Textual has devtools in the upcoming css branch, and its pretty awesome!
Textual is still very early and not really ready for prime time, but it's quite amazing how easy some things such as creating keybindings is. The docs are coming, but missing right now so if you want to use textual be ready for reading source code and examples.
As @willmcgugan shows in this tweet it's pretty easy to setup, it requires having two terminals open, or using tmux, and currently you have to use the css branch.
Textual is a tui application framework. Unlike when you are building cli applications, when the tui takes over the terminal in full screen there is no where to print statement debug, and breakpoints don't work.
In the future it will likely be in main and not need this, but for now you need to get the css branch to get devtools.
git clone https://github.com/Textualize/textual git fetch --alll git checkout css
Now you can create a virtual environment, feel free to use whatever virtual environment tool you want, venv is built in to most python distributions though, and should just be there.
python3 -m venv .venv --prompt textual source .venv/bin/activate pip install .
Once textual is installed you can open up the devtools by running textual console.