uses both shell and python codes in one script.

[James Lu]

I would use IPython as a scripting language. It has a slow startup time
though.

On Thu, Oct 3, 2019 at 9:59 AM Chris Angelico <rosuav at gmail.com> wrote:

> On Thu, Oct 3, 2019 at 11:41 PM Hongyi Zhao <hongyi.zhao at gmail.com> wrote:
> >
> > On Thu, 03 Oct 2019 23:12:45 +1000, Chris Angelico wrote:
> >
> >
> > > Seems fine. Most of the code is elsewhere, and presumably has been
> > > written to support both Py2 and Py3; the file you're linking to is
> > > *just* a wrapper that finds an interpreter to use.
> > >
> > > Though this should be unnecessary. A simple shebang of "/usr/bin/env
> > > python3" will suffice for many many situations (and then if someone
> > > specifically wants to run it in a legacy interpreter, an explicit
> > > "python2 scriptname.py" or "python scriptname.py" will work).
> >
> > I'm very confusing on the following part in this script:
> >
> > ----
> > ''':' # begin python string; this line is interpreted by the shell as `:`
> > which python  >/dev/null 2>&1 && exec python  "$0" "$@"
> > which python3 >/dev/null 2>&1 && exec python3 "$0" "$@"
> > which python2 >/dev/null 2>&1 && exec python2 "$0" "$@"
> > >&2 echo "error: cannot find python"
> > exit 1
> > '''
> > ----
> >
> > Any hints for the meaning of several ' used above?
> >
>
> The hint is there in that line, and stems from the way two different
> parsers (Python and sh) interpret the line. In Python, three single
> quote characters start a triple-quoted string, which doesn't end till
> you get three more; since nothing is done with this string, Python
> parses it and then ignores it. In the shell, the first two are an
> empty string, then ':' is a colon, which introduces a label (the fact
> that it's in quotes is irrelevant to the shell). So there's an empty
> label followed by a shell comment. The shell parses this line and does
> nothing with it. Then it moves on to the next lines, and runs the
> shell script. Since this shell script ends with 'exit 1', it's
> guaranteed to halt execution (and usually it'll exec to python, which
> also halts execution), so the Python code won't be executed.
>
> This is a common trick when writing polyglot code. You make the
> relevant code for one language appear as a comment or string literal
> in another. For instance, you can open a C program with "#if 0", which
> will cause the following text to be ignored by the C preprocessor; but
> since that line begins with a hash, Python will ignore it (but
> continue executing).
>
> ChrisA
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>