TYP use typeddict to define cssdict (pandas-dev#40947)

Author: MarcoGorelli

pandas/_typing.py

@@ -36,7 +36,10 @@
 # and use a string literal forward reference to it in subsequent types
 # https://mypy.readthedocs.io/en/latest/common_issues.html#import-cycles
 if TYPE_CHECKING:
-    from typing import final
+    from typing import (
+        TypedDict,
+        final,
+    )
 
     from pandas._libs import (
         Period,
@@ -70,6 +73,8 @@
 else:
     # typing.final does not exist until py38
     final = lambda x: x
+    # typing.TypedDict does not exist until py38
+    TypedDict = dict
 
 
 # array-like

pandas/io/formats/style_render.py

@@ -12,7 +12,6 @@
     Sequence,
     Tuple,
     Union,
-    cast,
 )
 from uuid import uuid4
 
@@ -21,7 +20,10 @@
 from pandas._config import get_option
 
 from pandas._libs import lib
-from pandas._typing import FrameOrSeriesUnion
+from pandas._typing import (
+    FrameOrSeriesUnion,
+    TypedDict,
+)
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.generic import ABCSeries
@@ -45,10 +47,14 @@
 CSSPair = Tuple[str, Union[str, int, float]]
 CSSList = List[CSSPair]
 CSSProperties = Union[str, CSSList]
-CSSStyles = List[Dict[str, CSSProperties]]  # = List[CSSDict]
-# class CSSDict(TypedDict):  # available when TypedDict is valid in pandas
-#     selector: str
-#     props: CSSProperties
+
+
+class CSSDict(TypedDict):
+    selector: str
+    props: CSSProperties
+
+
+CSSStyles = List[CSSDict]
 
 
 class StylerRenderer:
@@ -583,15 +589,9 @@ def _format_table_styles(styles: CSSStyles) -> CSSStyles:
               {'selector': 'th', 'props': 'a:v;'}]
     """
     return [
-        item
-        for sublist in [
-            [  # this is a CSSDict when TypedDict is available to avoid cast.
-                {"selector": x, "props": style["props"]}
-                for x in cast(str, style["selector"]).split(",")
-            ]
-            for style in styles
-        ]
-        for item in sublist
+        {"selector": selector, "props": css_dict["props"]}
+        for css_dict in styles
+        for selector in css_dict["selector"].split(",")
     ]