Is it better to do it all at once? I'm very new to wxPython and I'm thinking it would be better to write the program in a way familiar to me, then apply the wxPython gui to it after I'm satisfied with the overall design of the app. Any advice?
If you're used to a more command line approach, this would be a bad idea. Responding to user input is a completely different paradigm, and you're not likely to get it right the first time.
If you're just talking about the difference between wxPython and another GUI, don't worry about it.
Is it a bad idea to design and develop a python applications backend and then once finished try to apply a GUI to it?