When using os.execlp, why `python` needs `python` as argv[0]

When python is executed, it creates sys.argv for you. The values in that list are based on the arguments passed to it by the operating system, but it leaves off the sys.executable value from that list.

In other words, when Python is invoked, it sets sys.argv to everything but it's own executable.

When you invoke a new executable via os.execlp(), you still need to include Python in that as that is what executable that the OS will run. The first two values of what you a pass to os.execlp() are still required, whatever you find in sys.argv later on.

The second python is a name for python, it can be any string, but it has to be there.

See the second paragraph of http://docs.python.org/3/library/os.html?highlight=os.exec#process-management:

The various exec* functions take a list of arguments for the new program loaded into the process. In each case, the first of these arguments is passed to the new program as its own name rather than as an argument a user may have typed on a command line. For the C programmer, this is the argv[0] passed to a program’s main(). For example, os.execv('/bin/echo', ['foo', 'bar']) will only print bar on standard output; foo will seem to be ignored.