Clean up NotImplemented/NotImplementedError

- if something is not implemented, just raise NotImplementedError
  directly, rather than trying to sometimes return a NotImplemented
  object.  Returning a NotImplemented object complicates the type
  signature without providing much of an advantage in terms of
  workflow.

- PGPSignature.target_signature is a property that was never
  implemented or used.  We can just drop this property entirely,
  implementing it only when it can actually be implemented.

pgpy/packet/fields.py

@@ -172,7 +172,7 @@ def __getitem__(self, key):
 
     def __delitem__(self, key):
         ##TODO: this
-        raise NotImplementedError
+        raise NotImplementedError()
 
     def __contains__(self, key):
         return key in {k for k, _ in itertools.chain(self._hashed_sp, self._unhashed_sp)}
@@ -384,10 +384,10 @@ def publen(self) -> int:
         return len(self)
 
     def verify(self, subj, sigbytes, hash_alg):
-        raise NotImplemented  # pragma: no cover
+        raise NotImplementedError()  # pragma: no cover
 
     def encrypt(self, symalg: Optional[SymmetricKeyAlgorithm], data: bytes, fpr: Fingerprint) -> CipherText:
-        raise NotImplemented
+        raise NotImplementedError()
 
     def _encrypt_helper(self, symalg: Optional[SymmetricKeyAlgorithm], plaintext: bytes) -> bytes:
         'Common code for re-shaping session keys before storing in PKESK'
@@ -405,7 +405,7 @@ def __iter__(self):
         yield self.data
 
     def __pubkey__(self):
-        return NotImplemented
+        raise NotImplementedError()
 
     def __bytearray__(self) -> bytearray:
         return self.data
@@ -1441,7 +1441,7 @@ def encrypt_keyblob(self, passphrase: str,
 
     @abc.abstractmethod
     def decrypt_keyblob(self, passphrase: Union[str, bytes]) -> None:
-        ...
+        raise NotImplementedError()
 
     def _decrypt_keyblob_helper(self, passphrase: Union[str, bytes]) -> Optional[bytearray]:
         if not self.s2k:  # pragma: no cover
@@ -1477,10 +1477,10 @@ def _decrypt_keyblob_helper(self, passphrase: Union[str, bytes]) -> Optional[byt
         return bytearray(pt)
 
     def sign(self, sigdata, hash_alg):
-        return NotImplemented  # pragma: no cover
+        raise NotImplementedError()  # pragma: no cover
 
     def decrypt(self, ct: CipherText, fpr: Fingerprint, get_symalg: bool) -> Tuple[Optional[SymmetricKeyAlgorithm],bytes]:
-        raise NotImplemented
+        raise NotImplementedError()
 
     def _decrypt_helper(self, plaintext: bytes, get_symalg: bool) -> Tuple[Optional[SymmetricKeyAlgorithm], bytes]:
 
@@ -1528,7 +1528,7 @@ def clear(self) -> None:
 
 class OpaquePrivKey(PrivKey, OpaquePubKey):  # pragma: no cover
     def __privkey__(self):
-        return NotImplemented
+        raise NotImplementedError()
 
     def _generate(self, key_size_or_oid: Optional[Union[int, EllipticCurveOID]]) -> None:
         raise NotImplementedError()

pgpy/pgp.py

@@ -364,10 +364,6 @@ def intended_recipients(self):
         """
         return map(lambda x: x.intended_recipient, self._signature.subpackets['IntendedRecipient'])
 
-    @property
-    def target_signature(self):
-        return NotImplemented
-
     @property
     def type(self) -> SignatureType:
         """
@@ -1010,7 +1006,7 @@ def __format__(self, format_spec: str) -> str:
         if isinstance(self._uid, UserID):
             return self._uid.uid
 
-        raise NotImplementedError
+        raise NotImplementedError()
 
 
 class PGPMessage(Armorable):
@@ -1103,7 +1099,7 @@ def type(self) -> Literal['cleartext', 'literal', 'encrypted']:
         if isinstance(self._message, (SKEData, IntegrityProtectedSKEData)):
             return 'encrypted'
 
-        raise NotImplementedError
+        raise NotImplementedError()
 
     def __init__(self) -> None:
         """
@@ -2225,8 +2221,6 @@ def _sign(self, subject: PGPSubject, sig: PGPSignature, **prefs) -> PGPSignature
         sig._signature.hash2 = bytearray(h2.finalize()[:2])
 
         _sig = self._key.sign(sigdata, getattr(hashes, sig.hash_algorithm.name)())
-        if _sig is NotImplemented:
-            raise NotImplementedError(self.key_algorithm)
 
         sig._signature.signature.from_signer(_sig)
         sig._signature.update_hlen()
@@ -2811,8 +2805,6 @@ def _filter_sigs(sigs: Iterable[PGPSignature]) -> Iterator[PGPSignature]:
                     sigv.add_sigsubj(sig, self, subj, issues)
                 else:
                     verified = self._key.verify(sig.hashdata(subj), sig.__sig__, getattr(hashes, sig.hash_algorithm.name)())
-                    if verified is NotImplemented:
-                        raise NotImplementedError(sig.key_algorithm)
 
                     sigv.add_sigsubj(sig, self, subj, SecurityIssues.WrongSig if not verified else SecurityIssues.OK)