Meaning of "test_data: n_test" in python 3.8

I am following a tutorial on Neural_Networks and it uses python2.7 and the line of code I am confused with is the if statement ''' if test_data: n_test = len(test_data) ''' and I ever came across this syntax in python 3.8 and please explain me what does test_data: n_test means. Thank You