diff --git a/pandas/core/groupby/ops.py b/pandas/core/groupby/ops.py
index 2a7fd079679a4..19cb71cdc5528 100644
--- a/pandas/core/groupby/ops.py
+++ b/pandas/core/groupby/ops.py
@@ -790,7 +790,7 @@ def _get_axes(group):
         return group.axes
 
 
-def _is_indexed_like(obj, axes):
+def _is_indexed_like(obj, axes) -> bool:
     if isinstance(obj, Series):
         if len(axes) > 1:
             return False
diff --git a/pandas/core/indexes/frozen.py b/pandas/core/indexes/frozen.py
index a6c39d049c50c..4791ea2b70691 100644
--- a/pandas/core/indexes/frozen.py
+++ b/pandas/core/indexes/frozen.py
@@ -7,7 +7,6 @@
 - .levels & .codes (FrozenNDArray)
 
 """
-
 import warnings
 
 import numpy as np
@@ -31,7 +30,7 @@ class FrozenList(PandasObject, list):
     # Side note: This has to be of type list. Otherwise,
     #            it messes up PyTables type checks.
 
-    def union(self, other):
+    def union(self, other) -> "FrozenList":
         """
         Returns a FrozenList with other concatenated to the end of self.
 
@@ -49,7 +48,7 @@ def union(self, other):
             other = list(other)
         return type(self)(super().__add__(other))
 
-    def difference(self, other):
+    def difference(self, other) -> "FrozenList":
         """
         Returns a FrozenList with elements from other removed from self.
 
@@ -101,7 +100,9 @@ def __hash__(self):
     def _disabled(self, *args, **kwargs):
         """This method will not function because object is immutable."""
         raise TypeError(
-            "'%s' does not support mutable operations." % self.__class__.__name__
+            "'{cls}' does not support mutable operations.".format(
+                cls=self.__class__.__name__
+            )
         )
 
     def __str__(self):
@@ -132,7 +133,9 @@ def __new__(cls, data, dtype=None, copy=False):
 
     def _disabled(self, *args, **kwargs):
         """This method will not function because object is immutable."""
-        raise TypeError("'%s' does not support mutable operations." % self.__class__)
+        raise TypeError(
+            "'{cls}' does not support mutable operations.".format(cls=self.__class__)
+        )
 
     __setitem__ = __setslice__ = __delitem__ = __delslice__ = _disabled
     put = itemset = fill = _disabled
diff --git a/pandas/io/formats/css.py b/pandas/io/formats/css.py
index 92fe87cddb35b..cd58ef51d1b65 100644
--- a/pandas/io/formats/css.py
+++ b/pandas/io/formats/css.py
@@ -11,6 +11,23 @@ class CSSWarning(UserWarning):
     pass
 
 
+def _side_expander(prop_fmt: str):
+    def expand(self, prop, value):
+        tokens = value.split()
+        try:
+            mapping = self.SIDE_SHORTHANDS[len(tokens)]
+        except KeyError:
+            warnings.warn(
+                'Could not expand "{prop}: {val}"'.format(prop=prop, val=value),
+                CSSWarning,
+            )
+            return
+        for key, idx in zip(self.SIDES, mapping):
+            yield prop_fmt.format(key), tokens[idx]
+
+    return expand
+
+
 class CSSResolver:
     """A callable for parsing and resolving CSS to atomic properties
 
@@ -213,22 +230,6 @@ def atomize(self, declarations):
     }
     SIDES = ("top", "right", "bottom", "left")
 
-    def _side_expander(prop_fmt):
-        def expand(self, prop, value):
-            tokens = value.split()
-            try:
-                mapping = self.SIDE_SHORTHANDS[len(tokens)]
-            except KeyError:
-                warnings.warn(
-                    'Could not expand "{prop}: {val}"'.format(prop=prop, val=value),
-                    CSSWarning,
-                )
-                return
-            for key, idx in zip(self.SIDES, mapping):
-                yield prop_fmt.format(key), tokens[idx]
-
-        return expand
-
     expand_border_color = _side_expander("border-{:s}-color")
     expand_border_style = _side_expander("border-{:s}-style")
     expand_border_width = _side_expander("border-{:s}-width")
diff --git a/pandas/tests/io/msgpack/test_extension.py b/pandas/tests/io/msgpack/test_extension.py
index 85ed43fa01079..6d1f8cb694601 100644
--- a/pandas/tests/io/msgpack/test_extension.py
+++ b/pandas/tests/io/msgpack/test_extension.py
@@ -48,7 +48,7 @@ def default(obj):
             typecode = 123  # application specific typecode
             data = tobytes(obj)
             return ExtType(typecode, data)
-        raise TypeError("Unknown type object {obj!r}".format(obj))
+        raise TypeError("Unknown type object {obj!r}".format(obj=obj))
 
     def ext_hook(code, data):
         print("ext_hook called", code, data)