From 83b1fcc1026d4a7b141c19e8599c0f4f60198e64 Mon Sep 17 00:00:00 2001
From: Will McGugan <willmcgugan@gmail.com>
Date: Mon, 1 May 2023 16:57:40 +0100
Subject: [PATCH] raise attribute error (#2443)

* raise attribute error

* fix compute defaults
---
 CHANGELOG.md                         |  9 +++++----
 src/textual/reactive.py              | 10 +++++++++-
 src/textual/widgets/_progress_bar.py |  1 -
 tests/test_reactive.py               |  3 +++
 4 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index dc3903ab4..75bf2b7c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,14 +7,15 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Changed
+
+- Setting attributes with a `compute_` method will now raise an `AttributeError` https://github.com/Textualize/textual/issues/2383
+- Unknown psuedo-selectors will now raise a tokenizer error (previously they were silently ignored) https://github.com/Textualize/textual/pull/2445
+
 ### Added
 
 - Watch methods can now optionally be private https://github.com/Textualize/textual/issues/2382
 
-### Changed
-
-- Unknown psuedo-selectors will now raise a tokenizer error (previously they were silently ignored) https://github.com/Textualize/textual/pull/2445
-
 ## [0.22.3] - 2023-04-29
 
 ### Fixed
diff --git a/src/textual/reactive.py b/src/textual/reactive.py
index 5e2515de8..08259200d 100644
--- a/src/textual/reactive.py
+++ b/src/textual/reactive.py
@@ -174,6 +174,12 @@ class Reactive(Generic[ReactiveType]):
         _rich_traceback_omit = True
 
         self._initialize_reactive(obj, self.name)
+
+        if hasattr(obj, self.compute_name):
+            raise AttributeError(
+                f"Can't set {obj}.{self.name!r}; reactive attributes with a compute method are read-only"
+            )
+
         name = self.name
         current_value = getattr(obj, name)
         # Check for validate function
@@ -276,7 +282,9 @@ class Reactive(Generic[ReactiveType]):
                 compute_method = getattr(obj, f"compute_{compute}")
             except AttributeError:
                 continue
-            current_value = getattr(obj, f"_reactive_{compute}")
+            current_value = getattr(
+                obj, f"_reactive_{compute}", getattr(obj, f"_default_{compute}", None)
+            )
             value = compute_method()
             setattr(obj, f"_reactive_{compute}", value)
             if value != current_value:
diff --git a/src/textual/widgets/_progress_bar.py b/src/textual/widgets/_progress_bar.py
index a51fc6fba..8eb6449fd 100644
--- a/src/textual/widgets/_progress_bar.py
+++ b/src/textual/widgets/_progress_bar.py
@@ -336,7 +336,6 @@ class ProgressBar(Widget, can_focus=False):
         self.show_percentage = show_percentage
         self.show_eta = show_eta
 
-        self.percentage = None
         self.total = total
 
     def compose(self) -> ComposeResult:
diff --git a/tests/test_reactive.py b/tests/test_reactive.py
index 2032406ae..1f6b47647 100644
--- a/tests/test_reactive.py
+++ b/tests/test_reactive.py
@@ -356,6 +356,9 @@ async def test_compute():
         app.start = 10
         assert app.count_double == 14
 
+        with pytest.raises(AttributeError):
+            app.count_double = 100
+
 
 async def test_watch_compute():
     """Check that watching a computed attribute works."""