Any script worth it's salt that depends on a newer feature of Python 3 can just check `sys.version_info`. This was a thing one had to consider back when new versions of Python 2.x were coming out, too.
It's not quite that simple. Not all code that runs on Python 2 will be syntax-compatible with Python 3, and vice versa. You can, with some effort, use a subset of the Python syntax that is compatible with both interpreters, but many libraries do not do that, but ship separate Python 2 and Python 3 versions if they support both.
Syntax changes can be an issue for backwards compatibility, but that's not what I was talking about.
It's easy to specify what the minimum version of Python you want to support is. Even if there's syntax which is incompatible with Python 2 in the file, a hashbang that specifies `python3` is pretty clear.
Yes, #!/usr/bin/python3 is clear, but I don't think it addresses the question you were responding to, which was whether the scripts the OP of this subthread was talking about, which he knows will run under Python 2 (because he downloaded them and ran them under Python 2--he mentioned this in another subthread), will know to check for the presence of Python 3 and use it. Any such script will have to check sys.version_info to know which version of Python it is running under (and for at least one version the shebang will be irrelevant anyway since it can only specify python or python3, not both), and it will have to be syntax-compatible with both versions. If the script isn't syntax-compatible with Python 3, then he'll have to go find and download a version that is, if it exists.
