Strengthening Laurent series conversion.

src/sage/rings/lazy_series_ring.py

@@ -34,6 +34,7 @@
 from sage.rings.polynomial.laurent_polynomial_ring import LaurentPolynomialRing
 from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
 from sage.rings.lazy_series import (LazyModuleElement,
+ LazyCauchyProductSeries,
  LazyLaurentSeries,
  LazyDirichletSeries)
 from sage.structure.global_options import GlobalOptions
@@ -253,6 +254,13 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
  ...
  ValueError: constant may only be specified if the degree is specified
 
+ sage: D = LazyDirichletSeriesRing(ZZ, 't')
+ sage: m = D(moebius)
+ sage: L(m)
+ Traceback (most recent call last):
+ ...
+ ValueError: unable to convert ... into Lazy Laurent Series Ring ...
+
  We support the old input format for ``constant``::
 
  sage: f = L(lambda i: i, valuation=-3, constant=-1, degree=3)
@@ -263,11 +271,6 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
  sage: f == g
  True
 
- sage: D = LazyDirichletSeriesRing(ZZ, 't')
- sage: m = D(moebius)
- sage: L(m)
- z - z^2 - z^3 - z^5 + z^6 - z^7 + O(z^8)
-
  .. TODO::
 
  Add a method to change the sparse/dense implementation.
@@ -318,7 +321,7 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
  return self.element_class(self, coeff_stream)
 
  # Handle when it is a lazy series
- if isinstance(x, LazyModuleElement):
+ if isinstance(x, LazyCauchyProductSeries):
  if x._coeff_stream._is_sparse is not self._sparse:
  # TODO: Implement a way to make a self._sparse copy
  raise NotImplementedError("cannot convert between sparse and dense")