Reactice watch is not necessarily called when the underlying object changes

[samwareing]

This issue occurs because of the assumption that the object a reactive watches will only change value when the __set__ operator is called on the reactive.

If I have an object that has a deeper structue, or can be changed by calling a function in some way, I would expect reactive to know that it something has changed and call the watch function.

In the example below, I have created a reactive that watches a dataclass object that is able to change itself via a the initialise_my_types method.

I then call that method via a textual action, and I can see the underlying value change in my debugger, but the watch function is not called and therefore the displayed value is out of date.

from dataclasses import dataclass, field

from textual.app import App, ComposeResult
from textual.containers import Container
from textual.reactive import reactive
from textual.widgets import Footer, Header, Static


@dataclass
class MyType:
    id: int = 0


@dataclass
class DataclassExample:
    my_types: list[MyType] = field(default_factory=list)

    def initialise_my_types(self) -> None:
        self.my_types.append(MyType())
        pass


class Display(Static):
    dce = reactive(DataclassExample())

    def watch_dce(self, dce: DataclassExample) -> None:
        self.update(f"{self.dce!r}")

    def initialise_my_types(self) -> None:
        self.dce.initialise_my_types()
        # a workaround is to call self.update() here


class ExampleApp(App):
    BINDINGS = [("i", "initialise_my_type", "Initialise example")]

    def compose(self) -> ComposeResult:
        yield Container(Display())
        yield Footer()

    def action_initialise_my_type(self) -> None:
        self.query_one(Display).initialise_my_types()


def main():
    app = ExampleApp()
    app.run()


if __name__ == "__main__":
    main()

The following screenshots show the value changing. There is a breakpoint in the watch function that is never hit.

Textual Diagnostics

Versions

Name	Value
Textual	0.17.1
Rich	13.3.3

Python

Name	Value
Version	3.10.9
Implementation	CPython
Compiler	Clang 14.0.0 (clang-1400.0.29.202)
Executable	/Users/samwareing/code/textual/practice/venv/bin/python3.10

Operating System

Name	Value
System	Darwin
Release	22.3.0
Version	Darwin Kernel Version 22.3.0: Mon Jan 30 20:38:37 PST 2023; root:xnu-8792.81.3~2/RELEASE_ARM64_T6000

Terminal

Name	Value
Terminal Application	vscode (1.77.0)
TERM	xterm-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=125, height=12
legacy_windows	False
min_width	1
max_width	125
is_terminal	True
encoding	utf-8
max_height	12
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

[willmcgugan]

That's the expected behaviour at the moment. Reactives will only detect changes when you set a new value, not when you mutate an existing value.

You can do self.my_types == self.my_types to prompt the watchers.

[samwareing]

Hi Will,

I actually tried this last week, it still does not work for this example.

I tried it again though, here's my change:

class Display(Static):
    dce = reactive(DataclassExample())

    def watch_dce(self, dce: DataclassExample) -> None:
        self.update(f"{self.dce!r}")

    def initialise_my_types(self) -> None:
        self.dce.initialise_my_types()
        self.dce = self.dce

I think the below screenshot shows why the watcher is not prompted:

current_value already has the change.

A workaround that does work is to set always_update true.

edit: scratch that, I misread what you typed, will look at settings self.my_types.

[samwareing]

Hmm, still no joy with this:

@dataclass
class DataclassExample:
    my_types: list[MyType] = field(default_factory=list)

    def initialise_my_types(self) -> None:
        self.my_types.append(MyType())
        self.my_types = self.my_types


class Display(Static):
    dce = reactive(DataclassExample())

    def watch_dce(self, dce: DataclassExample) -> None:
        self.update(f"{self.dce!r}")

    def initialise_my_types(self) -> None:
        self.dce.initialise_my_types()
        self.dce = self.dce