python 从private key pem文件中加载public key

import rsa
import logging
from Crypto.PublicKey import RSA

class RsaUtil:
    def __init__(self, pem_file):
        with open(pem_file) as privatefile:
            keydata = privatefile.read()
        self._private_key = rsa.PrivateKey.load_pkcs1(keydata)
        try:
            fobj = open(pem_file, "r")
        except IOError as err:
            log.fatal("init rsautil failed: %s" % str(err))
        else:
            prikey = fobj.read()
            rsakey = RSA.importKey(prikey)
            self._public_key = rsakey.publickey()
            fobj.close()