pyo3-stub-gen

Python stub file (*.pyi) generator for PyO3 with maturin projects.

[!NOTE] Minimum supported Python version is 3.10. Do not enable 3.9 or older in PyO3 setting.

[!NOTE] Versions 0.15.0–0.17.1 unintentionally included a LGPL dependency. This was removed in 0.17.2, and the affected versions have been yanked.

Design

Our goal is to create a stub file *.pyi from Rust code, however, automated complete translation is impossible due to the difference between Rust and Python type systems and the limitation of proc-macro. We take semi-automated approach:

• Provide a default translator which will work most cases, not all cases
• Also provide a manual way to specify the translation.

If the default translator does not work, users can specify the translation manually, and these manual translations can be integrated with what the default translator generates. So the users can use the default translator as much as possible and only specify the translation for the edge cases.

pyo3-stub-gen crate provides the manual way to specify the translation, and pyo3-stub-gen-derive crate provides the default translator as proc-macro based on the mechanism of pyo3-stub-gen.

Usage

If you are looking for a working example, please see the examples directory.

Here we describe basic usage of pyo3-stub-gen crate based on examples/pure example.

Annotate Rust code with proc-macro

This crate provides a procedural macro #[gen_stub_pyfunction] and others to generate a Python stub file. It is used with PyO3's #[pyfunction] macro. Let's consider a simple example PyO3 project:

use pyo3::prelude::*;

#[pyfunction]
fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
    Ok((a + b).to_string())
}

#[pymodule]
fn your_module_name(m: &Bound<PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
    Ok(())
}

To generate a stub file for this project, please modify it as follows:

use pyo3::prelude::*;
use pyo3_stub_gen::{derive::gen_stub_pyfunction, define_stub_info_gatherer};

#[gen_stub_pyfunction]  // Proc-macro attribute to register a function to stub file generator.
#[pyfunction]
fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
    Ok((a + b).to_string())
}

#[pymodule]
fn your_module_name(m: &Bound<PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
    Ok(())
}

// Define a function to gather stub information.
define_stub_info_gatherer!(stub_info);

[!NOTE] The #[gen_stub_pyfunction] macro must be placed before #[pyfunction] macro.

#[gen_stub(skip)]

For functions or methods that you want to exclude from the generated stub file, use the #[gen_stub(skip)] attribute:

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

#[gen_stub_pyclass]
#[pyclass]
struct MyClass;

#[gen_stub_pymethods]
#[pymethods]
impl MyClass {
    #[gen_stub(skip)]
    fn internal_method(&self) {
        // This method will not appear in the .pyi file
    }
}

#[gen_stub(default=xx)]

For getters, setters, and class attributes, you can specify default values that will appear in the stub file:

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

#[gen_stub_pyclass]
#[pyclass]
struct Config {
    #[pyo3(get, set)]
    #[gen_stub(default = Config::default().timeout)]
    timeout: usize,
}

impl Default for Config {
    fn default() -> Self {
        Config { timeout: 30 }
    }
}

#[gen_stub_pymethods]
#[pymethods]
impl Config {
    #[getter]
    #[gen_stub(default = Config::default().timeout)]
    fn get_timeout(&self) -> usize {
        self.timeout
    }
}

Generate a stub file

And then, create an executable target in src/bin/stub_gen.rs to generate a stub file:

use pyo3_stub_gen::Result;

fn main() -> Result<()> {
    // `stub_info` is a function defined by `define_stub_info_gatherer!` macro.
    let stub = pure::stub_info()?;
    stub.generate()?;
    Ok(())
}

and add rlib in addition to cdylib in [lib] section of Cargo.toml:

[lib]
crate-type = ["cdylib", "rlib"]

This target generates a stub file pure.pyi when executed.

cargo run --bin stub_gen

The stub file is automatically found by maturin, and it is included in the wheel package. See also the maturin document for more details.

Manual Overriding

When the automatic Rust-to-Python type translation doesn't produce the desired result, you can manually specify type information using Python stub syntax. There are two main approaches:

1. Complete override - Replace entire function signature with #[gen_stub_pyfunction(python = "...")]
2. Partial override - Override specific arguments or return types with #[gen_stub(override_type(...))]

Method 1: Complete Override Using python Parameter

Use the python parameter to specify the complete function signature in Python stub syntax. This is ideal when you need to define complex types or when the entire signature needs custom definition.

