling.
He expects the unpickling-based work flow will not work for nested objects.

The necessary code would be

from sage.rings.polynomial.multi_polynomial_ring_base import MPolynomialRing_base
from sage.rings.polynomial.polynomial_ring import PolynomialRing_general
from sage.rings.polynomial.polynomial_element import Polynomial

def pickle_polynomial(p):
   return lambda R,d: R(d), (p.parent(), p.dict())
copyreg.pickle(Polynomial, pickle_polynomial)
def pickle_polynomial_ring(p):
   return lambda br,vn: PolynomialRing(br,vn), (p.base_ring(), p.varnames())
copyreg.pickle(MPolynomialRing_base, pickle_polynomial_ring)

We should also double-check what common base class Sage polynomials have. I received different results at two different times.

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