Why do people write the #!/usr/bin/env python shebang on the first line of a Python script?

If you have installed many versions of Python shebang line, then #!/usr/bin/env ensures that the interpreter will use the first installed version on your environment's $PATH.

If you are using Unix, an executable file that is meant to be interpreted can indicate what interpreter to use by having a #! at the start of the first line, followed by the interpreter (and any flags it may need).

For any other operating systems, of course, this rule does not apply (but that " python shebang line" does no harm, and will help if you ever copy that script to a platform with a Unix base, such as Linux, Mac, etc).