Python 3.5 doesn't know the syntax is valid in 3.6. It knows it's invalid python 3.5 syntax, and then throws the exception. We see the comment in the process.

Python 3.5 doesn't know the syntax is valid in 3.6. It knows it's invalid python 3.5 syntax, and then throws the exception. We see the comment in the process. % python2 since_3.6.py File "since_3.6.py", line 1 0_0 # Python >= 3.6 is required ^ SyntaxError: invalid syntax % python2 --version Python 2.7.18