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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 _proxy_conf_file(proxyfile, test):
    '''
    Check if proxy conf exists and update
    '''
    changes_old = []
    changes_new = []
    success = True
    if not os.path.exists(proxyfile):
        try:
            if not test:
                changes_new.append(_write_proxy_conf(proxyfile))
                msg = 'Salt Proxy: Wrote proxy conf {0}'.format(proxyfile)
            else:
                msg = 'Salt Proxy: Update required to proxy conf {0}' \
                    .format(proxyfile)
        except (OSError, IOError) as err:
            success = False
            msg = 'Salt Proxy: Error writing proxy file {0}'.format(err)
            log.error(msg)
            changes_new.append(msg)
        changes_new.append(msg)
        log.debug(msg)
    else:
        msg = 'Salt Proxy: {0} already exists, skipping'.format(proxyfile)
        changes_old.append(msg)
        log.debug(msg)
    return success, changes_new, changes_old