generate pytask from string and string from pytask (#188)

* pytask creation from bytestring: saving work

* pytask now works

* documentation and tests

* linting

* Lint and fix test case

src/common/lib/python/common_extension.c

@@ -68,6 +68,55 @@ PyObject *PyObjectID_make(object_id object_id) {
   return (PyObject *) result;
 }
 
+/**
+ * Convert a string to a Ray task specification Python object.
+ *
+ * This is called from Python like
+ *
+ * task = photon.task_from_string("...")
+ *
+ * @param task_string String representation of the task specification.
+ * @return Python task specification object.
+ */
+PyObject *PyTask_from_string(PyObject *self, PyObject *args) {
+  const char *data;
+  int size;
+  if (!PyArg_ParseTuple(args, "s#", &data, &size)) {
+    return NULL;
+  }
+  PyTask *result = PyObject_New(PyTask, &PyTaskType);
+  result = (PyTask *) PyObject_Init((PyObject *) result, &PyTaskType);
+  result->spec = malloc(size);
+  memcpy(result->spec, data, size);
+  /* TODO(pcm): Better error checking once we use flatbuffers. */
+  if (size != task_spec_size(result->spec)) {
+    PyErr_SetString(CommonError,
+                    "task_from_string: task specification string malformed");
+    return NULL;
+  }
+  return (PyObject *) result;
+}
+
+/**
+ * Convert a Ray task specification Python object to a string.
+ *
+ * This is called from Python like
+ *
+ * s = photon.task_to_string(task)
+ *
+ * @param task Ray task specification Python object.
+ * @return String representing the task specification.
+ */
+PyObject *PyTask_to_string(PyObject *self, PyObject *args) {
+  PyObject *arg;
+  if (!PyArg_ParseTuple(args, "O", &arg)) {
+    return NULL;
+  }
+  PyTask *task = (PyTask *) arg;
+  return PyBytes_FromStringAndSize((char *) task->spec,
+                                   task_spec_size(task->spec));
+}
+
 static PyObject *PyObjectID_id(PyObject *self) {
   PyObjectID *s = (PyObjectID *) self;
   return PyBytes_FromStringAndSize((char *) &s->object_id.id[0],