[IMPR] Assert internal state on Claim modifications

Add an assertion to ensure some operations cannot be
performed on a qualifier or a reference (such as
adding qualifiers/references or changing the rank)
and another one to ensure some operations can only
be performed on an existing (saved) claim.

Note that in the second case the operations would
already crash. But now the error messages are more
clear.

Change-Id: If7867b67ac0af72a5be0650ece2813fe1a964077
---
M pywikibot/page/_wikibase.py
1 file changed, 45 insertions(+), 8 deletions(-)

diff --git a/pywikibot/page/_wikibase.py b/pywikibot/page/_wikibase.py
index 9e857fc..d2af3ac 100644
--- a/pywikibot/page/_wikibase.py
+++ b/pywikibot/page/_wikibase.py
@@ -46,7 +46,7 @@
 from pywikibot.page._filepage import FilePage
 from pywikibot.page._page import BasePage
 from pywikibot.site import DataSite, Namespace
-from pywikibot.tools import cached
+from pywikibot.tools import cached, first_upper
 
 
 __all__ = (
@@ -271,7 +271,7 @@
 
         data = {}
 
-        # This initializes all data,
+        # This initializes all data
         for key, cls in self.DATA_ATTRIBUTES.items():
             value = cls.fromJSON(self._content.get(key, {}), self.repo)
             setattr(self, key, value)
@@ -1416,8 +1416,8 @@
         self._on_item = None  # The item it's on
 
     @property
-    def on_item(self):
-        """Return item this claim is attached to."""
+    def on_item(self) -> Optional[WikibaseEntity]:
+        """Return entity this claim is attached to."""
         return self._on_item
 
     @on_item.setter
@@ -1431,6 +1431,16 @@
                 for source in values:
                     source.on_item = item
 
+    def _assert_attached(self) -> None:
+        if self.on_item is None:
+            raise RuntimeError('The claim is not attached to an entity')
+
+    def _assert_mainsnak(self, message: str) -> None:
+        if self.isQualifier:
+            raise RuntimeError(first_upper(message.format('qualifier')))
+        if self.isReference:
+            raise RuntimeError(first_upper(message.format('reference')))
+
     def __repr__(self) -> str:
         """Return the representation string."""
         return '{cls_name}.fromJSON({}, {})'.format(
@@ -1664,6 +1674,7 @@
         :param snaktype: The new snak type ('value', 'somevalue', or
             'novalue').
         """
+        self._assert_attached()
         if value:
             self.setTarget(value)
 
@@ -1710,10 +1721,13 @@
 
     def setRank(self, rank) -> None:
         """Set the rank of the Claim."""
+        self._assert_mainsnak('Cannot set rank on a {}')
         self.rank = rank
 
     def changeRank(self, rank, **kwargs):
         """Change the rank of the Claim and save."""
+        self._assert_mainsnak('Cannot change rank on a {}')
+        self._assert_attached()
         self.rank = rank
         return self.repo.save_claim(self, **kwargs)
 
@@ -1747,6 +1761,7 @@
         :param claims: the claims to add
         :type claims: list of pywikibot.Claim
         """
+        self._assert_mainsnak('Cannot add sources to a {}')
         for claim in claims:
             if claim.on_item is not None:
                 raise ValueError(
@@ -1779,6 +1794,8 @@
         :param sources: the sources to remove
         :type sources: list of pywikibot.Claim
         """
+        self._assert_mainsnak('Cannot remove sources from a {}')
+        self._assert_attached()
         data = self.repo.removeSources(self, sources, **kwargs)
         self.on_item.latest_revision_id = data['pageinfo']['lastrevid']
         for source in sources:
@@ -1792,6 +1809,7 @@
         :param qualifier: the qualifier to add
         :type qualifier: pywikibot.page.Claim
         """
+        self._assert_mainsnak('Cannot add qualifiers to a {}')
         if qualifier.on_item is not None:
             raise ValueError(
                 'The provided Claim instance is already used in an entity')
@@ -1819,8 +1837,10 @@
         Remove the qualifiers.
 
         :param qualifiers: the qualifiers to remove
-        :type qualifiers: list Claim
+        :type qualifiers: list of pywikibot.Claim
         """
+        self._assert_mainsnak('Cannot remove qualifiers from a {}')
+        self._assert_attached()
         data = self.repo.remove_qualifiers(self, qualifiers, **kwargs)
         self.on_item.latest_revision_id = data['pageinfo']['lastrevid']
         for qualifier in qualifiers:
@@ -1879,9 +1899,7 @@
         :param target: qualifier target to check presence of
         :return: true if the qualifier was found, false otherwise
         """
-        if self.isQualifier or self.isReference:
-            raise ValueError('Qualifiers and references cannot have '
-                             'qualifiers.')
+        self._assert_mainsnak('{}s cannot have qualifiers')
         return any(qualifier.target_equals(target)
                    for qualifier in self.qualifiers.get(qualifier_id, []))
 

