src/sage/algebras/letterplace: define singular functions locally

These modules use singular_function() from sage.libs.singular.function
to create top-level aliases for a few singular functions. The docbuild
however tries to build documentation for these functions, and it can
raise an error if GNU info is not installed. There's no good reason to
define them globally in the first place (they are only used locally),
so an easy fix is to move the imports and variables local.

src/sage/algebras/letterplace/free_algebra_element_letterplace.pyx

@@ -17,13 +17,12 @@ AUTHOR:
 # ****************************************************************************
 
 from sage.groups.perm_gps.permgroup_named import CyclicPermutationGroup
-from sage.libs.singular.function import lib, singular_function
+from sage.libs.singular.function import lib
 from sage.rings.polynomial.multi_polynomial_ideal import MPolynomialIdeal
 from cpython.object cimport PyObject_RichCompare
 
 # Define some singular functions
 lib("freegb.lib")
-poly_reduce = singular_function("NF")
 
 #####################
 # Free algebra elements
@@ -695,6 +694,8 @@ cdef class FreeAlgebraElement_letterplace(AlgebraElement):
         bck = (libsingular_options['redTail'], libsingular_options['redSB'])
         libsingular_options['redTail'] = True
         libsingular_options['redSB'] = True
+        from sage.libs.singular.function import singular_function
+        poly_reduce = singular_function("NF")
         poly = poly_reduce(C(self._poly), gI, ring=C,
                            attributes={gI: {"isSB": 1}})
         libsingular_options['redTail'] = bck[0]

src/sage/algebras/letterplace/free_algebra_letterplace.pyx

@@ -121,7 +121,7 @@ TESTS::
     algebras with different term orderings, yet.
 """
 from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
-from sage.libs.singular.function import lib, singular_function
+from sage.libs.singular.function import lib
 from sage.libs.singular.function cimport RingWrap
 from sage.libs.singular.ring cimport singular_ring_delete, singular_ring_reference
 from sage.categories.algebras import Algebras
@@ -132,7 +132,6 @@ from sage.misc.cachefunc import cached_method
 #####################
 # Define some singular functions
 lib("freegb.lib")
-freeAlgebra = singular_function("freeAlgebra")
 
 # unfortunately we cannot set Singular attributes for MPolynomialRing_libsingular
 # Hence, we must constantly work around Letterplace's sanity checks,
@@ -892,6 +891,8 @@ cdef class FreeAlgebra_letterplace_libsingular():
 
     def __cinit__(self, MPolynomialRing_libsingular commutative_ring,
                   int degbound):
+        from sage.libs.singular.function import singular_function
+        freeAlgebra = singular_function("freeAlgebra")
         cdef RingWrap rw = freeAlgebra(commutative_ring, degbound)
         self._lp_ring = singular_ring_reference(rw._ring)
         # `_lp_ring` viewed as `MPolynomialRing_libsingular` with additional

src/sage/algebras/letterplace/letterplace_ideal.pyx

@@ -41,17 +41,14 @@ AUTHOR:
 #                  https://www.gnu.org/licenses/
 # ****************************************************************************
 from sage.rings.noncommutative_ideals import Ideal_nc
-from sage.libs.singular.function import lib, singular_function
+from sage.libs.singular.function import lib
 from sage.algebras.letterplace.free_algebra_letterplace cimport FreeAlgebra_letterplace, FreeAlgebra_letterplace_libsingular
 from sage.algebras.letterplace.free_algebra_element_letterplace cimport FreeAlgebraElement_letterplace
 from sage.rings.infinity import Infinity
 
 #####################
 # Define some singular functions
 lib("freegb.lib")
-singular_twostd = singular_function("twostd")
-poly_reduce = singular_function("NF")
-
 
 class LetterplaceIdeal(Ideal_nc):
     """
@@ -321,6 +318,8 @@ class LetterplaceIdeal(Ideal_nc):
         to_L = P.hom(L.gens(), L, check=False)
         from_L = L.hom(P.gens(), P, check=False)
         I = L.ideal([to_L(x._poly) for x in self.__GB.gens()])
+        from sage.libs.singular.function import singular_function
+        singular_twostd = singular_function("twostd")
         gb = singular_twostd(I)
         out = [FreeAlgebraElement_letterplace(A, from_L(X), check=False)
                for X in gb]
@@ -398,6 +397,8 @@ class LetterplaceIdeal(Ideal_nc):
         bck = (libsingular_options['redTail'], libsingular_options['redSB'])
         libsingular_options['redTail'] = True
         libsingular_options['redSB'] = True
+        from sage.libs.singular.function import singular_function
+        poly_reduce = singular_function("NF")
         sI = poly_reduce(sI, gI, ring=C, attributes={gI: {"isSB": 1}})
         libsingular_options['redTail'] = bck[0]
         libsingular_options['redSB'] = bck[1]