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

#[gen_stub_pyfunction(python = r#"
    import collections.abc
    import typing

    def fn_with_callback(callback: collections.abc.Callable[[str], typing.Any]) -> collections.abc.Callable[[str], typing.Any]:
        """Example using python parameter for complete override."""
"#)]
#[pyfunction]
pub fn fn_with_callback<'a>(callback: Bound<'a, PyAny>) -> PyResult<Bound<'a, PyAny>> {
    callback.call1(("Hello!",))?;
    Ok(callback)
}

This approach:

• ✅ Provides complete control over the generated stub
• ✅ Supports complex types like collections.abc.Callable
• ✅ Allows adding custom docstrings
• ✅ Import statements are automatically extracted

Method 2: Partial Override Using Attributes

For selective overrides, use #[gen_stub(override_type(...))] on specific arguments or #[gen_stub(override_return_type(...))] on the function. This is useful when most types translate correctly but a few need adjustment.

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

#[gen_stub_pyfunction]
#[pyfunction]
#[gen_stub(override_return_type(type_repr="collections.abc.Callable[[str], typing.Any]", imports=("collections.abc", "typing")))]
pub fn get_callback<'a>(
    #[gen_stub(override_type(type_repr="collections.abc.Callable[[str], typing.Any]", imports=("collections.abc", "typing")))]
    cb: Bound<'a, PyAny>,
) -> PyResult<Bound<'a, PyAny>> {
    Ok(cb)
}

This approach:

• ✅ Fine-grained control over individual types
• ✅ Preserves automatic generation for other parameters
• ✅ Explicit about which types need manual specification

Method 3: Separate Definitions Using Macros

How submit! works:

The #[gen_stub_pyfunction] and #[gen_stub_pyclass] macros automatically generate submit! blocks internally to register type information. You can also manually add submit! blocks to supplement or override this automatic registration.

When multiple type signatures exist for the same function or method, the stub generator automatically generates @overload decorators in the .pyi file. This enables proper type checking for functions that accept multiple type signatures.

Two approaches for overloads:

1. python_overload parameter: Define overloads inline with the function
2. submit! blocks: Keep stub definitions separate - useful for proc-macro/code generation

Function overloads:

Use the python_overload parameter to define multiple type signatures inline:

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

// Define overloads inline with python_overload parameter
#[gen_stub_pyfunction(
    python_overload = r#"
    @overload
    def process(x: int) -> int:
        """Process integer input"""
    "#
)]
#[pyfunction]
pub fn process(x: f64) -> f64 {
    x + 1.0
}

Generated stub:

@overload
def process(x: int) -> int:
    """Process integer input"""

@overload
def process(x: float) -> float: ...  # Auto-generated from Rust

Suppress auto-generation with no_default_overload = true:

use pyo3::prelude::*;
use pyo3_stub_gen::derive::*;

#[gen_stub_pyfunction(
    python_overload = r#"
    @overload
    def func(x: int) -> int: ...
    @overload
    def func(x: str) -> str: ...
    "#,
    no_default_overload = true  // Don't generate from Rust signature
)]
#[pyfunction]
pub fn func(ob: Bound<PyAny>) -> PyResult<PyObject> {
    // Runtime type checking
    todo!()
}

Class method overloads:

use pyo3::prelude::*;
use pyo3_stub_gen::{derive::*, inventory::submit};

#[gen_stub_pyclass]
#[pyclass]
pub struct Calculator {}

#[gen_stub_pymethods]
#[pymethods]
impl Calculator {
    fn add(&self, x: f64) -> f64 {
        x + 1.0
    }
}

// Alternative: Use submit! for method overloads (useful for proc-macro/code generation)
submit! {
    gen_methods_from_python! {
        r#"
        class Calculator:
            @overload
            def add(self, x: int) -> int:
                """Add integer (overload)"""
        "#
    }
}

Benefits:

• ✅ Inline overload definitions with python_overload parameter
• ✅ Automatic @overload decorator generation
• ✅ Deterministic ordering with index-based sorting
• ✅ submit! syntax available for proc-macro/code generation use cases

Advanced class method patterns:

For more advanced patterns, see examples/pure/src/manual_submit.rs:

• Fully manual method submission - Submit all method signatures without #[gen_stub_pymethods]
• Mixing proc-macro and manual submission - Use #[gen_stub(skip)] for methods that need complex type annotations

For comprehensive documentation, see Python Stub Syntax Support.

