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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 retrieve_sources():
 """Retrieve sources using spectool """

    spectool = find_executable('spectool')
    if not spectool:
        log.warn('spectool is not installed')
        return
    try:
        specfile = spec_fn()
    except Exception:
        return

    cmd = [spectool, "-g", specfile]
    output = subprocess.check_output(' '.join(cmd), shell=True)
    log.warn(output)