16203: preliminary implementation, dependent on 17399, 17438

src/sage/rings/power_series_ring.py

@@ -127,6 +127,7 @@
 import integral_domain
 import field
 import integer
+from sage.symbolic.expression import Expression
 import sage.structure.parent_gens as gens
 from infinity import infinity
 import sage.misc.latex as latex
@@ -724,6 +725,9 @@ def _element_constructor_(self, f, prec=infinity, check=True):
                 num = self.element_class(self, f.numerator(), prec, check=check)
                 den = self.element_class(self, f.denominator(), prec, check=check)
                 return self.coerce(num/den)
+        elif isinstance(f, Expression) and f.is_series():
+            return self.element_class(self, f.list(),
+                                      f.degree(f.default_variable()), check=check)            
         return self.element_class(self, f, prec, check=check)
 
     def construction(self):

src/sage/symbolic/ring.pyx

@@ -33,7 +33,9 @@ from sage.structure.parent_base import ParentWithBase
 from sage.rings.ring cimport CommutativeRing
 from sage.categories.morphism cimport Morphism
 
-from sage.rings.all import RR, CC
+from sage.rings.real_mpfr import RR
+from sage.rings.complex_field import ComplexField
+CC = ComplexField()
 
 pynac_symbol_registry = {}