remove the comments about "being expensive"

src/sage/rings/polynomial/polynomial_modn_dense_ntl.pyx

@@ -369,7 +369,7 @@ cdef class Polynomial_dense_mod_n(Polynomial):
 
     # Polynomial_singular_repr stuff, copied due to lack of multiple inheritance
     def _singular_(self, singular=singular_default, force=False):
-        self.parent()._singular_(singular, force=force).set_ring()  # this is expensive
+        self.parent()._singular_(singular, force=force).set_ring()
         if self.__singular is not None:
             try:
                 self.__singular._check_valid()
@@ -380,7 +380,7 @@ cdef class Polynomial_dense_mod_n(Polynomial):
         return self._singular_init_(singular)
 
     def _singular_init_(self, singular=singular_default, force=False):
-        self.parent()._singular_(singular, force=force).set_ring()  # this is expensive
+        self.parent()._singular_(singular, force=force).set_ring()
         self.__singular = singular(str(self))
         return self.__singular
 

src/sage/rings/polynomial/polynomial_rational_flint.pyx

@@ -334,7 +334,7 @@ cdef class Polynomial_rational_flint(Polynomial):
 
     def _singular_(self, singular=singular_default):
         """
-        Return a Singular representation of self.
+        Return a Singular representation of ``self``.
 
         INPUT:
 
@@ -347,7 +347,7 @@ cdef class Polynomial_rational_flint(Polynomial):
             sage: singular(f)
             3*x^2+2*x+5
         """
-        self._parent._singular_(singular).set_ring()  # Expensive!
+        self._parent._singular_(singular).set_ring()
         return singular(self._singular_init_())
 
     cpdef list list(self, bint copy=True):

src/sage/rings/polynomial/polynomial_singular_interface.py

@@ -49,6 +49,7 @@
 
 import sage.rings.finite_rings.finite_field_constructor
 
+
 def _do_singular_init_(singular, base_ring, char, _vars, order):
     r"""
     Implementation of :meth:`PolynomialRing_singular_repr._singular_init_`.
@@ -336,7 +337,7 @@ def _singular_(self, singular=singular):
             if sage.rings.finite_rings.finite_field_constructor.is_FiniteField(self.base_ring()) or \
                     (number_field.number_field_base.is_NumberField(self.base_ring()) and self.base_ring().is_absolute()):
                 R.set_ring()  # sorry for that, but needed for minpoly
-                if  singular.eval('minpoly') != f"({self.__minpoly})":
+                if singular.eval('minpoly') != f"({self.__minpoly})":
                     singular.eval(f"minpoly={self.__minpoly}")
                     self.__minpoly = singular.eval('minpoly')[1:-1]
             return R
@@ -490,8 +491,7 @@ def _singular_func(self, singular=singular):
         sage: R(h^20) == f^20
         True
     """
-    self.parent()._singular_(singular).set_ring()  # this is expensive
-
+    self.parent()._singular_(singular).set_ring()
     try:
         self.__singular._check_valid()
         if self.__singular.parent() is singular:
@@ -508,6 +508,6 @@ def _singular_init_func(self, singular=singular):
 
     Use ``self._singular_()`` instead.
     """
-    self.parent()._singular_(singular).set_ring()  # this is expensive
+    self.parent()._singular_(singular).set_ring()
     self.__singular = singular(str(self))
     return self.__singular

src/sage/rings/polynomial/polynomial_template.pxi

@@ -767,7 +767,7 @@ cdef class Polynomial_template(Polynomial):
 
     def _singular_(self, singular=singular_default):
         r"""
-        Return Singular representation of this polynomial
+        Return Singular representation of this polynomial.
 
         INPUT:
 
@@ -780,5 +780,5 @@ cdef class Polynomial_template(Polynomial):
             sage: singular(f)
             3*x^2+2*x-2
         """
-        self.parent()._singular_(singular).set_ring()  # this is expensive
+        self.parent()._singular_(singular).set_ring()
         return singular(self._singular_init_())