Hi David, 

Making the parameter vary at runtime is simple and just needs 
some extra XML and python code. The parameter should be an argument to 
the __init__ method (constructor) of your block and should have a "<param>" 
declaration in the matching XML file. Let's say the parameter name is 
"variable_param" with a declaration like:

<param>
   <name>Variable parameter</name>
   <key>variable_param</key>
   <value>42</value>
   <type>int</type>
</param>

Then you need to add the following in the XML file: 

<callback>set_variable_param($variable_param)</callback>

And you need to implement the "set_variable_param" method in your python class 
to take whatever action is needed when the parameter value changes. This method 
will be called every time the value of the expression in the underlined 
"Variable parameter" box in the GUI changes. The method will look like: 

   def set_variable_param(self, new_value): 
      whatever code 
      
This should make GRC underline the variable_param in the GUI and it will be variable 
at runtime. 

Thanks, 

--Patrick