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Comment by seniorsassycat | Hacker News Reader

How do these work for very old versions of python?

    $ python3.5 since-3.6.py
      File "since-3.6.py", line 1
        0_0  # Python >= 3.6 is required

This error means python 3.5 knows the syntax is valid in 3.6 which is plausible, but why would `python2.4 since-3.6.py` have a useful error message?

Edit: the comment is included in the source and python prints the whole line

    0_0 # use python 3.6 or greater

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> This error means python 3.5 knows the syntax is valid in 3.6

It doesn't. This relies on the interpreter printing the whole line that contains a syntax error, including the comment on it.

But that's not an error message; it's just a copy of the offending line in the file!

  $ python2
  Python 2.7.17 (default, Feb 27 2021, 15:10:58) 
  [GCC 7.5.0] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> 0_0  # Python >= 3.6 is required
    File "<stdin>", line 1
      0_0  # Python >= 3.6 is required

That's the point. Since it can't actually parse the file, this is as good as it gets. It at least gets something on the screen that says "Python >= 3.6 is required" which is better than nothing.

Python 3.5 doesn't know the syntax is valid in 3.6. It knows it's invalid python 3.5 syntax, and then throws the exception. We see the comment in the process.

    % python2 since_3.6.py 
      File "since_3.6.py", line 1
        0_0  # Python >= 3.6 is required
          ^
    SyntaxError: invalid syntax
    % python2 --version
    Python 2.7.18

The error message is a comment in the code, so python2.4 just has to print the line with the syntax error

  $ python2 --version
  Python 2.7.18

  $ python2 py36.py
    File "py36.py", line 1
      0_0
        ^
  SyntaxError: invalid syntax

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