Tkinter is the de facto standard GUI library for Python. It is cross-platform and included in virtually all distributions as part of the Python standard library.
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Pro Free for commercial use
Pro Included in the standard Python library
You probably already have it, since it's included in the standard Python distributions.
Pro Building executables is less complicated
Compared to other GUI libraries, building executables for TkInter applications are simpler because TkInter is included in Python and has no other dependencies. This results in less complicated packaging requirements and smaller binary size.
Pro Easy to learn and get productive with
TkInter is a small library with a gentle learning curve compared to what’s out there. It has a straightforward API and is often the go-to choice for building quick GUIs for Python scripts.
Con No advanced widgets
TkInter does not come with advanced widgets out of the box (e.g. date picker).
Con No reliable UI builder available
There is no tool in the same league as Qt Designer (PyQt) for TkInter.