Trac #30587: Remove import of 'ppl' at startup using lazy_import with…

… feature

Currently, sage imports `ppl` (from `pplpy`) at startup, via
`sage.geometry.cone`, `sage.geometry.integral_points`, and
`sage.geometry.lattice_polytopes`.

This should be changed, both
 - as preparation for other backends for polyhedral cones (in particular
`PyNormaliz`)
 - for the modularization of sagelib (#29705).

In addition we add documentation to the option `feature` of lazy import
and while we are at it, use this option for backend normaliz as well.

URL: https://trac.sagemath.org/30587
Reported by: mkoeppe
Ticket author(s): Jonathan Kliem, Matthias Koeppe
Reviewer(s): Matthias Koeppe, Jonathan Kliem

src/sage/features/__init__.py

@@ -374,7 +374,9 @@ def package_systems():
         # to obtain system package advice.
         try:
             proc = run('sage-guess-package-system', shell=True, stdout=PIPE, stderr=PIPE, universal_newlines=True, check=True)
-            _cache_package_systems = [PackageSystem(proc.stdout.strip())]
+            system_name = proc.stdout.strip()
+            if system_name != 'unknown':
+                _cache_package_systems = [PackageSystem(system_name)]
         except CalledProcessError:
             pass
         more_package_systems = [SagePackageSystem(), PipPackageSystem()]

src/sage/geometry/cone.py

@@ -212,11 +212,16 @@
 from sage.rings.all import QQ, ZZ
 from sage.structure.all import SageObject, parent
 from sage.structure.richcmp import richcmp_method, richcmp
-from ppl import (C_Polyhedron, Generator_System, Constraint_System,
-                 Linear_Expression, ray as PPL_ray, point as PPL_point,
-                 Poly_Con_Relation)
 from sage.geometry.integral_points import parallelotope_points
 
+from sage.misc.lazy_import import lazy_import
+from sage.features import PythonModule
+lazy_import('ppl', ['C_Polyhedron', 'Generator_System', 'Constraint_System',
+                    'Linear_Expression', 'Poly_Con_Relation'],
+                    feature=PythonModule("ppl", spkg="pplpy"))
+lazy_import('ppl', ['ray', 'point'], as_=['PPL_ray', 'PPL_point'],
+                    feature=PythonModule("ppl", spkg="pplpy"))
+
 
 def is_Cone(x):
     r"""

src/sage/geometry/lattice_polytope.py

@@ -114,8 +114,14 @@
 from sage.geometry.toric_lattice import ToricLattice, is_ToricLattice
 from sage.graphs.graph import DiGraph, Graph
 from sage.groups.perm_gps.permgroup_named import SymmetricGroup
-from ppl import (C_Polyhedron, Generator_System, Linear_Expression,
-                 point as PPL_point)
+
+from sage.misc.lazy_import import lazy_import
+from sage.features import PythonModule
+lazy_import('ppl', ['C_Polyhedron', 'Generator_System', 'Linear_Expression'],
+                    feature=PythonModule("ppl", spkg="pplpy"))
+lazy_import('ppl', 'point', as_='PPL_point',
+                    feature=PythonModule("ppl", spkg="pplpy"))
+
 from sage.matrix.constructor import matrix
 from sage.structure.element import is_Matrix
 from sage.misc.all import cached_method, flatten, tmp_filename

src/sage/geometry/polyhedron/backend_normaliz.py

@@ -26,6 +26,9 @@
 from sage.structure.element import Element
 from sage.misc.all import cached_method, prod
 from sage.features import PythonModule
+from sage.misc.lazy_import import lazy_import
+lazy_import('PyNormaliz', ['NmzResult', 'NmzCompute', 'NmzCone', 'NmzConeCopy'],
+                    feature=PythonModule("PyNormaliz", spkg="pynormaliz"))
 
 from sage.rings.all import ZZ, QQ
 from sage.arith.functions import LCM_list
@@ -273,11 +276,9 @@ def nfelem_handler(coords):
             # coords might be too short which is not accepted by Sage number field
             v = list(coords) + [0] * (self._normaliz_field.degree() - len(coords))
             return self._normaliz_field(v)
-
-        import PyNormaliz
-        return PyNormaliz.NmzResult(normaliz_cone, property,
-                                    RationalHandler=rational_handler,
-                                    NumberfieldElementHandler=nfelem_handler)
+        return NmzResult(normaliz_cone, property,
+                         RationalHandler=rational_handler,
+                         NumberfieldElementHandler=nfelem_handler)
 
     def _init_from_normaliz_cone(self, normaliz_cone, normaliz_field):
         """
@@ -416,9 +417,7 @@ def _cone_from_normaliz_data(self, data, verbose=False):
         if verbose:
             print("# Calling {}".format(_format_function_call('PyNormaliz.NmzCone', **data)))
 
