i'm using a python client that makes request to a push server, and has the option to use certificates with the lib OpenSSL for python(pyopenssl), but this client ask me for the private key(can be on the same file or different path. already checked with another server that i have both cert and key). but for the "real" server they are using self-signed cert and they don't have any private key in the same file or another one as far as they told me, they just share with me this cert file, is there any way to use this kind of cert with OpeenSSL in python?, thanks
In order to use a client certificate, you must have a private key. You will either sign some quantity with your private key or engage in encrypted key agreement in order to mutually agree on a key.  Either way, the private key is required.
It's possible though that rather than using client certificates they use something like username/password and what they have given you is not a certificate for you to use, but the certificate you should expect from them.
OpenSSL ask for private key