Shebang

It's common on Unix-like systems to have the first line of a script include a special comment, #!, called a shebang. This allows the program loader to identify which interpreter should be used to run the program. Shebangs have an additional purpose of conveniently documenting at the top of a file whether the Python code therein is Python 2 or Python 3.

The exact details of your shebang command depend on the location of Python on your system. Typical Python 3 shebangs use the Unix env program to locate Python 3 on your PATH environment variable, which importantly is compatible with Python virtual environments:

#!/usr/bin/env python3

Executable Python programs on Linux and Mac

On Mac or Linux, we must mark our script as executable using the chmod command before the shebang will have any effect:

$ chmod +x words.py

Having done that, we can now run our script directly:

$ ./words.py http://sixty-north.com/c/t.txt

Executable Python programs on Windows

Starting with Python 3.3, Python on Windows...