How to execute a python script in a different directory?

The easiest answer is probably to change your working directory, then call the second .py file from where it is:

python a.py && cd testA && python ../b.py

Of course you might find it even easier to write a script that does it all for you, like so:

Save this as runTests.sh in the same directory as a.py is:

#!/bin/sh
python a.py
cd testA
python ../b.py

Make it executable:

chmod +x ./runTests.sh

Then you can simply enter your directory and run it:

./runTests.sh