Parse texture to custom shader

Hi, 
We wrote a cutom shader and it is now working. By clicking on a button is possible to load the textures used by the shader.
I would like to assign the textures automatically from a python script, however I can't seem to find a way to assign these textures. 
Are there any tips on how to do through python script what the "Select texture" button is doing?

Thanks, 
Mauro