Add standalone serialization function

This commit adds a standalone serialization function to the symengine
wrapper module. Previously there was a load_basic() function that could
be used for directly deserializing the payload without going through
pickle, but no corresponding function for serialization. A new function
save_basic() is added here to perform this function which will enable
users to directly roundtrip symengine objects through binary
serialization.

Fixes symengine#449

symengine/lib/symengine_wrapper.in.pyx

@@ -829,11 +829,6 @@ cdef list vec_pair_to_list(symengine.vec_pair& vec):
         result.append((c2py(a), c2py(b)))
     return result
 
-
-def load_basic(bytes s):
-    return c2py(symengine.wrapper_loads(s))
-
-
 repr_latex=[False]
 
 cdef class Basic(object):
@@ -1225,6 +1220,14 @@ cdef class Basic(object):
         return d
 
 
+def load_basic(bytes s):
+    return c2py(symengine.wrapper_loads(s))
+
+
+cpdef save_basic(Basic basic):
+    return symengine.wrapper_dumps(basic)
+
+
 def series(ex, x=None, x0=0, n=6, as_deg_coef_pair=False):
     # TODO: check for x0 an infinity, see sympy/core/expr.py
     # TODO: nonzero x0

symengine/tests/test_pickling.py

@@ -1,4 +1,5 @@
 from symengine import symbols, sin, sinh, have_numpy, have_llvm, cos, Symbol
+from symengine.lib.symengine_wrapper import load_basic, save_basic
 from symengine.test_utilities import raises
 import pickle
 import unittest
@@ -11,6 +12,12 @@ def test_basic():
     expr2 = pickle.loads(s)
     assert expr == expr2
 
+def test_basic_direct():
+    x, y, z = symbols('x y z')
+    expr = sin(cos(x + y)/z)**2
+    s = save_basic(expr)
+    expr2 = load_basic(s)
+    assert expr == expr2
 
 class MySymbolBase(Symbol):
     def __init__(self, name, attr):