python == python2 OR python == python3 ? How to package, distribute python py2k scripts?

Question

Depending on system, python==python2 or python==python3.

Executable Python scripts, starts with:

#!/usr/bin/env python
#!/usr/bin/env python2
#!/usr/bin/env python3...

For python py3k it is provided in documentation I should/can use it with version number, so I do this:

#!/usr/bin/env python3

But I've found a problem with py2k scripts.

While in py2k documentation it is written to use : #! /usr/bin/env python ,

on some *nix-es python py3k is default, so python==python3. (For example ArchLinux python package , here python package files).

How to package (configure,make) and/or prepare python scripts for distribution to handle that ?

I ask about making software packages that can be run easily by users (without modyfing their environment)

Can I do the same trick for python py2k scripts as for python py3k scripts and set it as : #!/usr/bin/env python2 ? Can I be sure that each python py2k distribution contains python2 file, so #!/usr/bin/env python2 will work ?

If yes, why it is not proposed as standard, for example in python py2k documentation ?

Answer 1

A script can check its Python version and, if that's Python 3, re-start itself using Python 2. Add the following near the head of the script:

if sys.version > '3':
  python2 = os.popen('which python2 2> /dev/null').read().rstrip()
  if python2:
    args = sys.argv[:]
    args.insert(0,python2)
    os.execv(python2,args)
  else:
    sys.exit("%s requires Python Version 2 (python2 not in PATH)" % os.path.basename(__file__))

This uses the system's which command to locate python2 in the environment's PATH. It then relaunches itself with that (or aborts if unable to find it).

Note that the script does need to be valid Python 3 syntax for it to start in Python 3.

Also, any output should be flushed before the execv call or it will be lost. Adding, for example, sys.stdout.flush() just before the call to execv will flush any print statements.

Answer 2

On older versions, there may just be python instead of python2. To make your sheebang line clearer, you could create a link python2 -> python so you can use #!/usr/bin/env python2.

Answer 3

I think the "standard" is defined in https://www.python.org/dev/peps/pep-0394/

This PEP provides a convention to ensure that Python scripts can continue to be portable across *nix systems, regardless of the default version of the Python interpreter (i.e. the version invoked by the python command).

• python2 will refer to some version of Python 2.x.
• python3 will refer to some version of Python 3.x.
• for the time being, all distributions should ensure that python refers to the same target as python2.
• however, end users should be aware that python refers to python3 on at least Arch Linux (that change is what prompted the creation of this PEP), so python should be used in the shebang line only for scripts that are source compatible with both Python 2 and 3.
• in preparation for an eventual change in the default version of Python, Python 2 only scripts should either be updated to be source compatible with Python 3 or else to use python2 in the shebang line.