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Personally I would try to find a way to avoid the symbolic ring (with var, etc.) altogether.

If you want to use the symbolic ring, then you have to do the conversion to a LinearFunction somehow.

Assuming this setup:

sage: var('x,y')
sage: expr = 2*x + y
sage: p = MixedIntegerLinearProgram(maximization=False)
sage: pvar = p.new_variable(real=True, nonnegative=True)

You can do it e.g. like this:

def linear_function_from_symbolic(expr, pvar):
    constant_coeff = pvar.mip().base_ring()(expr.subs(dict(zip(expr.variables(), [0]*len(expr.variables())))))
    return sum(expr.coefficient(v)*pvar[v] for v in expr.variables()) + constant_coeff

and use it as follows:

sage: linear_function_from_symbolic(expr, pvar)
2*x_0 + x_1

You can convert constraints too:

def linear_constraint_from_symbolic(expr, pvar):
    return expr.operator()(linear_function_from_symbolic(expr.lhs(), pvar),
                           linear_function_from_symbolic(expr.rhs(), pvar))

for example:

sage: linear_constraint_from_symbolic(x <= 1, pvar)
x_0 <= 1

It is reasonable that there is no coercion from the Symbolic Ring to Linear functions over RDF, because it would not be well-defined with respect to the ordering of variables: e.g. coercion of two functions of different variables would depend on the order in which you do it. The function above "suffers" from the same (but probably you don't care).