Fix AttributeError in PowerSeriesRing for division

src/sage/categories/commutative_rings.py

@@ -15,6 +15,7 @@
 from sage.categories.category_with_axiom import CategoryWithAxiom
 from sage.categories.cartesian_product import CartesianProductsCategory
 from sage.structure.sequence import Sequence
+from sage.structure.element import coercion_model
 
 
 class CommutativeRings(CategoryWithAxiom):
@@ -539,6 +540,41 @@ def derivation(self, arg=None, twist=None):
                 codomain = self
             return self.derivation_module(codomain, twist=twist)(arg)
 
+        def _pseudo_fraction_field(self):
+            r"""
+            This method is used by the coercion model to determine if `a / b`
+            should be treated as `a * (1/b)`, for example when dividing an element
+            of `\ZZ[x]` by an element of `\ZZ`.
+
+            The default is to return the same value as ``self.fraction_field()``,
+            but it may return some other domain in which division is usually
+            defined (for example, ``\ZZ/n\ZZ`` for possibly composite `n`).
+
+            EXAMPLES::
+
+                sage: ZZ._pseudo_fraction_field()
+                Rational Field
+                sage: ZZ['x']._pseudo_fraction_field()
+                Fraction Field of Univariate Polynomial Ring in x over Integer Ring
+                sage: Integers(15)._pseudo_fraction_field()
+                Ring of integers modulo 15
+                sage: Integers(15).fraction_field()
+                Traceback (most recent call last):
+                ...
+                TypeError: self must be an integral domain.
+
+            TESTS::
+
+                sage: R.<x> = QQ[[]]
+                sage: S.<y> = R[[]]
+                sage: parent(y/(1+x))
+                Power Series Ring in y over Laurent Series Ring in x over Rational Field
+            """
+            try:
+                return self.fraction_field()
+            except (NotImplementedError, TypeError):
+                return coercion_model.division_parent(self)
+
     class ElementMethods:
         pass
 

src/sage/categories/fields.py

@@ -528,6 +528,22 @@
             """
             return self
 
+        def _pseudo_fraction_field(self):
+            """
+            The fraction field of ``self`` is always available as ``self``.
+
+            EXAMPLES::
+
+                sage: QQ._pseudo_fraction_field()
+                Rational Field
+                sage: K = GF(5)
+                sage: K._pseudo_fraction_field()
+                Finite Field of size 5
+                sage: K._pseudo_fraction_field() is K
+                True
+            """
+            return self
+
         def ideal(self, *gens, **kwds):
             """
             Return the ideal generated by ``gens``.