reactive initialize

Author: willmcgugan

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+# Unreleased
+
+### Added
+
+- Added `Content.simplify`
+- Added `textual.reactive.Initialize`
+
 ## [5.2.0] - 2025-08-01
 
 ### Added

src/textual/_callback.py

@@ -30,7 +30,10 @@ def count_parameters(func: Callable) -> int:
         param_count = _count_parameters(func) - 1
     else:
         param_count = _count_parameters(func)
-    func._param_count = param_count
+    try:
+        func._param_count = param_count
+    except TypeError:
+        pass
     return param_count
 
 

src/textual/reactive.py

@@ -56,6 +56,29 @@ class TooManyComputesError(ReactiveError):
     """Raised when an attribute has public and private compute methods."""
 
 
+class Initialize(Generic[ReactableType, ReactiveType]):
+    """Initialize a reactive by calling a method parent object.
+
+    Example:
+        ```python
+            class InitializeApp(App):
+
+                def get_names(self) -> list[str]:
+                    return ["foo", "bar", "baz"]
+
+                # The `names` property will call `get_names` to get its default when first referenced.
+                names = reactive(Initialize(get_names))
+        ```
+
+    """
+
+    def __init__(self, callback: Callable[[ReactableType], ReactiveType]) -> None:
+        self.callback = callback
+
+    def __call__(self, obj: ReactableType) -> ReactiveType:
+        return self.callback(obj)
+
+
 async def await_watcher(obj: Reactable, awaitable: Awaitable[object]) -> None:
     """Coroutine to await an awaitable returned from a watcher"""
     _rich_traceback_omit = True
@@ -118,7 +141,7 @@ class Reactive(Generic[ReactiveType]):
 
     def __init__(
         self,
-        default: ReactiveType | Callable[[], ReactiveType],
+        default: ReactiveType | Callable[[], ReactiveType] | Initialize[ReactiveType],
         *,
         layout: bool = False,
         repaint: bool = True,
@@ -190,7 +213,11 @@ def _initialize_reactive(self, obj: Reactable, name: str) -> None:
         else:
             default_or_callable = self._default
             default = (
-                default_or_callable()
+                (
+                    default_or_callable(obj)
+                    if isinstance(default_or_callable, Initialize)
+                    else default_or_callable()
+                )
                 if callable(default_or_callable)
                 else default_or_callable
             )
@@ -421,7 +448,7 @@ class reactive(Reactive[ReactiveType]):
 
     def __init__(
         self,
-        default: ReactiveType | Callable[[], ReactiveType],
+        default: ReactiveType | Callable[[], ReactiveType] | Initialize[ReactiveType],
         *,
         layout: bool = False,
         repaint: bool = True,
@@ -456,7 +483,7 @@ class var(Reactive[ReactiveType]):
 
     def __init__(
         self,
-        default: ReactiveType | Callable[[], ReactiveType],
+        default: ReactiveType | Callable[[], ReactiveType] | Initialize[ReactiveType],
         init: bool = True,
         always_update: bool = False,
         bindings: bool = False,

tests/test_reactive.py

@@ -7,13 +7,29 @@
 from textual.app import App, ComposeResult
 from textual.message import Message
 from textual.message_pump import MessagePump
-from textual.reactive import Reactive, TooManyComputesError, reactive, var
+from textual.reactive import Initialize, Reactive, TooManyComputesError, reactive, var
 from textual.widget import Widget
 
 OLD_VALUE = 5_000
 NEW_VALUE = 1_000_000
 
 
+async def test_initialize():
+    """Test that the default accepts an Initialize instance."""
+
+    class InitializeApp(App):
+
+        def get_names(self) -> list[str]:
+            return ["foo", "bar", "baz"]
+
+        names = reactive(Initialize(get_names))
+
+    app = InitializeApp()
+
+    async with app.run_test():
+        assert app.names == ["foo", "bar", "baz"]
+
+
 async def test_watch():
     """Test that changes to a watched reactive attribute happen immediately."""