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?
Since you are new to GUI programming, your approach is perfectly valid. It will likely result in a less than optimal UI, but that's OK for now. And in fact, there are some very successful multi-million dollar commercial projects that are built this way.
Arguably a better approach is to first design the UI since that is the most important part. After that is completel you can then create a back-end that can support that UI. This approach still results in separate front- and back-ends but puts the emphasis on the needs of the user, where it should be.
Is it a bad idea to design and develop a python applications backend and then once finished try to apply a GUI to it?