[ty] Show Final source in final assignment diagnostic (#24194)

charliermarsh · web-flow · commit eda2355832f7 · 2026-03-26T12:55:57.000-04:00
## Summary Addresses a TODO from #23880 to show the `Final` annotation in the final assignment diagnostic.
diff --git a/crates/ty_python_semantic/resources/mdtest/snapshots/final.md_-_`typing.Final`_-_Full_diagnostics_(174fdd8134fb325b).snap b/crates/ty_python_semantic/resources/mdtest/snapshots/final.md_-_`typing.Final`_-_Full_diagnostics_(174fdd8134fb325b).snap
@@ -39,7 +39,36 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md
 24 |     c.x = 2  # error: [invalid-assignment]
 25 | from typing import Final
 26 | 
-27 | UNINITIALIZED: Final[int]  # error: [final-without-value]
+27 | class C:
+28 |     x: Final[int]  # error: [final-without-value]
+29 | 
+30 |     def f(self):
+31 |         self.x = 2  # error: [invalid-assignment]
+32 | from typing import Final
+33 | 
+34 | class C:
+35 |     x: Final[int] = 1
+36 | 
+37 |     def __init__(self):
+38 |         self.x = 2  # error: [invalid-assignment]
+39 | from typing import Final
+40 | 
+41 | class Base:
+42 |     x: Final[int] = 1
+43 | 
+44 | class Child(Base):
+45 |     def f(self):
+46 |         self.x = 2  # error: [invalid-assignment]
+47 | from typing import Final
+48 | 
+49 | class C:
+50 |     x: int
+51 | 
+52 |     def f(self):
+53 |         self.x: Final[int] = 1  # error: [invalid-assignment]
+54 | from typing import Final
+55 | 
+56 | UNINITIALIZED: Final[int]  # error: [final-without-value]
 ```
 
 # Diagnostics
@@ -79,8 +108,12 @@ info: rule `invalid-assignment` is enabled by default
 
 ```
 error[invalid-assignment]: Cannot assign to final attribute `x` on type `Self@f`
-  --> src/mdtest_snippet.py:17:9
+  --> src/mdtest_snippet.py:14:8
    |
+13 | class C:
+14 |     x: Final[int] = 1
+   |        ---------- Attribute declared as `Final` here
+15 |
 16 |     def f(self):
 17 |         self.x = 2  # error: [invalid-assignment]
    |         ^^^^^^ `Final` attributes can only be assigned in the class body or `__init__`
@@ -92,8 +125,12 @@ info: rule `invalid-assignment` is enabled by default
 
 ```
 error[invalid-assignment]: Cannot assign to final attribute `x` on type `C`
-  --> src/mdtest_snippet.py:24:5
+  --> src/mdtest_snippet.py:21:8
    |
+20 | class C:
+21 |     x: Final[int] = 1
+   |        ---------- Attribute declared as `Final` here
+22 |
 23 | def __init__(c: C):
 24 |     c.x = 2  # error: [invalid-assignment]
    |     ^^^ `Final` attributes can only be assigned in the class body or `__init__`
@@ -103,13 +140,92 @@ info: rule `invalid-assignment` is enabled by default
 
