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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 updated(bank, key, cachedir):
    '''
    Return the epoch of the mtime for this cache file
    '''
    key_file = os.path.join(cachedir, os.path.normpath(bank), '{0}.p'.format(key))
    if not os.path.isfile(key_file):
        log.warning('Cache file "%s" does not exist', key_file)
        return None
    try:
        return int(os.path.getmtime(key_file))
    except IOError as exc:
        raise SaltCacheError(
            'There was an error reading the mtime for "{0}": {1}'.format(
                key_file, exc
            )
        )