How to use the fastecdsa.keys function in fastecdsa

def get_public_key(self) -> PublicKey:
        public_key_impl = keys.get_public_key(self.impl, self.curve)
        return ECCPublicKey(public_key_impl, self.curve)

def sign_with_k(bytes_msg: bytes,
                bytes_private: bytes,
                k: int) -> Optional[bytes]:

    private_key = int.from_bytes(bytes_private, "big")

    order = CURVE.q

    Q = CURVE.G * k
    bytes_Q_x = encode_public(Q.x, Q.y)

    pub_key = keys.get_public_key(private_key, CURVE)
    bytes_pub_x = encode_public(pub_key.x, pub_key.y)

    hasher = sha256()
    hasher.update(bytes_Q_x + bytes_pub_x + bytes_msg)
    r = hasher.digest()
    r = int.from_bytes(r, "big") % order
    s = (k - r * private_key) % order
    if r == 0 or s == 0:
        return None

    return encode_signature(r, s)

def get_pub_key(priv_key: str) -> str:
    '''
    Returns the public key associated with
    the private key. 'x' is used to split
    up the x and y coordinates of the public
    key
    '''
    pub_key = keys.get_public_key(int(priv_key, 16), curve.P256)
    return '{:x}'.format(pub_key.x) + 'x' + '{:x}'.format(pub_key.y)