feat: Adds @register_theme decorator

altair/__init__.py

@@ -617,6 +617,7 @@
     "mixins",
     "param",
     "parse_shorthand",
+    "register_theme",
     "renderers",
     "repeat",
     "sample",

altair/utils/plugin_registry.py

@@ -115,7 +115,7 @@ def __init__(
         self.entry_point_group: str = entry_point_group
         self.plugin_type: IsPlugin
         if plugin_type is not callable and isinstance(plugin_type, type):
-            msg = (
+            msg: Any = (
                 f"Pass a callable `TypeIs` function to `plugin_type` instead.\n"
                 f"{type(self).__name__!r}(plugin_type)\n\n"
                 f"See also:\n"

altair/utils/theme.py

@@ -3,9 +3,9 @@
 from __future__ import annotations
 
 import sys
-from typing import TYPE_CHECKING, Callable
+from typing import TYPE_CHECKING, Any, Dict
 
-from .plugin_registry import PluginRegistry
+from .plugin_registry import Plugin, PluginRegistry
 
 if sys.version_info >= (3, 11):
     from typing import LiteralString
@@ -16,10 +16,10 @@
     from altair.utils.plugin_registry import PluginEnabler
     from altair.vegalite.v5.theme import AltairThemes, VegaThemes
 
-ThemeType = Callable[..., dict]
+ThemeType = Plugin[Dict[str, Any]]
 
 
-class ThemeRegistry(PluginRegistry[ThemeType, dict]):
+class ThemeRegistry(PluginRegistry[ThemeType, Dict[str, Any]]):
     def enable(
         self, name: LiteralString | AltairThemes | VegaThemes | None = None, **options
     ) -> PluginEnabler:

altair/vegalite/v5/__init__.py

@@ -21,4 +21,4 @@
     renderers,
 )
 from .schema import *
-from .theme import themes
+from .theme import register_theme, themes

altair/vegalite/v5/theme.py

@@ -2,21 +2,33 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Final, Literal, get_args
+import sys
+from functools import wraps
+from typing import TYPE_CHECKING, Any, Callable, Dict, Final, Literal, TypeVar, get_args
 
 from altair.utils.theme import ThemeRegistry
 from altair.vegalite.v5.schema._typing import VegaThemes
 
-if TYPE_CHECKING:
-    import sys
+if sys.version_info >= (3, 10):
+    from typing import ParamSpec
+else:
+    from typing_extensions import ParamSpec
+
 
+if TYPE_CHECKING:
+    if sys.version_info >= (3, 11):
+        from typing import LiteralString
+    else:
+        from typing_extensions import LiteralString
     if sys.version_info >= (3, 10):
         from typing import TypeAlias
     else:
         from typing_extensions import TypeAlias
 
+P = ParamSpec("P")
+R = TypeVar("R", bound=Dict[str, Any])
 AltairThemes: TypeAlias = Literal["default", "opaque"]
-VEGA_THEMES: list[str] = list(get_args(VegaThemes))
+VEGA_THEMES: list[LiteralString] = list(get_args(VegaThemes))
 
 
 class VegaTheme:
@@ -60,3 +72,75 @@ def __repr__(self) -> str:
     themes.register(theme, VegaTheme(theme))
 
 themes.enable("default")
+
+
+def register_theme(
+    name: LiteralString, *, enable: bool
+) -> Callable[[Callable[P, R]], Callable[P, R]]:
+    """
+    Decorator for registering a theme function.
+
+    Parameters
+    ----------
+    name
+        Unique name assigned in ``alt.themes``.
+    enable
+        Auto-enable the wrapped theme.
+
+    Examples
+    --------
+    Register and enable a theme::
+
+        from __future__ import annotations
+
+        from typing import Any
+        import altair as alt
+
+
+        @alt.register_theme("param_font_size", enable=True)
+        def custom_theme() -> dict[str, Any]:
+            sizes = 12, 14, 16, 18, 20
+            return {
+                "autosize": {"contains": "content", "resize": True},
+                "background": "#F3F2F1",
+                "config": {
+                    "axisX": {"labelFontSize": sizes[1], "titleFontSize": sizes[1]},
+                    "axisY": {"labelFontSize": sizes[1], "titleFontSize": sizes[1]},
+                    "font": "'Lato', 'Segoe UI', Tahoma, Verdana, sans-serif",
+                    "headerColumn": {"labelFontSize": sizes[1]},
+                    "headerFacet": {"labelFontSize": sizes[1]},
+                    "headerRow": {"labelFontSize": sizes[1]},
+                    "legend": {"labelFontSize": sizes[0], "titleFontSize": sizes[1]},
+                    "text": {"fontSize": sizes[0]},
+                    "title": {"fontSize": sizes[-1]},
+                },
+                "height": {"step": 28},
+                "width": 350,
+            }
+
+    Until another theme has been enabled, all charts will use defaults set in ``custom_theme``::
+
+        from vega_datasets import data
+
+        source = data.stocks()
+        lines = (
+            alt.Chart(source, title=alt.Title("Stocks"))
+            .mark_line()
+            .encode(x="date:T", y="price:Q", color="symbol:N")
+        )
+        lines.interactive(bind_y=False)
+
+    """
+
+    def decorate(func: Callable[P, R], /) -> Callable[P, R]:
+        themes.register(name, func)
+        if enable:
+            themes.enable(name)
+
+        @wraps(func)
+        def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
+            return func(*args, **kwargs)
+
+        return wrapper
+
+    return decorate

tests/vegalite/v5/test_theme.py

@@ -1,19 +1,32 @@
+from __future__ import annotations
+
 import pytest
 
 import altair.vegalite.v5 as alt
-from altair.vegalite.v5.theme import VEGA_THEMES
+from altair.vegalite.v5.theme import VEGA_THEMES, register_theme, themes
 
 
 @pytest.fixture
 def chart():
     return alt.Chart("data.csv").mark_bar().encode(x="x:Q")
 
 
-def test_vega_themes(chart):
+def test_vega_themes(chart) -> None:
     for theme in VEGA_THEMES:
         with alt.themes.enable(theme):
             dct = chart.to_dict()
         assert dct["usermeta"] == {"embedOptions": {"theme": theme}}
         assert dct["config"] == {
             "view": {"continuousWidth": 300, "continuousHeight": 300}
         }
+
+
+def test_register_theme_decorator() -> None:
+    @register_theme("unique name", enable=True)
+    def custom_theme() -> dict[str, int]:
+        return {"height": 400, "width": 700}
+
+    assert themes.active == "unique name"
+    registered = themes.get()
+    assert registered is not None
+    assert registered() == {"height": 400, "width": 700} == custom_theme()