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Comment by tyingq | Hacker News Reader

Put an f-string into a script and get python2 not to choke on it, or try catching that SyntaxError as you describe...without using eval, futures, or similar. You can't catch something that already happened.

Yes, people are able to write backwards compatible scripts... by avoiding some features, or by using more than a single file.

What I'm describing was just a desire for a very simple "this script requires python version >= X" error, with the ability to write unfettered python version X code below it, in a single file script.

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This isn't pretty, but it's possible:

  import sys
  if sys.version_info[0] < 3:
      print("Needs Python 3+")
      sys.exit(1)
  else:
      prog = r'''
  
  def help():
      print("Help: ...")
  
  def do_spam(*args):
      print("spamming")
  
  def main(args):
      match args:
          case [subcommand, *args]:
              return globals()[f'do_{subcommand}'](*args)
          case _:
              return help()
  
  if __name__ == '__main__':
      try:
          status = main(sys.argv[1:])
          sys.exit(int(status if status is not None else 0))
      except Exception as e:
          import traceback
          traceback.print_exc(e)
          sys.exit(1)
  '''
      compile(prog, '', 'exec')
      exec(prog)

Sure, a variation on eval. I still think some sort of BEGIN equivalent would have been nice. But, the optimal time for that has passed, so I'm just complaining anyway :)

i meant exec as in a shell script to replace the current PID, not python exec()

What I'm describing was just a desire for a very simple "this script requires python version >= X" error, with the ability to write unfettered python version X code below it, in a single file script.

  import sys
  if sys.version_info.major < 3:
    raise Exception("this script requires python version >= 3")

plus an extra line or two if you're worried about minor versions.

  > cat test.py 
  import sys
  if sys.version_info.major < 3:
      raise Exception("this script requires python version >= 3")
  
  print(f"test")
  
  > python2 test.py
    File "test.py", line 5
      print(f"test")
                  ^
  SyntaxError: invalid syntax

Doesn't seem that simple

It needs to also do:

   from __future__ import absolute_import, division, print_function, unicode_literals

This will give you syntax much closer to Python 3 in Python 2.7. The remaining differences can be papered over with function and class helpers. Well, except for metaclasses, but I doubt you'd have need for those in a single-file script.