diff --git a/altair/__init__.py b/altair/__init__.py
index 073a1a53e..55feb3cdb 100644
--- a/altair/__init__.py
+++ b/altair/__init__.py
@@ -56,6 +56,7 @@
     "Categorical",
     "Chart",
     "ChartDataType",
+    "ChartType",
     "Color",
     "ColorDatum",
     "ColorDef",
@@ -580,6 +581,7 @@
     "expr",
     "graticule",
     "hconcat",
+    "is_chart_type",
     "jupyter",
     "layer",
     "limit_rows",
diff --git a/altair/utils/_transformed_data.py b/altair/utils/_transformed_data.py
index 1d886eb5e..7a616b9c5 100644
--- a/altair/utils/_transformed_data.py
+++ b/altair/utils/_transformed_data.py
@@ -180,8 +180,8 @@ def name_views(
         List of the names of the charts and subcharts
     """
     exclude = set(exclude) if exclude is not None else set()
-    if isinstance(chart, _chart_class_mapping[Chart]) or isinstance(
-        chart, _chart_class_mapping[FacetChart]
+    if isinstance(
+        chart, (_chart_class_mapping[Chart], _chart_class_mapping[FacetChart])
     ):
         if chart.name not in exclude:
             if chart.name in (None, Undefined):
diff --git a/altair/utils/save.py b/altair/utils/save.py
index 11db77bf1..609486bc6 100644
--- a/altair/utils/save.py
+++ b/altair/utils/save.py
@@ -10,14 +10,14 @@
 
 
 def write_file_or_filename(
-    fp: Union[str, pathlib.PurePath, IO],
+    fp: Union[str, pathlib.Path, IO],
     content: Union[str, bytes],
     mode: str = "w",
     encoding: Optional[str] = None,
 ) -> None:
     """Write content to fp, whether fp is a string, a pathlib Path or a
     file-like object"""
-    if isinstance(fp, str) or isinstance(fp, pathlib.PurePath):
+    if isinstance(fp, (str, pathlib.Path)):
         with open(file=fp, mode=mode, encoding=encoding) as f:
             f.write(content)
     else:
@@ -25,14 +25,12 @@ def write_file_or_filename(
 
 
 def set_inspect_format_argument(
-    format: Optional[str], fp: Union[str, pathlib.PurePath, IO], inline: bool
+    format: Optional[str], fp: Union[str, pathlib.Path, IO], inline: bool
 ) -> str:
     """Inspect the format argument in the save function"""
     if format is None:
-        if isinstance(fp, str):
-            format = fp.split(".")[-1]
-        elif isinstance(fp, pathlib.PurePath):
-            format = fp.suffix.lstrip(".")
+        if isinstance(fp, (str, pathlib.Path)):
+            format = pathlib.Path(fp).suffix.lstrip(".")
         else:
             raise ValueError(
                 "must specify file format: "
@@ -71,7 +69,7 @@ def set_inspect_mode_argument(
 
 def save(
     chart,
-    fp: Union[str, pathlib.PurePath, IO],
+    fp: Union[str, pathlib.Path, IO],
     vega_version: Optional[str],
     vegaembed_version: Optional[str],
     format: Optional[Literal["json", "html", "png", "svg", "pdf"]] = None,
@@ -140,7 +138,7 @@ def save(
 
     if json_kwds is None:
         json_kwds = {}
-
+    encoding = kwargs.get("encoding", "utf-8")
     format = set_inspect_format_argument(format, fp, inline)  # type: ignore[assignment]
 
     def perform_save():
@@ -152,9 +150,7 @@ def perform_save():
 
         if format == "json":
             json_spec = json.dumps(spec, **json_kwds)
-            write_file_or_filename(
-                fp, json_spec, mode="w", encoding=kwargs.get("encoding", "utf-8")
-            )
+            write_file_or_filename(fp, json_spec, mode="w", encoding=encoding)
         elif format == "html":
             if inline:
                 kwargs["template"] = "inline"
@@ -170,10 +166,7 @@ def perform_save():
                 **kwargs,
             )
             write_file_or_filename(
-                fp,
-                mimebundle["text/html"],
-                mode="w",
-                encoding=kwargs.get("encoding", "utf-8"),
+                fp, mimebundle["text/html"], mode="w", encoding=encoding
             )
         elif format in ["png", "svg", "pdf", "vega"]:
             mimebundle = spec_to_mimebundle(
@@ -193,7 +186,6 @@ def perform_save():
             elif format == "pdf":
                 write_file_or_filename(fp, mimebundle["application/pdf"], mode="wb")
             else:
-                encoding = kwargs.get("encoding", "utf-8")
                 write_file_or_filename(
                     fp, mimebundle["image/svg+xml"], mode="w", encoding=encoding
                 )
diff --git a/altair/utils/schemapi.py b/altair/utils/schemapi.py
index 9325b3a70..10b2597e6 100644
--- a/altair/utils/schemapi.py
+++ b/altair/utils/schemapi.py
@@ -1193,7 +1193,7 @@ def _freeze(val):
                     return frozenset((k, _freeze(v)) for k, v in val.items())
                 elif isinstance(val, set):
                     return frozenset(map(_freeze, val))
-                elif isinstance(val, list) or isinstance(val, tuple):
+                elif isinstance(val, (list, tuple)):
                     return tuple(map(_freeze, val))
                 else:
                     return val
diff --git a/altair/vegalite/v5/api.py b/altair/vegalite/v5/api.py
index 0bd9ae389..37a13c2ae 100644
--- a/altair/vegalite/v5/api.py
+++ b/altair/vegalite/v5/api.py
@@ -8,6 +8,8 @@
 from toolz.curried import pipe as _pipe
 import itertools
 import sys
+import pathlib
+import typing
 from typing import cast, List, Optional, Any, Iterable, Union, Literal, IO
 
