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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 create_key_file(service, key):
 """Create a file containing key."""

    keyfile = _keyfile_path(service)
    if os.path.exists(keyfile):
        log('Keyfile exists at %s.' % keyfile, level=WARNING)
        return

    with open(keyfile, 'w') as fd:
        fd.write(key)

    log('Created new keyfile at %s.' % keyfile, level=INFO)