Trac #20406: get_solver should allow passing a function (a solver fac…

…tory) as the solver argument

For example, to use GLPK's exact simplex algorithm:

{{{
from sage.numerical.backends.generic_backend import get_solver
def glpk_exact_solver():
    b = get_solver(solver="GLPK")
    b.solver_parameter("simplex_or_intopt", "exact_simplex_only")
    return b

delsarte_bound_additive_hamming_space(19,15,7,solver=glpk_exact_solver)
}}}

URL: http://trac.sagemath.org/20406
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

src/sage/numerical/backends/generic_backend.pyx

@@ -1385,6 +1385,9 @@ cpdef GenericBackend get_solver(constraint_generation = False, solver = None, ba
         or ``None``. If ``solver=None`` (default),
         the default solver is used (see ``default_mip_solver`` method).
 
+        ``solver`` can also be a callable, in which case it is called,
+        and its result is returned.
+
     - ``base_ring`` -- If not ``None``, request a solver that works over this
         (ordered) field.  If ``base_ring`` is not a field, its fraction field
         is used.
@@ -1432,6 +1435,26 @@ cpdef GenericBackend get_solver(constraint_generation = False, solver = None, ba
         <...sage.numerical.backends.interactivelp_backend.InteractiveLPBackend...>
         sage: p.base_ring()
         Rational Field
+
+    Passing a callable as the 'solver'::
+
+        sage: from sage.numerical.backends.glpk_backend import GLPKBackend
+        sage: p = get_solver(GLPKBackend); p
+        <...sage.numerical.backends.glpk_backend.GLPKBackend...>
+
+    Passing a callable that customizes a backend::
+
+        sage: def glpk_exact_solver():
+        ....:     from sage.numerical.backends.generic_backend import get_solver
+        ....:     b = get_solver(solver="GLPK")
+        ....:     b.solver_parameter("simplex_or_intopt", "exact_simplex_only")
+        ....:     return b
+        sage: delsarte_bound_additive_hamming_space(11,3,4,solver=glpk_exact_solver) # long time
+        glp_exact...
+        ...
+        OPTIMAL SOLUTION FOUND
+        8
+
     """
     if solver is None:
 
@@ -1452,6 +1475,12 @@ cpdef GenericBackend get_solver(constraint_generation = False, solver = None, ba
         if solver == "Coin" and constraint_generation:
             solver = "Glpk"
 
+    elif callable(solver):
+        kwds = {}
+        if base_ring is not None:
+            kwds['base_ring']=base_ring
+        return solver(**kwds)
+
     else:
         solver = solver.capitalize()
 
@@ -1484,4 +1513,4 @@ cpdef GenericBackend get_solver(constraint_generation = False, solver = None, ba
         return InteractiveLPBackend(base_ring=base_ring)
 
     else:
-        raise ValueError("'solver' should be set to 'GLPK', 'Coin', 'CPLEX', 'CVXOPT', 'Gurobi', 'PPL', 'InteractiveLP', or None (in which case the default one is used).")
+        raise ValueError("'solver' should be set to 'GLPK', 'Coin', 'CPLEX', 'CVXOPT', 'Gurobi', 'PPL', 'InteractiveLP', None (in which case the default one is used), or a callable.")

src/sage/numerical/mip.pyx

@@ -267,6 +267,10 @@ cdef class MixedIntegerLinearProgram(SageObject):
       - If ``solver=None`` (default), the default solver is used (see
         :func:`default_mip_solver`)
 
+      - ``solver`` can also be a callable,
+        see :func:`sage.numerical.backends.generic_backend.get_solver` for
+        examples.
+
     - ``maximization``
 
       - When set to ``True`` (default), the ``MixedIntegerLinearProgram``
@@ -358,8 +362,12 @@ cdef class MixedIntegerLinearProgram(SageObject):
           - PPL (``solver="PPL"``). See the `PPL
             <http://bugseng.com/products/ppl>`_ web site.
 
-          -If ``solver=None`` (default), the default solver is used (see
-           ``default_mip_solver`` method.
+          - If ``solver=None`` (default), the default solver is used, see
+            :func:`default_mip_solver`.
+
+          - ``solver`` can also be a callable,
+            see :func:`sage.numerical.backends.generic_backend.get_solver` for
+            examples.
 
         - ``maximization``
 
@@ -536,8 +544,11 @@ cdef class MixedIntegerLinearProgram(SageObject):
             sage: q.number_of_constraints()
             1
         """
+        def copying_solver(**kwdargs):
+            return (<GenericBackend> self._backend).copy()
+
         cdef MixedIntegerLinearProgram p = \
-            MixedIntegerLinearProgram(solver="GLPK")
+            MixedIntegerLinearProgram(solver=copying_solver)
         try:
             p._variables = copy(self._variables)
         except AttributeError:
@@ -554,7 +565,6 @@ cdef class MixedIntegerLinearProgram(SageObject):
         except AttributeError:
             pass
 
-        p._backend = (<GenericBackend> self._backend).copy()
         return p
 
     def __getitem__(self, v):