PGPSignature API change: keyserver,policy_uri return None if no subpa…

…cket present

This is a change in the API to more accurately reflect what is
happening in a signature.  It's possible to have a subpacket that
contains the empty string, and this allows a consumer to distinguish
between the two cases.

docs/source/changelog.rst

@@ -38,6 +38,8 @@ The following properties of PGPSignature now return None if the
 corresponding subpacket is not present (they used to return an empty
 string in that case):
 
+* keyserver
+* policy_uri
 * signer
 * signer_fingerprint
 

pgpy/pgp.py

@@ -214,13 +214,13 @@ def key_flags(self):
         return set()
 
     @property
-    def keyserver(self):
+    def keyserver(self) -> Optional[str]:
         """
-        The preferred key server specified in this signature, if any. Otherwise, an empty ``str``.
+        The preferred key server specified in this signature, if any. Otherwise, None.
         """
         if 'PreferredKeyServer' in self._signature.subpackets:
             return next(iter(self._signature.subpackets['h_PreferredKeyServer'])).uri
-        return ''
+        return None
 
     @property
     def keyserverprefs(self):
@@ -243,13 +243,13 @@ def notation(self):
         return {nd.name: nd.value for nd in self._signature.subpackets['NotationData']}
 
     @property
-    def policy_uri(self):
+    def policy_uri(self) -> Optional[str]:
         """
-        The policy URI specified in this signature, if any. Otherwise, an empty ``str``.
+        The policy URI specified in this signature, if any. Otherwise, None.
         """
         if 'Policy' in self._signature.subpackets:
             return next(iter(self._signature.subpackets['Policy'])).uri
-        return ''
+        return None
 
     @property
     def revocable(self) -> bool:

tests/test_03_armor.py

@@ -199,11 +199,11 @@
          ('is_expired',     False),
          ('key_algorithm',  PubKeyAlgorithm.RSAEncryptOrSign),
          ('key_flags',      set()),
-         ('keyserver',      ''),
          ('keyserverprefs', []),
+         ('keyserver',      None),
          ('magic',          "SIGNATURE"),
          ('notation',       {}),
-         ('policy_uri',     ''),
+         ('policy_uri',     None),
          ('revocable',      True),
          ('revocation_key', None),
          ('signer',         KeyID('FCAE54F74BA27CF7')),