21529: fix bug in factoring of general symbolic expressions

src/sage/symbolic/expression.pyx

@@ -10100,6 +10100,14 @@ cdef class Expression(CommutativeRingElement):
             x^3*y^17 + x*y^19 + y^20)*(x^10 - x^9*y + x^8*y^2 - x^7*y^3 +
             x^6*y^4 - x^5*y^5 + x^4*y^6 - x^3*y^7 + x^2*y^8 - x*y^9 +
             y^10)*(x^6 - x^3*y^3 + y^6)*(x^2 - x*y + y^2)*(x + y)
+
+        TESTS:
+
+        Check that :trac:`21529` is fixed::
+
+            sage: f(x) = function('f')(x)
+            sage: (f(x).diff(x)^2-1).factor()
+            (diff(f(x), x) + 1)*(diff(f(x), x) - 1)
         """
         from sage.calculus.calculus import symbolic_expression_from_maxima_string, symbolic_expression_from_string
         if len(dontfactor) > 0:
@@ -10114,7 +10122,7 @@ cdef class Expression(CommutativeRingElement):
                 f = self.polynomial(QQ)
                 w = repr(f.factor())
                 return symbolic_expression_from_string(w)
-            except TypeError:
+            except (TypeError, NotImplementedError):
                 pass
             return self.parent()(self._maxima_().factor())