Trac #28582: some cleaning for is_ring

as a first step towards deprecation

URL: https://trac.sagemath.org/28582
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Travis Scrimshaw

src/doc/en/thematic_tutorials/coercion_and_categories.rst

@@ -155,7 +155,6 @@ This base class provides a lot more methods than a general parent::
      'is_integrally_closed',
      'is_noetherian',
      'is_prime_field',
-     'is_ring',
      'is_subring',
      'krull_dimension',
      'ngens',

src/sage/categories/rings.py

@@ -266,7 +266,6 @@ def is_ring(self):
 
                 sage: Parent(QQ,category=Rings()).is_ring()
                 True
-
             """
             return True
 

src/sage/matroids/constructor.py

@@ -109,6 +109,7 @@
 from sage.graphs.all import Graph
 from sage.structure.element import is_Matrix
 from sage.rings.all import ZZ, QQ
+from sage.categories.all import Fields, Rings
 from sage.rings.finite_rings.finite_field_base import FiniteField
 import sage.matroids.matroid
 import sage.matroids.basis_exchange_matroid
@@ -694,11 +695,11 @@ def Matroid(groundset=None, data=None, **kwds):
     base_ring = None
     if 'field' in kwds:
         base_ring = kwds.pop('field')
-        if check and not base_ring.is_field():
+        if check and not base_ring in Fields:
             raise TypeError("{} is not a field".format(base_ring))
     elif 'ring' in kwds:
         base_ring = kwds.pop('ring')
-        if check and not base_ring.is_ring():
+        if check and not base_ring in Rings:
             raise TypeError("{} is not a ring".format(base_ring))
 
     # "key" is the kind of data we got

src/sage/rings/polynomial/infinite_polynomial_ring.py

@@ -238,6 +238,7 @@
 
 import six
 from sage.rings.ring import CommutativeRing
+from sage.categories.rings import Rings
 from sage.structure.all import SageObject, parent
 from sage.structure.factory import UniqueFactory
 from sage.misc.cachefunc import cached_method
@@ -684,17 +685,14 @@ def __init__(self, R, names, order):
             names = ['x']
         for n in names:
             if not (isinstance(n, six.string_types) and n.isalnum() and (not n[0].isdigit())):
-                raise ValueError("generator names must be alpha-numeric strings not starting with a  digit, but %s isn't"%n)
-        if len(names)!=len(set(names)):
+                raise ValueError("generator names must be alpha-numeric strings not starting with a  digit, but %s isn't" % n)
+        if len(names) != len(set(names)):
             raise ValueError("generator names must be pairwise different")
         self._names = tuple(names)
         if not isinstance(order, six.string_types):
             raise TypeError("The monomial order must be given as a string")
-        try:
-            if not (hasattr(R,'is_ring') and R.is_ring() and hasattr(R,'is_commutative') and R.is_commutative()):
-                raise TypeError
-        except Exception:
-            raise TypeError("The given 'base ring' (= %s) must be a commutative ring"%(R))
+        if not R in Rings().Commutative():
+            raise TypeError("The given 'base ring' (= %s) must be a commutative ring" % R)
 
         # now, the input is accepted
         if hasattr(R,'_underlying_ring'):

src/sage/rings/ring.pyx

@@ -179,7 +179,7 @@ cdef class Ring(ParentWithGens):
         sage: QQ.cardinality()
         +Infinity
      """
-    def __init__(self, base, names=None, normalize=True, category = None):
+    def __init__(self, base, names=None, normalize=True, category=None):
         """
         Initialize ``self``.
 
@@ -997,17 +997,6 @@ cdef class Ring(ParentWithGens):
         else:
             return False
 
-    def is_ring(self):
-        """
-        Return ``True`` since ``self`` is a ring.
-
-        EXAMPLES::
-
-            sage: QQ.is_ring()
-            True
-        """
-        return True
-
     def is_noetherian(self):
         """
         Return ``True`` if this ring is Noetherian.
@@ -2549,6 +2538,4 @@ def is_Ring(x):
         sage: is_Ring(MS)
         True
     """
-    # TODO: use the idiom `x in _Rings` as soon as all rings will be
-    # in the category Rings()
-    return isinstance(x, Ring) or x in _Rings
+    return x in _Rings