Advanced: Using RustType Marker

Within Python stub syntax, you can reference Rust types directly using the pyo3_stub_gen.RustType["TypeName"] marker. This leverages the PyStubType trait implementation of the Rust type.

use pyo3::prelude::*;
use pyo3_stub_gen::{derive::*, inventory::submit};

#[pyfunction]
pub fn sum_list(values: Vec<i32>) -> i32 {
    values.iter().sum()
}

submit! {
    gen_function_from_python! {
        r#"
        def sum_list(values: pyo3_stub_gen.RustType["Vec<i32>"]) -> pyo3_stub_gen.RustType["i32"]:
            """Sum a list of integers"""
        "#
    }
}

The RustType marker automatically expands to the appropriate Python type:

• RustType["Vec<i32>"] → typing.Sequence[int] (for arguments)
• RustType["i32"] → int (for return values)

This is particularly useful for:

• Generic types like Vec<T>, HashMap<K, V>
• Custom types that implement PyStubType
• Ensuring consistency between Rust and Python type mappings

When to Use Which Method

For complete examples, see the examples/pure directory, particularly:

• overriding.rs - Type override examples
• overloading.rs - Function overload examples
• rust_type_marker.rs - RustType marker examples

Type Aliases

Type aliases allow you to define semantic names for complex or frequently used types in your stub files. They improve code readability and maintainability by providing meaningful names for type combinations.

Basic Usage

Use the type_alias! macro to define type aliases:

use pyo3_stub_gen::type_alias;
use std::collections::HashMap;

// Simple type alias
type_alias!("your_module", SimpleAlias = Option<usize>);

// Collection types
type_alias!("your_module", StrIntMap = HashMap<String, i32>);

// Nested option types
type_alias!("your_module", MaybeString = Option<Option<String>>);

Direct Union Syntax

The type_alias! macro supports direct union syntax, eliminating the need for a separate impl_stub_type! declaration in most cases:

use pyo3_stub_gen::type_alias;

// Simple union types
type_alias!("your_module", NumberOrStringAlias = i32 | String);

// Multiple types in a union
type_alias!("your_module", TripleUnion = i32 | String | bool);

// Unions of generic types
type_alias!("your_module", GenericUnion = Option<i32> | Vec<String>);

// Complex nested unions
type_alias!("your_module", ComplexUnion = Option<Vec<i32>> | Option<Vec<String>>);

Alternative: Using with impl_stub_type!

For reusable union types that you want to reference in multiple places, you can still use the two-step impl_stub_type! + type_alias! pattern:

use pyo3_stub_gen::{impl_stub_type, type_alias};

// Define a reusable union type
struct NumberOrString;
impl_stub_type!(NumberOrString = i32 | String);

// Use it in multiple type aliases
type_alias!("your_module", NumberOrStringAlias = NumberOrString);
type_alias!("your_module", AnotherAlias = NumberOrString);

This approach is useful when you need to use the same union type in multiple contexts, avoiding repetition.

Generated Output

Type aliases are rendered in Python stub files using the TypeAlias annotation (Python 3.11+ compatible):

from typing import TypeAlias

__all__ = [
    "MaybeDecimal",
    "NumberOrStringAlias",
    "SimpleAlias",
    "StrIntMap",
    "StructUnion",
]

MaybeDecimal: TypeAlias = typing.Optional[DecimalHolder]
NumberOrStringAlias: TypeAlias = builtins.int | builtins.str
SimpleAlias: TypeAlias = typing.Optional[builtins.int]
StrIntMap: TypeAlias = builtins.dict[builtins.str, builtins.int]
StructUnion: TypeAlias = ComparableStruct | HashableStruct

Type Alias Syntax Configuration

By default, pyo3-stub-gen generates type aliases using the pre-Python 3.12 syntax with TypeAlias:

from typing import TypeAlias

MyAlias: TypeAlias = int | str

For projects targeting Python 3.12 or higher, you can use the newer type statement syntax by adding the following to your pyproject.toml:

[tool.pyo3-stub-gen]
use-type-statement = true

This will generate:

type MyAlias = int | str

[!NOTE] When using use-type-statement = true, ensure your project's minimum Python version is 3.12 or higher. The type statement is not available in earlier Python versions.

Python Stub Syntax for Type Aliases

For complex type aliases that require Python-specific syntax, you can use gen_type_alias_from_python!:

use pyo3_stub_gen::derive::gen_type_alias_from_python;

gen_type_alias_from_python!(
    "your_module",
    r#"
    import collections.abc
    from typing import TypeAlias
    CallbackType: TypeAlias = collections.abc.Callable[[str], None]
    "#
);

The parser accepts both the pre-3.12 and 3.12+ syntaxes:

use pyo3_stub_gen::derive::gen_type_alias_from_python;

// Pre-3.12 syntax
gen_type_alias_from_python!(
    "your_module",
    r#"
    from typing import TypeAlias
    CallbackType: TypeAlias = collections.abc.Callable[[str], None]
    "#
);

// Python 3.12+ syntax
gen_type_alias_from_python!(
    "your_module",
    r#"
    import collections.abc
    type OptionalCallback = collections.abc.Callable[[str], None] | None
    "#
);

The output format is controlled by the use-type-statement configuration, regardless of which syntax you use in the input.

