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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_install_sources(self):
 """Construct arguments to use alternative package indexes if there were sources supplied, empty string if there were not. """

        if not self._sources:
            return ''
        parts = ['--no-index']
        for source in self._sources:
            parts.append("--find-links '{0}'".format(source))
        return ' '.join(parts)