Deprecate is_SymmetricFunction

src/sage/combinat/sf/classical.py

@@ -155,7 +155,7 @@ def _element_constructor_(self, x):
         ##############
         # Dual bases #
         ##############
-        elif sfa.is_SymmetricFunction(x) and hasattr(x, 'dual'):
+        elif isinstance(x, sfa.SymmetricFunctionAlgebra_generic.Element) and hasattr(x, 'dual'):
             # Check to see if it is the dual of some other basis
             # If it is, try to coerce its corresponding element
             # in the other basis

src/sage/combinat/sf/sfa.py

@@ -304,12 +304,20 @@ def is_SymmetricFunction(x):
         sage: from sage.combinat.sf.sfa import is_SymmetricFunction
         sage: s = SymmetricFunctions(QQ).s()
         sage: is_SymmetricFunction(2)
+        doctest:warning...
+        DeprecationWarning: The function is_SymmetricFunction is deprecated;
+        use 'isinstance(..., SymmetricFunctionAlgebra_generic.Element)' instead.
+        See https://github.com/sagemath/sage/issues/38279 for details.
         False
         sage: is_SymmetricFunction(s(2))
         True
         sage: is_SymmetricFunction(s([2,1]))
         True
     """
+    from sage.misc.superseded import deprecation
+    deprecation(38279,
+                "The function is_SymmetricFunction is deprecated; "
+                "use 'isinstance(..., SymmetricFunctionAlgebra_generic.Element)' instead.")
     return isinstance(x, SymmetricFunctionAlgebra_generic.Element)
 
 #####################################################################
@@ -3429,7 +3437,7 @@ def plethysm(self, x, include=None, exclude=None):
 
         tHA = HopfAlgebrasWithBasis(R).TensorProducts()
         tensorflag = Px in tHA
-        if not is_SymmetricFunction(x):
+        if not isinstance(x, SymmetricFunctionAlgebra_generic.Element):
             if R.has_coerce_map_from(Px) or x in R:
                 x = R(x)
                 Px = R

src/sage/rings/lazy_series.py

@@ -6981,9 +6981,8 @@ def functorial_composition(self, *args):
         """
         if len(args) != self.parent()._arity:
             raise ValueError("arity must be equal to the number of arguments provided")
-        from sage.combinat.sf.sfa import is_SymmetricFunction
-        if not all(isinstance(g, LazySymmetricFunction)
-                   or is_SymmetricFunction(g)
+        from sage.combinat.sf.sfa import SymmetricFunctionAlgebra_generic
+        if not all(isinstance(g, (LazySymmetricFunction, SymmetricFunctionAlgebra_generic.Element))
                    or not g for g in args):
             raise ValueError("all arguments must be (possibly lazy) symmetric functions")
 
@@ -7200,9 +7199,8 @@ def arithmetic_product(self, *args):
         """
         if len(args) != self.parent()._arity:
             raise ValueError("arity must be equal to the number of arguments provided")
-        from sage.combinat.sf.sfa import is_SymmetricFunction
-        if not all(isinstance(g, LazySymmetricFunction)
-                   or is_SymmetricFunction(g)
+        from sage.combinat.sf.sfa import SymmetricFunctionAlgebra_generic
+        if not all(isinstance(g, (LazySymmetricFunction, SymmetricFunctionAlgebra_generic.Element))
                    or not g for g in args):
             raise ValueError("all arguments must be (possibly lazy) symmetric functions")