AbstractFamily.{keys,values,items}: New

src/sage/sets/family.py

@@ -431,6 +431,39 @@ def hidden_keys(self):
         """
         return []
 
+    @abstract_method
+    def keys(self):
+        """
+        Return the keys of the family.
+
+        EXAMPLES::
+
+            sage: f = Family({3: 'a', 4: 'b', 7: 'd'})
+            sage: sorted(f.keys())
+            [3, 4, 7]
+        """
+
+    @abstract_method(optional=True)
+    def values(self):
+        """
+        Return the elements (values) of this family.
+
+        EXAMPLES::
+
+            sage: f = Family(["c", "a", "b"], lambda x: x + x)
+            sage: sorted(f.values())
+            ['aa', 'bb', 'cc']
+        """
+
+    def items(self):
+        """
+        Return an iterator for key-value pairs.
+
+        A key can only appear once, but if the function is not injective, values may
+        appear multiple times.
+        """
+        return zip(self.keys(), self.values())
+
     def zip(self, f, other, name=None):
         r"""
         Given two families with same index set `I` (and same hidden