Creates and register the comm target when it does not exist (#698)

Author: JohanMabille

src/xcomm.cpp

@@ -84,7 +84,15 @@ namespace xpyt
 
     const xeus::xtarget* xcomm::target(const py::object& target_name) const
     {
-        return xeus::get_interpreter().comm_manager().target(target_name.cast<std::string>());
+        auto& comm_manager = xeus::get_interpreter().comm_manager();
+        auto res = comm_manager.target(target_name.cast<std::string>());
+        if (!res)
+        {
+            comm_manager.register_comm_target(
+                target_name.cast<std::string>(), [](xeus::xcomm&&, const xeus::xmessage&) {}
+            );
+        }
+        return comm_manager.target(target_name.cast<std::string>());
     }
 
     xeus::xguid xcomm::id(const py::kwargs& kwargs) const

src/xcomm.hpp

@@ -40,6 +40,9 @@ namespace xpyt
 
     private:
 
+        // Warning: this function creates and register the target with a dummy
+        // comm_open handler when the target does not exist, so that create_comm
+        // has the same behavior as ipykernel.
         const xeus::xtarget* target(const py::object& target_name) const;
         xeus::xguid id(const py::kwargs& kwargs) const;
         cpp_callback_type cpp_callback(const python_callback_type& callback) const;