-        PythonModule("PyNormaliz", spkg="pynormaliz").require()
-        import PyNormaliz
-        cone = PyNormaliz.NmzCone(**data)
+        cone = NmzCone(**data)
         assert cone, "{} did not return a cone".format(_format_function_call('PyNormaliz.NmzCone', **data))
         return cone
 
@@ -1117,11 +1116,9 @@ def _make_normaliz_cone(data, verbose=False):
             sage: NmzResult(nmz_cone, "ExtremeRays")                                           # py3 # optional - pynormaliz
             [[1, 2, 0], [2, 1, 0]]
         """
-        PythonModule("PyNormaliz", spkg="pynormaliz").require()
-        import PyNormaliz
         if verbose:
             print("# Calling PyNormaliz.NmzCone(**{})".format(data))
-        cone = PyNormaliz.NmzCone(**data)
+        cone = NmzCone(**data)
         assert cone, "NmzCone(**{}) did not return a cone".format(data)
         return cone
 
@@ -1253,8 +1250,7 @@ def __copy__(self):
 
         # Make a copy of the cone.
         cone = self._normaliz_cone
-        import PyNormaliz
-        conecopy = PyNormaliz.NmzConeCopy(cone)
+        conecopy = NmzConeCopy(cone)
         other._normaliz_cone = conecopy
         return other
 
@@ -1803,9 +1799,7 @@ def _ehrhart_quasipolynomial_normaliz(self, variable='t'):
         """
         cone = self._normaliz_cone
         # Normaliz needs to compute the EhrhartSeries first
-        PythonModule("PyNormaliz", spkg="pynormaliz").require()
-        import PyNormaliz
-        assert PyNormaliz.NmzCompute(cone, ["EhrhartSeries"])
+        assert NmzCompute(cone, ["EhrhartSeries"])
         e = self._nmz_result(cone, "EhrhartQuasiPolynomial")
 
         from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing

src/sage/geometry/polyhedron/backend_ppl.py

@@ -6,15 +6,16 @@
 from sage.rings.integer import Integer
 from sage.arith.functions import LCM_list
 from sage.misc.functional import denominator
-from ppl import (
-    C_Polyhedron, Constraint_System, Generator_System,
-    Linear_Expression,
-    line, ray, point
-)
 from .base import Polyhedron_base
 from .base_QQ import Polyhedron_QQ
 from .base_ZZ import Polyhedron_ZZ
 
+from sage.misc.lazy_import import lazy_import
+from sage.features import PythonModule
+lazy_import('ppl', ['C_Polyhedron', 'Generator_System', 'Constraint_System',
+                    'Linear_Expression', 'line', 'ray', 'point'],
+                    feature=PythonModule("ppl", spkg="pplpy"))
+
 
 #########################################################################
 class Polyhedron_ppl(Polyhedron_base):

src/sage/misc/lazy_import.pyx

@@ -372,7 +372,11 @@ cdef class LazyImport(object):
             sage: repr(lazy_ZZ)
             'Integer Ring'
         """
-        return repr(self.get_object())
+        try:
+            obj = self.get_object()
+            return repr(obj)
+        except FeatureNotPresentError as e:
+            return "Failed lazy import:\n" + str(e)
 
     def __str__(self):
         """
@@ -990,6 +994,9 @@ def lazy_import(module, names, as_=None, *,
       ``deprecation`` should be either a trac number (integer) or a
       pair ``(trac_number, message)``
 
+    - ``feature`` -- a python module (optional), if it cannot be imported
+      an appropriate error is raised
+
     .. SEEALSO:: :mod:`sage.misc.lazy_import`, :class:`LazyImport`
 
     EXAMPLES::
@@ -1052,6 +1059,22 @@ def lazy_import(module, names, as_=None, *,
         doctest:...: DeprecationWarning: This is an example.
         See http://trac.sagemath.org/14275 for details.
         5-adic Field with capped relative precision 20
+
+    An example of an import relying on a feature::
+
+        sage: from sage.features import PythonModule
+        sage: lazy_import('ppl', 'equation', feature=PythonModule('ppl', spkg='pplpy'))
+        sage: equation
+        <built-in function equation>
+        sage: lazy_import('PyNormaliz', 'NmzListConeProperties', feature=PythonModule('PyNormaliz', spkg='pynormaliz'))  # optional - pynormaliz
+        sage: NmzListConeProperties  # optional - pynormaliz
+        <built-in function NmzListConeProperties>
+        sage: lazy_import('foo', 'not_there', feature=PythonModule('foo', spkg='non-existing-package'))
+        sage: not_there
+        Failed lazy import:
+        foo is not available.
+        Importing not_there failed: No module named 'foo'...
+        No equivalent system packages for ... are known to Sage...
     """
     if as_ is None:
         as_ = names