Allow the use of string s-expression CV-policies in pyarb

doc/concepts/morphology.rst

@@ -693,6 +693,8 @@ and *B*, while *A* | *B* is a policy which gives all the boundary points from
 The domain of *A* + *B* and *A* | *B* is the union of the domains of *A* and
 *B*.
 
+.. _morph-cv-sexpr:
+
 Reading CV policies from strings
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

doc/python/decor.rst

@@ -167,3 +167,9 @@ Cable cell decoration
 
         :param policy: The cv_policy.
         :type policy: :py:class:`cv_policy`
+
+    .. method:: discretization(policy)
+
+        Set the cv_policy used to discretise the cell into control volumes for simulation.
+
+        :param str policy: :ref:`string representation <morph-cv-sexpr>` of a cv_policy.

python/cells.cpp

@@ -691,7 +691,13 @@ void register_cells(pybind11::module& m) {
             "The group of spike detectors has the label 'label', used for forming connections between cells.")
         .def("discretization",
             [](arb::decor& dec, const arb::cv_policy& p) { dec.set_default(p); },
-            pybind11::arg_v("policy", "A cv_policy used to discretise the cell into compartments for simulation"));
+            pybind11::arg_v("policy", "A cv_policy used to discretise the cell into compartments for simulation"))
+        .def("discretization",
+            [](arb::decor& dec, const std::string& p) {
+                dec.set_default(arborio::parse_cv_policy_expression(p).unwrap());
+            },
+            pybind11::arg_v("policy", "An s-expression string representing a cv_policy used to discretise the "
+                                      "cell into compartments for simulation"));
 
     // arb::cable_cell