[IMPR] use roundrobin_generators to combine generators when limit is given

Change-Id: I11957c120c911b701581e60d6c3b965980478999
---
M pywikibot/pagegenerators/__init__.py
M pywikibot/pagegenerators/_factory.py
2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/pywikibot/pagegenerators/__init__.py b/pywikibot/pagegenerators/__init__.py
index fa30b7d..4608b6c 100644
--- a/pywikibot/pagegenerators/__init__.py
+++ b/pywikibot/pagegenerators/__init__.py
@@ -432,8 +432,10 @@
 
 -intersect          Work on the intersection of all the provided generators.
 
--limit              When used with any other argument -limit:n specifies a set
-                    of pages, work on no more than n pages in total.
+-limit              When used with any other argument ``-limit:n``
+                    specifies a set of pages, work on no more than n
+                    pages in total. If used with multiple generators,
+                    pages are yielded in a roundrobin way.
 
 -namespaces         Filter the page generator to only yield pages in the
 -namespace          specified namespaces. Separate multiple namespace
diff --git a/pywikibot/pagegenerators/_factory.py b/pywikibot/pagegenerators/_factory.py
index 0b42c73..f32c8e3 100644
--- a/pywikibot/pagegenerators/_factory.py
+++ b/pywikibot/pagegenerators/_factory.py
@@ -54,7 +54,11 @@
     WikidataSPARQLPageGenerator,
 )
 from pywikibot.tools.collections import DequeGenerator
-from pywikibot.tools.itertools import filter_unique, intersect_generators
+from pywikibot.tools.itertools import (
+    filter_unique,
+    intersect_generators,
+    roundrobin_generators,
+)
 
 
 HANDLER_RETURN_TYPE = Union[None, bool, Iterable['pywikibot.page.BasePage']]
@@ -200,6 +204,10 @@
 
         .. versionchanged:: 7.3
            set the instance variable :attr:`is_preloading` to True or False.
+        .. versionchanged:: 8.0
+           if ``limit`` option is set and multiple generators are given,
+           pages are yieded in a :func:`roundrobin
+           <tools.itertools.roundrobin_generators>` way.
 
         :param gen: Another generator to be combined with
         :param preload: preload pages using PreloadingGenerator
@@ -247,7 +255,8 @@
             # By definition no duplicates are possible.
             dupfiltergen = intersect_generators(*self.gens)
         else:
-            dupfiltergen = _filter_unique_pages(itertools.chain(*self.gens))
+            combine = roundrobin_generators if self.limit else itertools.chain
+            dupfiltergen = _filter_unique_pages(combine(*self.gens))
 
         # Add on subpage filter generator
         if self.subpage_max_depth is not None:

