Paginator works for 'last' only if it's used with 'before'.

openlobby/core/api/paginator.py

@@ -5,6 +5,10 @@ from graphene.relay import PageInfo
 PER_PAGE = 10
 
 
+class MissingBeforeValueError(Exception):
+    pass
+
+
 def encode_cursor(num):
     return base64.b64encode(str(num).encode('utf-8')).decode('utf-8')
 
@@ -27,7 +31,9 @@ class Paginator:
             slice_to = slice_from + first
 
         elif last is not None:
-            if before is not None:
+            if before is None:
+                raise MissingBeforeValueError('Pagination "last" works only in combination with "before" argument.')
+
             slice_to = decode_cursor(before) - 1
             slice_from = slice_to - last
             if slice_from < 0:

openlobby/core/tests/test_paginator.py

 import pytest
 
-from ..api.paginator import PER_PAGE, encode_cursor, decode_cursor, Paginator
+from ..api.paginator import (
+    PER_PAGE,
+    encode_cursor,
+    decode_cursor,
+    Paginator,
+    MissingBeforeValueError,
+)
 
 
 @pytest.mark.parametrize('num, cursor', [
@@ -41,10 +47,8 @@ def test_paginator__custom_per_page():
 @pytest.mark.parametrize('kw, slice_from, slice_to', [
     ({'first': 15}, 0, 15),
     ({'first': 5, 'after': encode_cursor(3)}, 3, 8),
-    ({'last': 3}, 7, 10),
     ({'last': 8, 'before': encode_cursor(20)}, 11, 19),
     # overflow of slice_from
-    ({'last': 15}, 0, 10),
     ({'last': 100, 'before': encode_cursor(42)}, 0, 41),
     # preffer first before last if both provided
     ({'first': 20, 'last': 4}, 0, 20),
@@ -55,6 +59,11 @@ def test_paginator__input_combinations(kw, slice_from, slice_to):
     assert paginator.slice_to == slice_to
 
 
+def test_paginator__last_without_before():
+    with pytest.raises(MissingBeforeValueError):
+        Paginator(last=1)
+
+
 @pytest.mark.parametrize('kw, total, previous, next, start, end', [
     ({}, 10, False, False, 1, 10),
     ({}, 15, False, True, 1, 10),
@@ -66,10 +75,6 @@ def test_paginator__input_combinations(kw, slice_from, slice_to):
     ({'first': 3, 'after': encode_cursor(7)}, 20, True, True, 8, 10),
     ({'first': 3, 'after': encode_cursor(17)}, 20, True, False, 18, 20),
     ({'first': 5, 'after': encode_cursor(17)}, 20, True, False, 18, 20),
-    ({'last': 1}, 10, True, False, 10, 10),
-    ({'last': 5}, 10, True, False, 6, 10),
-    ({'last': 15}, 10, False, False, 1, 10),
-    ({'last': 10}, 10, False, False, 1, 10),
     ({'last': 4, 'before': encode_cursor(10)}, 20, True, True, 6, 9),
     ({'last': 4, 'before': encode_cursor(5)}, 20, False, True, 1, 4),
     ({'last': 4, 'before': encode_cursor(3)}, 20, False, True, 1, 2),
@@ -89,7 +94,7 @@ def test_paginator__get_page_info(kw, total, previous, next, start, end):
 @pytest.mark.parametrize('kw, num, cursor', [
     ({}, 3, encode_cursor(3)),
     ({'first': 6, 'after': encode_cursor(1)}, 3, encode_cursor(4)),
-    ({'last': 6}, 3, encode_cursor(7)),
+    ({'last': 6, 'before': encode_cursor(11)}, 3, encode_cursor(7)),
 ])
 def test_paginator__get_edge_cursor(kw, num, cursor):
     paginator = Paginator(**kw)