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Comment by tyingq | Hacker News Reader

I had commented on something related to this a couple of weeks ago here: https://news.ycombinator.com/item?id=28930278

The top reply had a great suggestion. Putting this at the top of a python 3.6+ required script results in a reasonable error for those with older /usr/bin/python interpreters, instead of a generic syntax error stack trace.

  #!/usr/bin/python
  0_0  # Python >= 3.6 is required

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Why use a hack when you can do it cleanly and give a good error message to your user ?

    import sys

    if sys.version_info < (3, 6):
        print('This script requires python 3.6+)

    else:
        from module import main
        main()

If you want all of it to be in one file, you can use a zipapp: https://docs.python.org/fr/3/library/zipapp.html

If you don't want to make the zipapp manually, or want it to automatically extract everything on first run, use shiv:

http://shiv.readthedocs.io

There's still lots of use cases for sharing a single file script where you would like people with Python2 as /usr/bin/python to get a reasonable error. Zipapps don't work well as gists, for example.

Edit: Zipapps feel hacky to me. I get that not everyone would share that opinion.

A zipapp is not a hack, it's an official feature of the python stdlib (as shown in the parent comment), and is made for the specific use case of grouping several files into one module for ease of sharing.

All of this works fine with python 2.7 as long as you put the rest of your code in your main() entry point, which you should do anyway. So an f-string would cause no problem.

Also, "informative stack trace" is only informative to another dev, and an experienced one at that, given how many juniors I encounter that can't read a traceback.

Indeed, you script users may not be devs, and for those, a traceback would be confusing. For this reason, a script should only output a traceback in dev mode, but in production, if the dev flag is not set (-X dev), you should never see a traceback. It's a good practice to suppress it with sys.excepthook, and log it into a file instead.

Using a syntax error like you do for this purpose IS the big hack.

Now I'm not against a hack. I can see the value of it for your own scripts, of things that you code quick and dirty style. After all, I don't always want to have a clean entry point and more than one file.

Can you trap initial parse errors with sys.excepthook? I don't think you can, at least not in a single file script.

It is not necessary, as I indicated before, if you isolate your entry point, you will not run into syntax errors (unless you publish broken code).

Well, where broken code is code that assumes /usr/bin/python would be python3.

Again, not a problem if you isolate your entry point. Python will not parse the incompatible code since it's never imported.

You keep repeating the same thing a if you ignored this essential part.

>You keep repeating the same thing a if you ignored this essential part.

Well, yes, and you keep insisting that everything is fine, as-is. There was a missed opportunity here.

Yep. I'd add that making sure your script returns a proper error code and to output error logging to `stderr` is good practice too. No additional imports required in this case as you already have `sys`, by just replacing the `print('...')` with `sys.exit('...')`.

While this looks funny and smart, it's not really a good solution as it lacks well communicated documentation. Python has a proper way to check for version, which is backward-compatible till version 2.0, so around 20+ years.

  import sys
  if sys.version_info.major < 3:
      raise Exception('Need Python 3 or higher')

EDIT: correction, sys.version_info.major is only there since Python 2.7. For older versions sys.version_info[0] should be used.

>so around 20+ years.

Right, but if there's anything below that that would cause a syntax error, like an f-string, that Exception you created never happens. The end user just sees a syntax error.

It's unfortunate there's not a better way to handle it, especially with so many people still having /usr/bin/python linked to a 2.x version.

I think this is worse because once you have a python 3 only syntax in your script (such as f-string), python 2 will simply throw syntax error because it need to parse the file to execute. The original solution looks hacky, but it can actually inform user to use python 3.

I usually put this at the top, so it raises a syntax error if using an old version.

    f"Python 3.6+ is required"

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