[Python-ideas] Re: Auto dedent -c arguments.

30 Apr 2023

      I can't think of a case where the check for "\n" would result in a false
negative. It likely makes sense to do that to keep python startup as
lightweight as possible.

I've made a PR that implements a proof-of-concept:
https://github.com/python/cpython/pull/103998

It's currently done via the Python C-API, but in terms of keeping things
lightweight, I might need some help on how to effectively use the wide
c-string API to implement this.

On Mon, Apr 10, 2023 at 4:46 AM Barry <barry@barrys-emacs.org> wrote:
...
On 10 Apr 2023, at 02:18, Jonathan Crall <erotemic@gmail.com> wrote:

There's no question that there are lots of ways you can work around the
issue (I think the `if 1:` method is the easiest and most portable, but it
still is boilerplate which can be confusing if you don't know why it's
there). The question is: would enough people benefit from this to add the
feature to CPython?
A PR that has an implementation might push this forward.
I wonder if in the C code of -c treating a string that starts with \n
could trigger the dedent?
Barry
On Sun, Apr 9, 2023 at 4:09 PM Barry Scott <barry@barrys-emacs.org> wrote:
...
On 04/04/2023 15:18, Jonathan Crall wrote:
I have what I think is a fairly low impact quality of life improvement to
suggest for the python CLI.
When I'm not working in Python I tend to be working in bash. But often I
want to break out and do something quick in Python. I find the `python -c `
CLI very useful for this. For one liners it's perfect. E.g.
NEW_VAR=$(python -c "import pathlib;
print(pathlib.Path('$MYVAR').parent.parent)")
And even if I want to do something multi-line it's pretty easy
NEW_VAR=$(python -c "
import pathlib
for _ in range(10):
    print('this is a demo, bear with me')
")
But the problem is when I'm writing bash inside a function or some other
nested code, I would like to have nice indentation in my bash file, but if
I write something like this:
mybashfunc(){
    python -c "
    import pathlib
    for _ in range(10):
        print('this is a demo, bear with me')
    "
}
I get `IndentationError: unexpected indent`.
This means I have to write the function ugly like this:
mybashfunc(){
    python -c "
import pathlib
for _ in range(10):
    print('this is a demo, bear with me')
"
}
Or use a helper function like this:
codeblock()
{
    __doc__='
    copy-pastable implementation
    Prevents indentation errors in bash
    '
    echo "$1" | python -c "import sys; from textwrap import dedent;
print(dedent(sys.stdin.read()).strip('\n'))"
}
mybashfunc(){
    python -c $(codeblock "
    import pathlib
    for _ in range(10):
        print('this is a demo, bear with me')
    ")
}
Or more recently I found that this is a low-impact workaround:
mybashfunc(){
    python -c "if 1:
    import pathlib
    for _ in range(10):
        print('this is a demo, bear with me')
    "
}
But as a certain Python dev may say: "There must be a better way."
Would there be any downside to the Python CLI automatically dedenting the
input string given to -c? I can't think of any case off the top of my head
where it would make a previously valid program invalid. Unless I'm missing
something this would strictly make previously invalid strings valid.
Thoughts?
You can solve this with a small module. I named mine run_dedent
mkdir -p run_dedent
echo pass >run_dedent/__init__.py
cat <<EOF >run_dedent/__main__.py
import sys
import textwrap
cmd = textwrap.dedent(sys.argv[1])
exec(cmd)
EOF
python3 -m run_dedent "
        import sys
        print(sys.version_info)
        "
Barry
--
-Dr. Jon Crall (him)
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-Dr. Jon Crall (him)