sage.rings.{finite_rings,polynomial}: Modularization fixes for imports

Author: Matthias Koeppe

src/sage/rings/finite_rings/conway_polynomials.py

@@ -94,7 +94,10 @@ def exists_conway_polynomial(p, n):
         sage: exists_conway_polynomial(6,6)
         False
     """
-    return ConwayPolynomials().has_polynomial(p,n)
+    try:
+        return ConwayPolynomials().has_polynomial(p,n)
+    except ImportError:
+        return False
 
 class PseudoConwayLattice(WithEqualityById, SageObject):
     r"""
@@ -174,9 +177,13 @@ def __init__(self, p, use_database=True):
         from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
         self.ring = PolynomialRing(FiniteField(p), 'x')
         if use_database:
-            C = ConwayPolynomials()
-            self.nodes = {n: self.ring(C.polynomial(p, n))
-                          for n in C.degrees(p)}
+            try:
+                C = ConwayPolynomials()
+            except ImportError:
+                self.nodes = {}
+            else:
+                self.nodes = {n: self.ring(C.polynomial(p, n))
+                              for n in C.degrees(p)}
         else:
             self.nodes = {}
 

src/sage/rings/finite_rings/element_pari_ffelt.pyx

@@ -28,13 +28,16 @@ from .element_base cimport FinitePolyExtElement
 from .integer_mod import IntegerMod_abstract
 
 import sage.rings.integer
-from sage.modules.free_module_element import FreeModuleElement
 from sage.rings.integer cimport Integer
 from sage.rings.polynomial.polynomial_element import Polynomial
 from sage.rings.polynomial.multi_polynomial_element import MPolynomial
 from sage.rings.rational import Rational
 from sage.structure.richcmp cimport rich_to_bool
 
+try:
+    from sage.modules.free_module_element import FreeModuleElement
+except ImportError:
+    FreeModuleElement = ()
 
 from sage.interfaces.abc import GapElement
 

src/sage/rings/finite_rings/finite_field_constructor.py

@@ -189,6 +189,12 @@
 except ImportError:
     FiniteField_givaro = None
 
+try:
+    from .finite_field_ntl_gf2e import FiniteField_ntl_gf2e
+except ImportError:
+    FiniteField_ntl_gf2e = None
+
+
 from sage.structure.factory import UniqueFactory
 
 
@@ -619,9 +625,9 @@ def create_key_and_extra_args(self, order, name=None, modulus=None, names=None,
                 name = normalize_names(1, name)
 
                 if impl is None:
-                    if order < zech_log_bound:
+                    if order < zech_log_bound and FiniteField_givaro is not None:
                         impl = 'givaro'
-                    elif p == 2:
+                    elif p == 2 and FiniteField_ntl_gf2e is not None:
                         impl = 'ntl'
                     else:
                         impl = 'pari_ffelt'
@@ -772,7 +778,6 @@ def create_object(self, version, key, **kwds):
                 if impl == 'givaro':
                     K = FiniteField_givaro(order, name, modulus, repr, elem_cache)
                 elif impl == 'ntl':
-                    from .finite_field_ntl_gf2e import FiniteField_ntl_gf2e
                     K = FiniteField_ntl_gf2e(order, name, modulus)
                 elif impl == 'pari_ffelt' or impl == 'pari':
                     from .finite_field_pari_ffelt import FiniteField_pari_ffelt