How to capture all Error messages from Python and save them in a txt file

I would like to capture all errors that are created during the runtime or during testing(using pytest) of a Python program. So far I have managed to capture all output of pytest using a plugin for pytest which saves the output to a .txt file line by line.

But now I would like to have all the errors created by Python in a .txt file. As I am new to Python and also new to such capturing of program output, do you have any ideas how could I achieve something like this?

Thanks for all the answers in advance.