How can i get Certificate issuer information in python?

The pyOpenSSL library seems to be outdated and here is what they say in official docs

Note: The Python Cryptographic Authority strongly suggests the use of pyca/cryptography where possible.

there are some workarounds using Python's standard library, but most of those seem to be messy.

Here is the way to do it through pyca/cryptography. It looks pretty straightforward and clean

from cryptography.x509 import load_pem_x509_certificate

cert = load_pem_x509_certificate(certificate_content)
certificate_serial_number = cert.serial_number
certificate_issuer_info = cert.issuer.rfc4514_string()
certificate_subject_info = cert.subject.rfc4514_string()

the result will print the whole line with issuer details

CN=name,O=Org\, Inc.,L=Tucson,ST=Arizona,C=US
CN=Sectigo RSA Organization Validation Secure Server CA,O=Sectigo Limited,L=Salford,ST=Greater Manchester,C=GB

If you need any particular of the details, those are available through the attributes of cert.issuer object.

If you use requests, a simple code is here:

#!/usr/bin/python
# -*- coding: utf-8 -*-


from requests.packages.urllib3.contrib import pyopenssl as reqs


def https_cert_subject_alt_names(host, port):
    """Read subject domains in https cert from remote server"""

    x509 = reqs.OpenSSL.crypto.load_certificate(
        reqs.OpenSSL.crypto.FILETYPE_PEM,
        reqs.ssl.get_server_certificate((host, port))
    )
    return reqs.get_subj_alt_name(x509)

if __name__ == '__main__':
    domains = https_cert_subject_alt_names("www.yahoo.com", 443)
    print(domains)

The result is as follow:

[('DNS', '*.www.yahoo.com'), 
 ('DNS', 'www.yahoo.com'), 
 ('DNS', 'add.my.yahoo.com'), 
 ('DNS', 'au.yahoo.com'), 
 ('DNS', 'be.yahoo.com'), 
 ('DNS', 'br.yahoo.com'), 
 ('DNS', 'ca.my.yahoo.com'), 
 ('DNS', 'ca.rogers.yahoo.com'), 
 ('DNS', 'ca.yahoo.com'), 
 ('DNS', 'ddl.fp.yahoo.com'), 
 ('DNS', 'de.yahoo.com'), 
 ...
 ('DNS', 'mbp.yimg.com')]

Updated answer

If you can establish a connection to the remote server you can use the ssl standard library module:

import ssl, socket

hostname = 'google.com'
ctx = ssl.create_default_context()
with ctx.wrap_socket(socket.socket(), server_hostname=hostname) as s:
    s.connect((hostname, 443))
    cert = s.getpeercert()

subject = dict(x[0] for x in cert['subject'])
issued_to = subject['commonName']
issuer = dict(x[0] for x in cert['issuer'])
issued_by = issuer['commonName']

>>> issued_to
u'*.google.com'
>>> issued_by
u'Google Internet Authority G2'

Original answer

Use pyOpenSSL.

from OpenSSL import crypto

cert_file = '/path/to/your/certificate'
cert = crypto.load_certificate(crypto.FILETYPE_PEM, open(cert_file).read())
subject = cert.get_subject()
issued_to = subject.CN    # the Common Name field
issuer = cert.get_issuer()
issued_by = issuer.CN

You can also access additional components, e.g. organisation (subject.O/issuer.O), organisational unit (subject.OU/issuer.OU).

Your certificate file might be in another format, so you could try crypto.FILETYPE_ASN1 instead of crypto.FILETYPE_PEM.