fix constant polynomials bug

src/sage/modular/modform/element.py

@@ -3596,6 +3596,8 @@ def _homogeneous_to_polynomial(self, names, gens):
             x0
             sage: M.1._homogeneous_to_polynomial('E4, E6', gens)
             E6
+            sage: M(1/2).to_polynomial()
+            1/2
             sage: p = ((M.0)**3 + (M.1)**2)._homogeneous_to_polynomial('x', gens); p
             x0^3 + x1^2
             sage: M(p) == (M.0)**3 + (M.1)**2
@@ -3608,6 +3610,8 @@ def _homogeneous_to_polynomial(self, names, gens):
         M = self.parent()
         k = self.weight() #only if self is homogeneous
         poly_parent = M.polynomial_ring(names, gens)
+        if k == 0:
+            return poly_parent(self[k])
         monomials = M._monomials_of_weight(k, gens, poly_parent)
 
         # initialize the matrix of coefficients
@@ -3664,4 +3668,4 @@ def to_polynomial(self, names='x', gens=None):
             gens = M.gen_forms()
 
         # sum the polynomial of each homogeneous part
-        return sum(M(self[k])._homogeneous_to_polynomial(names, gens) for k in self.weights_list())
+        return sum(M(self[k])._homogeneous_to_polynomial(names, gens) for k in self.weights_list())

src/sage/modular/modform/ring.py

@@ -444,13 +444,18 @@ def from_polynomial(self, pol, gens=None):
             1 + q + q^2 + 27*q^3 + q^4 + 6*q^5 + O(q^6)
             sage: M.0 + M.1 + M.2
             1 + q + q^2 + 27*q^3 + q^4 + 6*q^5 + O(q^6)
+            sage: P = x.parent()
+            sage: M.from_polynomial(P(1/2))
+            1/2
 
         ..TODO::
 
             * add conversion for symbolic expressions?
         """
         if not isinstance(pol, (MPolynomial, Polynomial)):
             raise TypeError('`pol` must be a polynomial')
+        if pol.is_constant():
+            return self(pol.constant_coefficient())
         if gens is None:
             gens = self.gen_forms()
         dict = self._generators_variables_dictionnary(pol.parent(), gens)