Sass sourcemaps with django-pipeline in local development environment

gistfile1.py

import re
import simplejson

from pipeline.compilers.sass import SASSCompiler

from django.conf import settings

class SASSWithSourcemapsCompiler(SASSCompiler):
    """
    For sourcemaps to work properly, we need to replace paths like these:
    ../../../../lessons/static/lessons/course/scss/paid_unit_list

    with valid local static URLs. So on every request in development we open up the sourcemap file and regex-replace
    the prefix with our static URL prefix.
    """
    REPLACE_REGEX = re.compile(r'(\.\./)+\D+/static/')

    def match_file(self, filename):
        return filename.endswith(('.scss', '.sass'))

    def compile_file(self, infile, outfile, *args, **kwargs):
        command_result = None

        # We only compile Sass via Pipline in production
        # In development, we use "sass --sourcemap --watch XYZ" so the compilation doesn't run on every request
        if not settings.DEBUG:
            command_result = super(SASSWithSourcemapsCompiler, self).compile_file(infile, outfile, *args, **kwargs)

        # In development, we assume a sourcemap file exists in the same directory as the Sass file
        # Now we need to rewrite the source URLs so things work nicely in DevTools
        if settings.DEBUG:
            sourcemap_path = outfile + '.map'
            sourcemap_file = open(sourcemap_path, 'r+')
            sourcemap = simplejson.load(sourcemap_file)

            # The line that matters
            sourcemap['sources'] = [self.REPLACE_REGEX.sub(settings.STATIC_URL, sass_partial_url)
                for sass_partial_url in sourcemap['sources']]

            sourcemap_file.seek(0)
            simplejson.dump(sourcemap, sourcemap_file)
            sourcemap_file.truncate()
            sourcemap_file.close()

        return command_result