Watchers called with same old and new value when using mutate_reactive with data binding

[mhmaguire]

watcher receive the same new and old value when using mutate_reactive to update a reactive which is data bound to a child widget.

from textual.widgets import Static, Button
from textual.widget import Widget
from textual.app import App
from textual.reactive import reactive


class Child(Widget):

    strings: reactive[list[str]] = reactive(list, recompose=True)

    def compose(self):
        for s in self.strings:
            yield Static(s)

    def watch_strings(self, old, new):
        self.log('strings', old=old, new=new)
        

class Example(App):

    strings: reactive[list[str]] = reactive(list)

    def compose(self):
        yield Button('Click Me')
        yield Child().data_bind(Example.strings)

    def on_mount(self):
        self.strings.append('hello')
        self.mutate_reactive(Example.strings)

    def on_button_pressed(self, event):
        self.strings.append('message')
        self.mutate_reactive(Example.strings)


if __name__ == "__main__":
    app = Example()
    app.run()
    

Textual Diagnostics

Versions

Name	Value
Textual	3.0.1
Rich	14.0.0

Python

Name	Value
Version	3.12.9
Implementation	CPython
Compiler	GCC 13.3.0
Executable	/home/mhmaguire/projects/sketch/tmp/textual_example/.venv/bin/python3

Operating System

Name	Value
System	Linux
Release	6.8.0-55-generic
Version	#57-Ubuntu SMP PREEMPT_DYNAMIC Wed Feb 12 23:42:21 UTC 2025

Terminal

Name	Value
Terminal Application	GNOME Terminal
TERM	xterm-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=160, height=45
legacy_windows	False
min_width	1
max_width	160
is_terminal	True
encoding	utf-8
max_height	45
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

[github-actions[bot]]

We found the following entry in the FAQ which you may find helpful:

• How do I center a widget in a screen?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

This is an automated reply, generated by FAQtory

[willmcgugan]

If you are calling mutate_reactive it is because you are mutating the value, rather than assigning a new value. Since there is only a single value, the old and new value will be the same instance. Textual won't attempt to copy the old value.

If you need the old value, then don't use mutate_reactive. Assign a new value. i.e. self.strings = self.strings + ['message']