[pyupgrade] Handle multiple base classes for PEP 695 generics (UP046) (#15659)

## Summary

Addresses the second follow up to #15565 in #15642. This was easier than
expected by using this cool destructuring syntax I hadn't used before,
and by assuming
[PYI059](https://docs.astral.sh/ruff/rules/generic-not-last-base-class/)
(`generic-not-last-base-class`).

## Test Plan

Using an existing test, plus two new tests combining multiple base
classes and multiple generics. It looks like I deleted a relevant test,
which I did, but I meant to rename this in #15565. It looks like instead
I copied it and renamed the copy.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP046_0.py

@@ -49,6 +49,57 @@ class MultipleBaseClasses(list, Generic[T]):
     var: T
 
 
+# these are just for the MoreBaseClasses and MultipleBaseAndGenerics cases
+class Base1: ...
+
+
+class Base2: ...
+
+
+class Base3: ...
+
+
+class MoreBaseClasses(Base1, Base2, Base3, Generic[T]):
+    var: T
+
+
+class MultipleBaseAndGenerics(Base1, Base2, Base3, Generic[S, T, *Ts, P]):
+    var: S
+    typ: T
+    tup: tuple[*Ts]
+    pep: P
+
+
+class A(Generic[T]): ...
+
+
+class B(A[S], Generic[S]):
+    var: S
+
+
+class C(A[S], Generic[S, T]):
+    var: tuple[S, T]
+
+
+class D(A[int], Generic[T]):
+    var: T
+
+
+class NotLast(Generic[T], Base1):
+    var: T
+
+
+class Sandwich(Base1, Generic[T], Base2):
+    var: T
+
+
+# runtime `TypeError` to inherit from `Generic` multiple times, but we still
+# emit a diagnostic
+class TooManyGenerics(Generic[T], Generic[S]):
+    var: T
+    var: S
+
+
 # These cases are not handled
 class D(Generic[T, T]):  # duplicate generic variable, runtime error
     pass
@@ -71,11 +122,6 @@ class MixedGenerics[U]:
         return (u, t)
 
 
-# TODO(brent) we should also handle multiple base classes
-class Multiple(NotGeneric, Generic[T]):
-    pass
-
-
 # TODO(brent) default requires 3.13
 V = TypeVar("V", default=Any, bound=str)
 

crates/ruff_linter/src/rules/pyupgrade/rules/pep695/non_pep695_generic_class.rs

@@ -1,11 +1,12 @@
 use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, ViolationMetadata};
 use ruff_python_ast::visitor::Visitor;
-use ruff_python_ast::StmtClassDef;
-use ruff_python_ast::{Expr, ExprSubscript};
+use ruff_python_ast::{Arguments, ExprSubscript, StmtClassDef};
+use ruff_python_semantic::SemanticModel;
 use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
+use crate::fix::edits::{remove_argument, Parentheses};
 use crate::settings::types::PythonVersion;
 
 use super::{check_type_vars, in_nested_context, DisplayTypeVars, TypeVarReferenceVisitor};
@@ -21,10 +22,9 @@ use super::{check_type_vars, in_nested_context, DisplayTypeVars, TypeVarReferenc
 ///
 /// ## Known problems
 ///
-/// The rule currently skips generic classes with multiple base classes. It also skips
-/// generic classes nested inside of other
-/// functions or classes. Finally, this rule skips type parameters with the `default` argument
-/// introduced in [PEP 696] and implemented in Python 3.13.
+/// The rule currently skips generic classes nested inside of other functions or classes. It also
+/// skips type parameters with the `default` argument introduced in [PEP 696] and implemented in
+/// Python 3.13.
 ///
 /// This rule can only offer a fix if all of the generic types in the class definition are defined
 /// in the current module. For external type parameters, a diagnostic is emitted without a suggested
@@ -65,6 +65,11 @@ use super::{check_type_vars, in_nested_context, DisplayTypeVars, TypeVarReferenc
 /// [`unused-private-type-var`](unused-private-type-var.md) for a rule to clean up unused
 /// private type variables.
 ///
+/// This rule will correctly handle classes with multiple base classes, as long as the single
+/// `Generic` base class is at the end of the argument list, as checked by
+/// [`generic-not-last-base-class`](generic-not-last-base-class.md). If a `Generic` base class is
+/// found outside of the last position, a diagnostic is emitted without a suggested fix.
+///
 /// This rule only applies to generic classes and does not include generic functions. See
 /// [`non-pep695-generic-function`](non-pep695-generic-function.md) for the function version.
 ///
@@ -118,22 +123,12 @@ pub(crate) fn non_pep695_generic_class(checker: &mut Checker, class_def: &StmtCl
         return;
     };
 
