code: def decode_privkey_hex(privkey_hex):
      
      if not isinstance(privkey_hex, (str, unicode)):
          raise ValueError("private key is not a string")
  
      # force uncompressed
      priv = str(privkey_hex)
      if len(priv) > 64:
          if priv[-2:] != '01':
              raise ValueError("private key does not end in '01'")
  
          priv = priv[:64]
  
      pk_i = int(priv, 16)
      return pk_i
docs: Decode a private key for ecdsa signature