René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Take `\u0026Py\u003cPyType\u003e` instead of `PyTypeRef` in `vm.invoke_exception()`","articleBody":"## Summary\n\n`VirtualMachine::invoke_exception` currently takes its class argument by owned value:\n\n```rust\n// crates/vm/src/exceptions.rs\npub fn invoke_exception(\n    \u0026self,\n    cls: PyTypeRef,          // PyRef\u003cPyType\u003e\n    args: Vec\u003cPyObjectRef\u003e,\n) -\u003e PyResult\u003cPyBaseExceptionRef\u003e {\n    let res = PyType::call(\u0026cls, args.into_args(self), self)?;\n    res.downcast::\u003cPyBaseException\u003e().map_err(|obj| {\n        self.new_type_error(format!(\n            \"calling {} should have returned an instance of BaseException, not {}\",\n            cls,\n            obj.class()\n        ))\n    })\n}\n```\n\nIt should take a borrow instead:\n\n```rust\npub fn invoke_exception(\n    \u0026self,\n    cls: \u0026Py\u003cPyType\u003e,\n    args: Vec\u003cPyObjectRef\u003e,\n) -\u003e PyResult\u003cPyBaseExceptionRef\u003e { ... }\n```\n\n## Why the change is justified\n\n**1. `invoke_exception` never takes ownership of `cls`.**\nThe body only ever *borrows* it:\n- `PyType::call(\u0026cls, ...)` — `PyType::call` is defined as `fn call(zelf: \u0026Py\u003cSelf\u003e, ...)`, i.e. it already takes a borrow.\n- `cls` in the `new_type_error` format string is used via `Display`, by reference.\n\nThere is no point where `cls` is moved, stored, or otherwise consumed. The owned `PyTypeRef` parameter is therefore strictly stronger than what the function needs. `PyTypeRef` (`= PyRef\u003cPyType\u003e`) derefs to `Py\u003cPyType\u003e`, so `\u0026Py\u003cPyType\u003e` is the exact borrowed form.\n\n**2. The owned parameter forces needless refcount churn at call sites.**\nBecause the signature demands ownership, most callers only holding a borrow must `.to_owned()` — an atomic refcount increment (and later decrement) — purely to satisfy the type, even though the reference is dropped immediately after the call returns:\n\n| Call site | Current argument |\n|-----------|------------------|\n| `crates/vm/src/stdlib/_io.rs:945` | `vm.ctx.exceptions.blocking_io_error.to_owned()` |\n| `crates/vm/src/stdlib/_io.rs:1156` | `vm.ctx.exceptions.blocking_io_error.to_owned()` |\n| `crates/vm/src/stdlib/_io.rs:1202` | `vm.ctx.exceptions.blocking_io_error.to_owned()` |\n| `crates/vm/src/stdlib/sys.rs:779` | `vm.ctx.exceptions.system_exit.to_owned()` |\n| `crates/vm/src/exception_group.rs:74` | `vm.ctx.exceptions.base_exception_group.to_owned()` |\n| `crates/vm/src/exceptions.rs:2010` | `typ.to_owned()` |\n| `crates/capi/src/pyerrors.rs:152` | `exc_type.to_owned()` |\n\nWith `\u0026Py\u003cPyType\u003e` every one of these drops the `.to_owned()` and passes the borrow directly.\n\n**3. It matches the idiomatic borrow used elsewhere.**\n`\u0026Py\u003cT\u003e` is the conventional \"borrowed typed object\" throughout the VM, and specifically the method `invoke_exception` delegates to — `PyType::call(zelf: \u0026Py\u003cSelf\u003e, ...)` — already takes exactly that. The change makes the helper consistent with what it wraps.\n\n**4. Callers that legitimately own a `PyTypeRef` are unaffected in cost.**\n`ExceptionCtor::instantiate` / `instantiate_value` (`crates/vm/src/exceptions.rs:447` and `:475`) own their `cls` via the `self` match; they just pass `\u0026cls` instead of moving it — a trivial edit and no clone either way.\n\n## Proposed change\n\n- Change the parameter to `cls: \u0026Py\u003cPyType\u003e`.\n- Update the ~9 call sites: drop `.to_owned()` where a borrow is already in hand, and pass `\u0026cls` from the `ExceptionCtor` sites.\n\nNo behavior change; this is a pure API/ergonomics cleanup that removes several unnecessary refcount bumps.","author":{"url":"https://github.com/youknowone","@type":"Person","name":"youknowone"},"datePublished":"2026-07-19T04:48:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/8320/RustPython/issues/8320"}