code: def unlock(self, passphrase):
          
          if self.is_public:
              # we can't unprotect public keys because only private key material is ever protected
              warnings.warn("Public keys cannot be passphrase-protected", stacklevel=3)
              yield self
              return
  
          if not self.is_protected:
              # we can't unprotect private keys that are not protected, because there is no ciphertext to decrypt
              warnings.warn("This key is not protected with a passphrase", stacklevel=3)
              yield self
              return
  
          try:
              for sk in itertools.chain([self], self.subkeys.values()):
                  sk._key.unprotect(passphrase)
              del passphrase
              yield self
  
          finally:
              # clean up here by deleting the previously decrypted secret key material
              for sk in itertools.chain([self], self.subkeys.values()):
                  sk._key.keymaterial.clear()
docs: Context manager method for unlocking passphrase-protected private keys. Has no effect if the key is not both
          private and passphrase-protected.
  
          When the context managed block is exited, the unprotected private key material is removed.
  
          Example::
  
              privkey = PGPKey()
              privkey.parse(keytext)
  
              assert privkey.is_protected
              assert privkey.is_unlocked is False
              # privkey.sign("some text") <- this would raise an exception
  
              with privkey.unlock("TheCorrectPassphrase"):
                  # privkey is now unlocked
                  assert privkey.is_unlocked
                  # so you can do things with it
                  sig = privkey.sign("some text")
  
              # privkey is no longer unlocked
              assert privkey.is_unlocked is False
  
          Emits a :py:obj:`~warnings.UserWarning` if the key is public or not passphrase protected.
  
          :param str passphrase: The passphrase to be used to unlock this key.
          :raises: :py:exc:`~pgpy.errors.PGPDecryptionError` if the passphrase is incorrect