Feature or enhancement

The Python CLI should automatically dedent the argument given to "-c".

I raised this issue on the Python-Ideas mailing list and it got some positive feedback, so I'm moving forward with it here and in a proof-of-concept PR.

Pitch

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?

Previous discussion

On the mailing list there were these responses:

Lucas Wiman said:

Very strong +1 to this. That would be useful and it doesn't seem like there's a downside. I often make bash functions that pipe files or database queries to Python for post-processing. I also sometimes resort to Ruby because it's easy to write one-liners in Ruby and annoying to write one-liners in python/bash.

I suppose there's some ambiguity in the contents of multi-line """strings""". Should indentation be stripped at all in that case? E.g.
python -c "
'''
some text
''''
"

But it seems simpler and easier to understand/document if you pre-process the input like using an algorithm like this:

• If the first nonempty line has indentation, and all subsequent lines either start with the same indentation characters or are empty, then remove that prefix from those lines.

I think that handles cases where editors strip trailing spaces or the first line is blank. So e.g.:
python -c "
some_code_here()
"
Then python receives something like "\n some_code_here\n"

python -c "
some_code here()

if some_some_other_code():
    still_more_code()

"
Then python receives something like "\n some_code_here\n\n if ..."

This wouldn't handle cases where indentation is mixed and there is a first line, e.g.:
python -c "first_thing()
if second_thing():
third_thing()
"
That seems simple enough to avoid, and raising a syntax error is reasonable in that case.

Best wishes,
Lucas

There was also positive feedback from @cameron-simpson and @barry-scott suggested a PR with an implementation might move this forward. I have a proof-of-concept PR and this is the corresponding issue for it.

Linked PRs

• gh-103997: Automatically dedent the argument to "-c" #103998