When comparing IUP vs Tkinter, the Slant community recommends IUP for most people. In the question“What are the best cross-platform GUI toolkits?” IUP is ranked 14th while Tkinter is ranked 20th.
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Pros
Pro Lightweight library and simple C interface
Pro Native controls on each supported OS
Pro Just UI
Easier to mix with other libraries since it doesn't try to control all that.
Pro Free/gratis
Pro MIT license
Pro Supports embedded OpenGL
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 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.
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 Fast
Cons
Con Just UI, no additional functionality support (like network etc)
Con No advanced widgets
TkInter does not come with advanced widgets out of the box (e.g. date picker).
Con Not good looking
You need a lot of code to make it look good.
Con No reliable UI builder available
There is no tool in the same league as Qt Designer (PyQt) for TkInter.