Callback models -- X Python Reference Manual

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# 6.2. Callback models

There are three callback models for associating Python callback logic with UIL-defined widgets. The Examples section below demonstrates all of the models. The models are:

`Xt.AddCallback`
  With this approach, Python initialization logic attaches widget callbacks to Python functions (and the UIL callback feature is not used). This is likely to be the most popular callback model; if you cannot know which model to use, then use this approach. Example code:

          ...
          main = hier.FetchWidget("main", top);
          main.NameToWidget("*button").AddCallback(
                            'activateCallback', pythonFunction, "context");
          ...

`PyRegister`
  `PyRegister` registers a Python function using `MrmRegister` so that the function will be called as specified in UIL callback statements. (This model imitates the manner in which UIL specifies callbacks to C functions and the C program registers the callbacks during initialization.) One use for this approach would be to prototype an application in Python and then convert to C code, keeping the same UIL.

  For example:

          mrm_hier = Mrm.OpenHierarchy(["reg.uid"])
          mrm_hier.PyRegister(MyPrint, "widget");

  `MyPrint`, now a Mrm-registered Python routine, can be called directly from UIL.

`PyEval`
  `PyEval` is a UIL-callable function (automatically-registered) that dynamically interprets its string argument as Python code. This approach allows the logic associated with each callback to be entirely specified in the UIL module. This approach is somewhat experimental. The `PyEval` function is described above. Example UIL code:

          procedure PyEval(string);
          ...
          XmNactivateCallback = procedure PyEval("MyPrint(g_var)");

  The string "MyPrint(g_var)" is evaluated by the Python interpreter whenever the activate callback is invoked. Evaluating the string is efficient because `PyEval` caches the compiled Python code object the first time the string is evaluated and compiled. Subsequently, the code object can be evaluated without having to be compiled.

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