diff --git a/functionalpy/test_sequence.py b/functionalpy/test_sequence.py
index 5d81c14..05dae2a 100644
--- a/functionalpy/test_sequence.py
+++ b/functionalpy/test_sequence.py
@@ -1,10 +1,13 @@
+from collections.abc import Mapping, Sequence
+from typing import Literal
+
 from functionalpy._sequence import Seq
 
 
 # TODO: https://github.com/MartinBernstorff/FunctionalPy/issues/40 tests: add hypothesis tests where it makes sense
 def test_chaining():
     sequence = Seq([1, 2])
-    result = (
+    result: list[int] = (
         sequence.filter(lambda x: x % 2 == 0)
         .map(lambda x: x * 2)
         .to_list()
@@ -14,66 +17,64 @@ def test_chaining():
 
 def test_map():
     sequence = Seq([1, 2])
-    result = sequence.map(lambda x: x * 2).to_list()
+    result: list[int] = sequence.map(lambda x: x * 2).to_list()
     assert result == [2, 4]
 
 
 def test_filter():
-    sequence = Seq((1, 2))
-    result = sequence.filter(lambda x: x % 2 == 0).to_list()
+    sequence = Seq([1, 2])
+    result: list[int] = sequence.filter(
+        lambda x: x % 2 == 0
+    ).to_list()
     assert result == [2]
 
 
 def test_reduce():
     sequence = Seq([1, 2])
-    result = sequence.reduce(lambda x, y: x + y)
+    result: int = sequence.reduce(lambda x, y: x + y)
     assert result == 3
 
 
-class TestGroupby:
-    def test_grouped_filter(self):
-        sequence = Seq([1, 2, 3, 4])
-
-        def is_even(num: int) -> str:
-            if num % 2 == 0:
-                return "even"
-            return "odd"
-
-        grouped = sequence.groupby(is_even)
-        assert grouped == {
-            "odd": [1, 3],
-            "even": [2, 4],
-        }
+def test_grouped_filter():
+    sequence = Seq([1, 2, 3, 4])
 
-    def test_grouped_map(self):
-        ...
+    def is_even(num: int) -> str:
+        if num % 2 == 0:
+            return "even"
+        return "odd"
 
-    def test_grouped_reduce(self):
-        ...
+    grouped: Mapping[str, Sequence[int]] = sequence.groupby(is_even)
+    assert grouped == {
+        "odd": [1, 3],
+        "even": [2, 4],
+    }
 
 
 def test_flatten():
-    test_input = ((1, 2), (3, 4))
+    test_input: list[list[int]] = [[1, 2], [3, 4]]
     sequence = Seq(test_input)
-    result = sequence.flatten()
+    result: Seq[int] = sequence.flatten()
     assert result.to_list() == [1, 2, 3, 4]
 
 
 class TestFlattenTypes:
     def test_flatten_tuple(self):
-        test_input = ((1, 2), (3, 4))
+        test_input: Sequence[tuple[int, ...]] = (
+            (1, 2),
+            (3, 4),
+        )
         sequence = Seq(test_input)
-        result = sequence.flatten()
+        result: Seq[Literal[1, 2, 3, 4]] = sequence.flatten()
         assert result.to_list() == [1, 2, 3, 4]
 
     def test_flatten_list(self):
-        test_input = [[1, 2], [3, 4]]
+        test_input: list[list[int]] = [[1, 2], [3, 4]]
         sequence = Seq(test_input)
-        result = sequence.flatten()
+        result: Seq[int] = sequence.flatten()
         assert result.to_list() == [1, 2, 3, 4]
 
     def test_flatten_str(self):
-        test_input = ["abcd"]
+        test_input: list[str] = ["abcd"]
         sequence = Seq(test_input)
-        result = sequence.flatten()
+        result: Seq[str] = sequence.flatten()
         assert result.to_list() == ["a", "b", "c", "d"]