diff --git a/pystreamapi/_itertools/tools.py b/pystreamapi/_itertools/tools.py
index 51b3835..547a8ba 100644
--- a/pystreamapi/_itertools/tools.py
+++ b/pystreamapi/_itertools/tools.py
@@ -55,8 +55,10 @@ def reduce(function, sequence, initial=_initial_missing, handler: ErrorHandler =
 
 
 def peek(iterable: Iterable, mapper):
-    """Generator wrapper that applies a function to every item of the iterable
-    and yields the item unchanged."""
+    """
+    Generator wrapper that applies a function to every item of the iterable
+    and yields the item unchanged.
+    """
     for item in iterable:
         mapper(item)
         yield item