-    // TODO(brent) only accept a single, Generic argument for now. I think it should be fine to have
-    // other arguments, but this simplifies the fix just to delete the argument list for now
-    let [Expr::Subscript(ExprSubscript {
-        value,
-        slice,
-        range,
-        ..
-    })] = arguments.args.as_ref()
+    let Some((generic_idx, generic_expr @ ExprSubscript { slice, range, .. })) =
+        find_generic(arguments, checker.semantic())
     else {
         return;
     };
 
-    if !checker.semantic().match_typing_expr(value, "Generic") {
-        return;
-    }
-
     let mut diagnostic = Diagnostic::new(
         NonPEP695GenericClass {
             name: name.to_string(),
@@ -141,6 +136,19 @@ pub(crate) fn non_pep695_generic_class(checker: &mut Checker, class_def: &StmtCl
         *range,
     );
 
+    // only handle the case where Generic is at the end of the argument list, in line with PYI059
+    // (generic-not-last-base-class). If it comes elsewhere, it results in a runtime error. In stubs
+    // it's not *strictly* necessary for `Generic` to come last in the bases tuple, but it would
+    // cause more complication for us to handle stubs specially, and probably isn't worth the
+    // bother. we still offer a diagnostic here but not a fix
+    //
+    // because `find_generic` also finds the *first* Generic argument, this has the additional
+    // benefit of bailing out with a diagnostic if multiple Generic arguments are present
+    if generic_idx != arguments.len() - 1 {
+        checker.diagnostics.push(diagnostic);
+        return;
+    }
+
     let mut visitor = TypeVarReferenceVisitor {
         vars: vec![],
         semantic: checker.semantic(),
@@ -179,12 +187,34 @@ pub(crate) fn non_pep695_generic_class(checker: &mut Checker, class_def: &StmtCl
             source: checker.source(),
         };
 
-        diagnostic.set_fix(Fix::unsafe_edit(Edit::replacement(
-            type_params.to_string(),
-            name.end(),
-            arguments.end(),
-        )));
+        diagnostic.try_set_fix(|| {
+            let removal_edit = remove_argument(
+                generic_expr,
+                arguments,
+                Parentheses::Remove,
+                checker.source(),
+            )?;
+            Ok(Fix::unsafe_edits(
+                Edit::insertion(type_params.to_string(), name.end()),
+                [removal_edit],
+            ))
+        });
     }
 
     checker.diagnostics.push(diagnostic);
 }
+
+/// Search `class_bases` for a `typing.Generic` base class. Returns the `Generic` expression (if
+/// any), along with its index in the class's bases tuple.
+fn find_generic<'a>(
+    class_bases: &'a Arguments,
+    semantic: &SemanticModel,
+) -> Option<(usize, &'a ExprSubscript)> {
+    class_bases.args.iter().enumerate().find_map(|(idx, expr)| {
+        expr.as_subscript_expr().and_then(|sub_expr| {
+            semantic
+                .match_typing_expr(&sub_expr.value, "Generic")
+                .then_some((idx, sub_expr))
+        })
+    })
+}