 # Have to rename it here as else it overlaps with schema.core.Type and schema.core.Dict
@@ -30,6 +32,10 @@
 from ...utils.data import DataType
 from ...utils.deprecation import AltairDeprecationWarning
 
+if sys.version_info >= (3, 13):
+    from typing import TypeIs
+else:
+    from typing_extensions import TypeIs
 if sys.version_info >= (3, 11):
     from typing import Self
 else:
@@ -1138,7 +1144,7 @@ def open_editor(self, *, fullscreen: bool = False) -> None:
 
     def save(
         self,
-        fp: Union[str, IO],
+        fp: Union[str, pathlib.Path, IO],
         format: Optional[Literal["json", "html", "png", "svg", "pdf"]] = None,
         override_data_transformer: bool = True,
         scale_factor: float = 1.0,
@@ -4098,3 +4104,18 @@ def graticule(**kwds):
 def sphere() -> core.SphereGenerator:
     """Sphere generator."""
     return core.SphereGenerator(sphere=True)
+
+
+ChartType = Union[
+    Chart, RepeatChart, ConcatChart, HConcatChart, VConcatChart, FacetChart, LayerChart
+]
+
+
+def is_chart_type(obj: Any) -> TypeIs[ChartType]:
+    """Return `True` if the object is an Altair chart. This can be a basic chart
+    but also a repeat, concat, or facet chart.
+    """
+    return isinstance(
+        obj,
+        typing.get_args(ChartType),
+    )
diff --git a/doc/user_guide/api.rst b/doc/user_guide/api.rst
index 99aa5f69b..08013023f 100644
--- a/doc/user_guide/api.rst
+++ b/doc/user_guide/api.rst
@@ -152,6 +152,7 @@ API Functions
    condition
    graticule
    hconcat
+   is_chart_type
    layer
    param
    repeat
diff --git a/pyproject.toml b/pyproject.toml
index 36c95b317..a7ad687f7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,7 +15,7 @@ build-backend = "hatchling.build"
 name = "altair"
 authors = [{ name = "Vega-Altair Contributors" }]
 dependencies = [
-    "typing_extensions>=4.0.1; python_version<\"3.11\"",
+    "typing_extensions>=4.10.0; python_version<\"3.13\"",
     "jinja2",
     # If you update the minimum required jsonschema version, also update it in build.yml
     "jsonschema>=3.0",
@@ -164,6 +164,7 @@ exclude = [
 ]
 
 [tool.ruff.lint]
+extend-safe-fixes=["SIM101"]
 select = [
     # flake8-bugbear
     "B",
@@ -177,6 +178,8 @@ select = [
     "F",
     # flake8-tidy-imports
     "TID",
+    # flake8-simplify
+    "SIM101"
 ]
 ignore = [  
     # Whitespace before ':'
diff --git a/tools/schemapi/schemapi.py b/tools/schemapi/schemapi.py
index 29da970ec..ce1268c04 100644
--- a/tools/schemapi/schemapi.py
+++ b/tools/schemapi/schemapi.py
@@ -1191,7 +1191,7 @@ def _freeze(val):
                     return frozenset((k, _freeze(v)) for k, v in val.items())
                 elif isinstance(val, set):
                     return frozenset(map(_freeze, val))
-                elif isinstance(val, list) or isinstance(val, tuple):
+                elif isinstance(val, (list, tuple)):
                     return tuple(map(_freeze, val))
                 else:
                     return val
diff --git a/tools/update_init_file.py b/tools/update_init_file.py
index 4f342f8ef..e4fa65a86 100644
--- a/tools/update_init_file.py
+++ b/tools/update_init_file.py
@@ -21,6 +21,10 @@
     cast,
 )
 
+if sys.version_info >= (3, 13):
+    from typing import TypeIs
+else:
+    from typing_extensions import TypeIs
 if sys.version_info >= (3, 11):
     from typing import Self
 else:
@@ -97,6 +101,7 @@ def _is_relevant_attribute(attr_name: str) -> bool:
         or attr is Protocol
         or attr is Sequence
         or attr is IO
+        or attr is TypeIs
         or attr_name == "TypingDict"
         or attr_name == "TypingGenerator"
         or attr_name == "ValueOrDatum"