Problems compiling Bootstrap 4

This might be a dumb question, but I'm having a hard time figuring this out from the docs and such. I'm trying to compile Bootstrap 4 and having zero success. For now I'm just trying to do this within the bootstrap/scss directory so path issues are less of a big deal. Within this directory I have added a main.scss file with one line:

@import "bootstrap.scss";

I have a script called test_scss.py that looks like this:

from webassets import Bundle, Environment
my_env = Environment(directory='.', url='/')
css = Bundle('main.scss', filters='scss', output='all.css')
my_env.register('css_all', css)
print(my_env['css_all'].urls())

When I run this command, I get an error trace like this:

Traceback (most recent call last):
  File "./test_scss.py", line 11, in <module>
    print(my_env['css_all'].urls())
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/bundle.py", line 806, in urls
    urls.extend(bundle._urls(new_ctx, extra_filters, *args, **kwargs))
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/bundle.py", line 765, in _urls
    *args, **kwargs)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/bundle.py", line 619, in _build
    force, disable_cache=disable_cache, extra_filters=extra_filters)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/bundle.py", line 543, in _merge_and_apply
    kwargs=item_data)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/merge.py", line 276, in apply
    return self._wrap_cache(key, func)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/merge.py", line 218, in _wrap_cache
    content = func().getvalue()
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/merge.py", line 251, in func
    getattr(filter, type)(data, out, **kwargs_final)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/filter/sass.py", line 196, in input
    self._apply_sass(_in, out, os.path.dirname(source_path))
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/filter/sass.py", line 190, in _apply_sass
    return self.subprocess(args, out, _in, cwd=child_cwd)
  File "/Users/benlindsay/miniconda/lib/python3.6/site-packages/webassets/filter/__init__.py", line 527, in subprocess
    proc.returncode, stdout, stderr))
webassets.exceptions.FilterError: scss: subprocess returned a non-success result code: 65, stdout=b'',
stderr=b'DEPRECATION WARNING: Importing from the current working directory will
not be automatic in future versions of Sass.  To avoid future errors, you can add it
to your environment explicitly by setting `SASS_PATH=.`, by using the -I command
line option, or by changing your Sass configuration options.

Error: Invalid CSS after "...lor}: #{$value}": expected "{", was ";"
        on line 4 of /Users/benlindsay/scratch/python/webassets/test-2/bootstrap/scss/_root.scss
        from line 11 of /Users/benlindsay/scratch/python/webassets/test-2/bootstrap/scss/bootstrap.scss
        from line 1 of standard input
  Use --trace for backtrace.

If I follow the instructions and set environment variable SASS_PATH=., that gets rid of that part of the error message, but I still get the error

Error: Invalid CSS after "...lor}: #{$value}": expected "{", was ";"
        on line 4 of /Users/benlindsay/scratch/python/webassets/test-2/bootstrap/scss/_root.scss
        from line 11 of /Users/benlindsay/scratch/python/webassets/test-2/bootstrap/scss/bootstrap.scss
        from line 1 of standard input
  Use --trace for backtrace.

I don't know SCSS syntax well yet, but I'd bet a lot of money this is me doing something wrong and not an error in the Bootstrap SCSS. Any hints on what I'm doing wrong would be much appreciated.

Thanks in advance!

miracle2k / webassets

Problems compiling Bootstrap 4 #504