 ```
 
+```
+error[final-without-value]: `Final` symbol `x` is not assigned a value
+  --> src/mdtest_snippet.py:28:5
+   |
+27 | class C:
+28 |     x: Final[int]  # error: [final-without-value]
+   |     ^^^^^^^^^^^^^
+29 |
+30 |     def f(self):
+   |
+info: rule `final-without-value` is enabled by default
+
+```
+
+```
+error[invalid-assignment]: Cannot assign to final attribute `x` on type `Self@f`
+  --> src/mdtest_snippet.py:28:8
+   |
+27 | class C:
+28 |     x: Final[int]  # error: [final-without-value]
+   |        ---------- Attribute declared as `Final` here
+29 |
+30 |     def f(self):
+31 |         self.x = 2  # error: [invalid-assignment]
+   |         ^^^^^^ `Final` attributes can only be assigned in the class body or `__init__`
+32 | from typing import Final
+   |
+info: rule `invalid-assignment` is enabled by default
+
+```
+
+```
+error[invalid-assignment]: Invalid assignment to final attribute
+  --> src/mdtest_snippet.py:35:8
+   |
+34 | class C:
+35 |     x: Final[int] = 1
+   |        ---------- Attribute declared as `Final` here
+36 |
+37 |     def __init__(self):
+38 |         self.x = 2  # error: [invalid-assignment]
+   |         ^^^^^^ `x` already has a value in the class body
+39 | from typing import Final
+   |
+info: rule `invalid-assignment` is enabled by default
+
+```
+
+```
+error[invalid-assignment]: Cannot assign to final attribute `x` on type `Self@f`
+  --> src/mdtest_snippet.py:42:8
+   |
+41 | class Base:
+42 |     x: Final[int] = 1
+   |        ---------- Attribute declared as `Final` here
+43 |
+44 | class Child(Base):
+45 |     def f(self):
+46 |         self.x = 2  # error: [invalid-assignment]
+   |         ^^^^^^ `Final` attributes can only be assigned in the class body or `__init__`
+47 | from typing import Final
+   |
+info: rule `invalid-assignment` is enabled by default
+
+```
+
+```
+error[invalid-assignment]: Cannot assign to final attribute `x` on type `Self@f`
+  --> src/mdtest_snippet.py:53:9
+   |
+52 |     def f(self):
+53 |         self.x: Final[int] = 1  # error: [invalid-assignment]
+   |         ^^^^^^ `Final` attributes can only be assigned in the class body or `__init__`
+54 | from typing import Final
+   |
+info: rule `invalid-assignment` is enabled by default
+
+```
+
 ```
 error[final-without-value]: `Final` symbol `UNINITIALIZED` is not assigned a value
-  --> src/mdtest_snippet.py:27:1
+  --> src/mdtest_snippet.py:56:1
    |
-25 | from typing import Final
-26 |
-27 | UNINITIALIZED: Final[int]  # error: [final-without-value]
+54 | from typing import Final
+55 |
+56 | UNINITIALIZED: Final[int]  # error: [final-without-value]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 info: rule `final-without-value` is enabled by default
diff --git a/crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md b/crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md
@@ -1138,6 +1138,29 @@ class E:
         self.x = 2  # Error: `self` is the second parameter, not the implicit receiver
 ```
 
+## Cross-module final attribute assignment
+
+Assigning to an inherited `Final` attribute where the base class is in a different module:
+
+`base.py`:
+
+```py
+from typing import Final
+
+class Base:
+    x: Final[int] = 1
+```
+
+`child.py`:
+
+```py
+from base import Base
+
+class Child(Base):
+    def f(self):
+        self.x = 2  # error: [invalid-assignment]
+```
+
 ## Full diagnostics
 
 <!-- snapshot-diagnostics -->
@@ -1186,6 +1209,55 @@ def __init__(c: C):
     c.x = 2  # error: [invalid-assignment]
 ```
 
+Class-body `Final` declaration without value:
+
+```py
+from typing import Final
+
+class C:
+    x: Final[int]  # error: [final-without-value]
+
+    def f(self):
+        self.x = 2  # error: [invalid-assignment]
+```
+
+`__init__` assignment after class-body value:
+
+```py
+from typing import Final
+
+class C:
+    x: Final[int] = 1
+
+    def __init__(self):
+        self.x = 2  # error: [invalid-assignment]
+```
+
+Inherited final attribute assignment:
+
+```py
+from typing import Final
+
+class Base:
+    x: Final[int] = 1
+
+class Child(Base):
+    def f(self):
+        self.x = 2  # error: [invalid-assignment]
+```
+
+Method-local `Final` annotation should not point at non-`Final` class annotation:
+
+```py
+from typing import Final
+
+class C:
+    x: int
+
+    def f(self):
+        self.x: Final[int] = 1  # error: [invalid-assignment]
+```
+
 `Final` declaration without value:
 
 ```py
diff --git a/crates/ty_python_semantic/src/types.rs b/crates/ty_python_semantic/src/types.rs
@@ -1057,6 +1057,15 @@ impl<'db> Type<'db> {
             Type::NominalInstance(instance) => Some(instance.class(db)),
             Type::ProtocolInstance(instance) => instance.to_nominal_instance().map(|i| i.class(db)),
             Type::TypeAlias(alias) => alias.value_type(db).nominal_class(db),
+            Type::NewTypeInstance(newtype) => newtype.concrete_base_type(db).nominal_class(db),
+            Type::TypeVar(typevar) => {
+                let TypeVarBoundOrConstraints::UpperBound(bound) =
+                    typevar.typevar(db).bound_or_constraints(db)?
+                else {
+                    return None;
+                };
+                bound.nominal_class(db)
+            }
             _ => None,
         }
     }
diff --git a/crates/ty_python_semantic/src/types/infer/builder/final_attribute.rs b/crates/ty_python_semantic/src/types/infer/builder/final_attribute.rs
