Fix an occurrence of lazy_import

src/sage/misc/lazy_import.pyx

@@ -36,6 +36,23 @@ that (s)he can remove the flag::
     UserWarning: Option ``at_startup=True`` for lazy import ZZ not needed anymore
     Integer Ring
 
+.. WARNING::
+
+    After the first usage, the imported object is directly injected into the
+    namespace; however, before that, the :class:`LazyImport` object
+    is not exactly equivalent to the actual imported object. For example::
+
+        sage: from sage.misc.lazy_import import LazyImport
+        sage: my_qqbar = LazyImport('sage.rings.qqbar', 'QQbar')
+        sage: my_qqbar(5) == QQbar(5)  # good
+        True
+        sage: isinstance(QQbar, Parent)
+        True
+        sage: isinstance(my_qqbar, Parent)  # fails!
+        False
+
+    To avoid this issue, you may execute the import inside the function instead.
+
 .. SEEALSO:: :func:`lazy_import`, :class:`LazyImport`
 
 AUTHOR:

src/sage/schemes/curves/projective_curve.py

@@ -159,7 +159,6 @@
 
 lazy_import('sage.interfaces.singular', 'singular')
 lazy_import('sage.rings.number_field.number_field', 'NumberField')
-lazy_import('sage.rings.qqbar', ['number_field_elements_from_algebraics', 'QQbar'])
 
 from .curve import Curve_generic
 
@@ -1400,7 +1399,7 @@
             sage: set_verbose(-1)
             sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
             sage: C = Curve([(x^2 + y^2 - y*z - 2*z^2)*(y*z - x^2 + 2*z^2)*z + y^5], P)
             sage: C.ordinary_model() # long time (5 seconds)
             Scheme morphism:
               From: Projective Plane Curve over Number Field in a
                     with defining polynomial y^2 - 2 defined
@@ -1450,9 +1449,10 @@
                       + (1/16*a + 1/16)*x*y*z^2 + (3/16*a + 3/16)*y^2*z^2
                       + (-3/16*a - 1/4)*y*z^3 + (1/16*a + 3/32)*z^4)
         """
-        # helper function for extending the base field
+        from sage.rings.qqbar import number_field_elements_from_algebraics, QQbar
 
         def extension(self):
+            # helper function for extending the base field
             F = self.base_ring()
             pts = self.change_ring(F.embeddings(QQbar)[0]).rational_points()
             L = [t for pt in pts for t in pt]
@@ -1804,7 +1804,7 @@
             sage: C = P.curve(z^2*y^3 - z*(33*x*z+2*x^2+8*z^2)*y^2
             ....:             + (21*z^2+21*x*z-x^2)*(z^2+11*x*z-x^2)*y
             ....:             + (x-18*z)*(z^2+11*x*z-x^2)^2)
             sage: G0 = C.fundamental_group()                    # needs sirocco
             sage: G.is_isomorphic(G0)                           # needs sirocco
             True
             sage: C = P.curve(z)