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP046_0.py.snap

@@ -124,3 +124,156 @@ UP046_0.py:41:24: UP046 [*] Generic class `MultipleGenerics` uses `Generic` subc
 42 42 |     var: S
 43 43 |     typ: T
 44 44 |     tup: tuple[*Ts]
+
+UP046_0.py:48:33: UP046 [*] Generic class `MultipleBaseClasses` uses `Generic` subclass instead of type parameters
+   |
+48 | class MultipleBaseClasses(list, Generic[T]):
+   |                                 ^^^^^^^^^^ UP046
+49 |     var: T
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+45 45 |     pep: P
+46 46 | 
+47 47 | 
+48    |-class MultipleBaseClasses(list, Generic[T]):
+   48 |+class MultipleBaseClasses[T: float](list):
+49 49 |     var: T
+50 50 | 
+51 51 | 
+
+UP046_0.py:62:44: UP046 [*] Generic class `MoreBaseClasses` uses `Generic` subclass instead of type parameters
+   |
+62 | class MoreBaseClasses(Base1, Base2, Base3, Generic[T]):
+   |                                            ^^^^^^^^^^ UP046
+63 |     var: T
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+59 59 | class Base3: ...
+60 60 | 
+61 61 | 
+62    |-class MoreBaseClasses(Base1, Base2, Base3, Generic[T]):
+   62 |+class MoreBaseClasses[T: float](Base1, Base2, Base3):
+63 63 |     var: T
+64 64 | 
+65 65 | 
+
+UP046_0.py:66:52: UP046 [*] Generic class `MultipleBaseAndGenerics` uses `Generic` subclass instead of type parameters
+   |
+66 | class MultipleBaseAndGenerics(Base1, Base2, Base3, Generic[S, T, *Ts, P]):
+   |                                                    ^^^^^^^^^^^^^^^^^^^^^ UP046
+67 |     var: S
+68 |     typ: T
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+63 63 |     var: T
+64 64 | 
+65 65 | 
+66    |-class MultipleBaseAndGenerics(Base1, Base2, Base3, Generic[S, T, *Ts, P]):
+   66 |+class MultipleBaseAndGenerics[S: (str, bytes), T: float, *Ts, **P](Base1, Base2, Base3):
+67 67 |     var: S
+68 68 |     typ: T
+69 69 |     tup: tuple[*Ts]
+
+UP046_0.py:73:9: UP046 [*] Generic class `A` uses `Generic` subclass instead of type parameters
+   |
+73 | class A(Generic[T]): ...
+   |         ^^^^^^^^^^ UP046
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+70 70 |     pep: P
+71 71 | 
+72 72 | 
+73    |-class A(Generic[T]): ...
+   73 |+class A[T: float]: ...
+74 74 | 
+75 75 | 
+76 76 | class B(A[S], Generic[S]):
+
+UP046_0.py:76:15: UP046 [*] Generic class `B` uses `Generic` subclass instead of type parameters
+   |
+76 | class B(A[S], Generic[S]):
+   |               ^^^^^^^^^^ UP046
+77 |     var: S
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+73 73 | class A(Generic[T]): ...
+74 74 | 
+75 75 | 
+76    |-class B(A[S], Generic[S]):
+   76 |+class B[S: (str, bytes)](A[S]):
+77 77 |     var: S
+78 78 | 
+79 79 | 
+
+UP046_0.py:80:15: UP046 [*] Generic class `C` uses `Generic` subclass instead of type parameters
+   |
+80 | class C(A[S], Generic[S, T]):
+   |               ^^^^^^^^^^^^^ UP046
+81 |     var: tuple[S, T]
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+77 77 |     var: S
+78 78 | 
+79 79 | 
+80    |-class C(A[S], Generic[S, T]):
+   80 |+class C[S: (str, bytes), T: float](A[S]):
+81 81 |     var: tuple[S, T]
+82 82 | 
+83 83 | 
+
+UP046_0.py:84:17: UP046 [*] Generic class `D` uses `Generic` subclass instead of type parameters
+   |
+84 | class D(A[int], Generic[T]):
+   |                 ^^^^^^^^^^ UP046
+85 |     var: T
+   |
+   = help: Use type parameters
+
+ℹ Unsafe fix
+81 81 |     var: tuple[S, T]
+82 82 | 
+83 83 | 
+84    |-class D(A[int], Generic[T]):
+   84 |+class D[T: float](A[int]):
+85 85 |     var: T
+86 86 | 
+87 87 | 
+
+UP046_0.py:88:15: UP046 Generic class `NotLast` uses `Generic` subclass instead of type parameters
+   |
+88 | class NotLast(Generic[T], Base1):
+   |               ^^^^^^^^^^ UP046
+89 |     var: T
+   |
+   = help: Use type parameters
+
+UP046_0.py:92:23: UP046 Generic class `Sandwich` uses `Generic` subclass instead of type parameters
+   |
+92 | class Sandwich(Base1, Generic[T], Base2):
+   |                       ^^^^^^^^^^ UP046
+93 |     var: T
+   |
+   = help: Use type parameters
+
+UP046_0.py:98:23: UP046 Generic class `TooManyGenerics` uses `Generic` subclass instead of type parameters
+    |
+ 96 | # runtime `TypeError` to inherit from `Generic` multiple times, but we still
+ 97 | # emit a diagnostic
+ 98 | class TooManyGenerics(Generic[T], Generic[S]):
+    |                       ^^^^^^^^^^ UP046
+ 99 |     var: T
+100 |     var: S
+    |
+    = help: Use type parameters