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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_path(download_name, filename='', test_exists=True):
 """ Get the local path to a file in a MHCflurry download Parameters download_name : string filename : string Relative path within the download to the file of interest test_exists : boolean If True (default) throw an error telling the user how to download the data if the file does not exist Returns string giving local absolute path """

    assert '/' not in download_name, "Invalid download: %s" % download_name
    path = join(get_downloads_dir(), download_name, filename)
    if test_exists and not exists(path):
        raise RuntimeError(
            "Missing MHCflurry downloadable file: %s. "
            "To download this data, run:\n\tmhcflurry-downloads fetch %s\n"
            "in a shell."
            % (quote(path), download_name))
    return path