I would suggest taking a look at wxPython which is a set of python bindings for the wxWidgets GUI toolkit.
This provides a native look and feel on all of Windows, OS-X & Linux and can be used to produce very professional looking a GUI from python.
To get an introduction to wxPython for Python 2.7 download and install it plus the documents & demo suite.
Python 3 support is an in progress project called Project Phoenix that, while it is not yet at the first official release, works well to provide a very good, cross platform, GUI for most projects. Installation of the latest build can be done with the line:
pip install --upgrade --trusted-host wxpython.org --pre -f http://wxpython.org/Phoenix/snapshot-builds/ wxPython_Phoenix
but installation of the documents and demos package requires downloading the source from github.
- Free, Gratis & Open Source
- Licence permits commercial use as well as personal & academic
- Cross Platform Windows, OS-X & Windows
- Native Look & Feel utilises native components where ever possible
- Large & Active user & developer communities
- Mature - wxPython initial release was in 1998!
- A lot of online help & books available
- Numerous Applications already using wxPython
PySpread Pythonic Spreadsheet
Cornice Cross Platform Image Browser
OpenDict Open Source multiplatform computer dictionary.
All screenshots from the respective programs web site.