Add return annotation to `annotations` last #6071

jackoconnordev · 2025-08-05T13:54:19Z

Description

Functions like functools.singledispatch are sensitive to the order of items in the __annotations__ map. CPython puts return types last.

Resolves #6068

Note: I'd prefer to include automated tests from CPython alongside this, but struggled. There are tests from Python 3.13.5's test_importlib/ which will fail without commit, as they make use of singledispatch that would serve to test this code change on every release.

Test script

def func(s: str) -> int:
    return int(s)


print(func.__annotations__)

RustPython before

{'return': <class 'int'>, 's': <class 'str'>, }

## CPython

{'s': <class 'str'>, 'return': <class 'int'>}

RustPython after

{'s': <class 'str'>, 'return': <class 'int'>}

Summary by CodeRabbit

Refactor
- Changed the processing order of return and parameter annotations during compilation. This does not affect the user interface or functionality.

Functions like `functools.singledispatch` are sensitive to the order of items in the `__annotations__` map. CPython puts returns last.

coderabbitai · 2025-08-05T13:54:26Z

Walkthrough

The change updates the visit_annotations method in the RustPython compiler so that parameter annotations are processed before the return annotation. The method's logic is otherwise unchanged, and its signature remains the same. This change affects only the order in which annotations are compiled and emitted.

Changes

Cohort / File(s)	Change Summary
Annotation Order Logic `compiler/codegen/src/compile.rs`	Adjusted order in `visit_annotations` to process parameter annotations before the return annotation. No other logic or signature changes.

Sequence Diagram(s)

sequenceDiagram
    participant Compiler
    participant Parameters
    participant Expr (Return Annotation)

    Compiler->>Parameters: Iterate parameter annotations
    loop For each parameter
        Compiler->>Parameters: Compile annotation
    end
    alt If return annotation exists
        Compiler->>Expr: Compile return annotation
    end
    Compiler-->>Compiler: Return total annotation count

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Ensure function `__annotations__` item order matches CPython: parameters first, then return (#6068)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes found.

Poem

A hop and a skip, the order is new,
Parameters first, as Python would do.
Return comes last, just as it should,
Now singledispatch will work as it could.
🐇✨
Rust and Python, in harmony,
Annotations aligned—code jubilee!

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Reviewing files that changed from the base of the PR and between f5a77a1 and 6f2f5c4.

📒 Files selected for processing (1)

compiler/codegen/src/compile.rs (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

**/*.rs

📄 CodeRabbit Inference Engine (.github/copilot-instructions.md)

**/*.rs: Follow the default rustfmt code style (cargo fmt to format)
Always run clippy to lint code (cargo clippy) before completing tasks. Fix any warnings or lints that are introduced by your changes
Follow Rust best practices for error handling and memory management
Use the macro system (pyclass, pymodule, pyfunction, etc.) when implementing Python functionality in Rust

Files:

compiler/codegen/src/compile.rs

🔇 Additional comments (1)

compiler/codegen/src/compile.rs (1)

2276-2283: LGTM! This change correctly fixes the annotation ordering to match CPython behavior.

The modification successfully addresses the compatibility issue by ensuring that return annotations are processed after parameter annotations, placing them last in the __annotations__ dictionary. This matches CPython's ordering where parameter annotations appear first, followed by the return annotation.

The logic itself remains unchanged - only the positioning within the method has been adjusted, which is exactly what was needed to resolve the issue with libraries like functools.singledispatch that depend on this specific ordering.

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youknowone · 2025-08-06T01:29:05Z

Thank you so much! To prevent regression, could you add a test about this? Adding the code described in the issue to snippets/extra_tests/syntax_annotations.py will be one of the ways.

jackoconnordev · 2025-08-06T08:08:36Z

Can do 🫡

Add return annotation to __annotations__ last

6f2f5c4

Functions like `functools.singledispatch` are sensitive to the order of items in the `__annotations__` map. CPython puts returns last.

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Description

Test script

RustPython before

RustPython after

Summary by CodeRabbit

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Walkthrough

Changes

Sequence Diagram(s)

Estimated code review effort

Assessment against linked issues

Assessment against linked issues: Out-of-scope changes

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