[cleanup] No longer accept BaseBot.init_page to return None

Change-Id: I10b2c6e7929b27db34b94c492da3a8b6d3d6a63e
---
M pywikibot/bot.py
1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/pywikibot/bot.py b/pywikibot/bot.py
index a2b65ca..cceffd1 100644
--- a/pywikibot/bot.py
+++ b/pywikibot/bot.py
@@ -1344,10 +1344,8 @@
         skip_page and treat.
 
         @param item: any item from self.generator
-        @return: return the page object to be processed further or None if
-            page is a pywikibot.Page already and superclass' init_page isn't
-            to be called
-        @rtype: pywikibot.Page or None
+        @return: return the page object to be processed further
+        @rtype: pywikibot.Page
         """
         return item
 
@@ -1415,8 +1413,15 @@
                         sys.exc_clear()
                     self._skip_counter += 1
                     continue
+
+                if initialized_page is None:
+                    issue_deprecation_warning(
+                        'Returning None from init_page() method',
+                        'return a pywikibot.page.BasePage object',
+                        since='20200406')
+                    page = item
                 else:
-                    page = initialized_page or item
+                    page = initialized_page
 
                 assert isinstance(page, pywikibot.page.BasePage), (
                     '"page" is not a pywikibot.page.BasePage object but {}.'