This approach is useful for:

• Types requiring specific imports (e.g., collections.abc.Callable)
• Complex generic types
• Types that are difficult to express with PyStubType

Benefits

• Readability: Provide semantic names for complex types
• Consistency: Ensure the same type combination is used throughout
• Maintainability: Update the type definition in one place
• Integration: Works with automatic __all__ generation and type checkers (mypy, pyright, ruff)

Note

Type aliases are stub-only constructs and do not exist at runtime. They are purely for static type checking and IDE support.

Advanced: mypy.stubtest integration

mypy stubtest validates that stub files match runtime behavior. You can add it to your test suite:

uv run stubtest your_module_name --ignore-missing-stub --ignore-disjoint-bases

Required flags for PyO3/maturin projects

• --ignore-missing-stub - Maturin creates internal native modules (.so files) that re-export to __init__.py. Stubtest looks for stubs for these internal modules, which don't exist (all types are in __init__.pyi). This flag prevents false positives.
• --ignore-disjoint-bases - PyO3 classes are disjoint bases at runtime, but pyo3-stub-gen does not generate @typing.disjoint_base decorators.

Known limitation: nested submodules

Stubtest does not work with PyO3 nested submodules. Nested #[pymodule] creates runtime attributes (not importable modules), but stub files use directory structure. For projects with nested submodules, disable stubtest for those packages. See examples/mixed/Taskfile.yml for an example.

API Reference Documentation

In addition to stub files, pyo3-stub-gen can generate Sphinx API reference documentation from the same Rust type metadata. This provides rendered HTML documentation with cross-references, type links, and docstrings — without writing any Python documentation manually.

Configuration

Add a [tool.pyo3-stub-gen.doc-gen] section to your pyproject.toml:

[tool.pyo3-stub-gen.doc-gen]
output-dir = "docs/api"
json-output = "api_reference.json"
index-title = "API Reference"

No changes are needed to src/bin/stub_gen.rs — stub.generate() automatically generates documentation when this section is present.

Available options:

Sphinx Setup

Add Sphinx and related packages to your dev dependencies:

[dependency-groups]
dev = ["myst-parser", "sphinx", "sphinx-rtd-theme"]

Create a docs/conf.py that loads the generated extension:

import sys
from pathlib import Path

# Add the API docs directory so Sphinx can find the generated extension
sys.path.insert(0, str(Path(__file__).parent / "api"))

project = "your_project"
extensions = [
    "pyo3_stub_gen_ext",       # Generated extension — reads api_reference.json
    "sphinx.ext.intersphinx",  # Enables cross-references to external projects
]

intersphinx_mapping = {
    "python": ("https://docs.python.org/3", None),
}

html_theme = "sphinx_rtd_theme"

Generated Files

Running cargo run --bin stub_gen produces the following in output-dir:

• api_reference.json — structured API data (JSON intermediate representation)
• pyo3_stub_gen_ext.py — Sphinx extension that renders the JSON into documentation
• index.rst — table of contents with toctree (when separate-pages = true)
• <module>.rst — one page per module (when separate-pages = true)

Each module .rst file contains a single directive that the extension expands:

pure
====

.. pyo3-api:: pure

Building the Documentation

cargo run --bin stub_gen                                        # Generate API data + Sphinx files
uv run --with sphinx sphinx-build -W -b html docs docs/_build   # Build HTML

The first command regenerates documentation whenever your Rust code changes. The second invokes Sphinx, which uses the generated extension to build HTML.

Sphinx Directives

The generated pyo3_stub_gen_ext.py extension provides two RST directives:

• .. pyo3-api:: module_name — render a single module's API reference
• .. pyo3-api-package:: package_name — render all modules in a package

Docstrings

Rust doc comments (/// ...) are rendered as MyST Markdown in the generated documentation. This supports cross-references (:class:`ClassName`), code blocks, admonitions, and other Sphinx/MyST features. The myst-parser extension is required for this.

Working Examples

See the following directories for complete setups:

• examples/pure/docs/ — single-module project
• examples/mixed/docs/ — multi-module project with submodules

For in-depth architecture details, see docs/docgen-architecture.md.

Contribution

To be written.

License

© 2024 Jij Inc.

This project is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

Links

• MusicalNinjas/pyo3-stubgen
  • Same motivation, but different approach.
  • This project creates a stub file by loading the compiled library and inspecting the __text_signature__ attribute generated by PyO3 in Python side.
• pybind11-stubgen
  • Stub file generator for pybind11 based C++ projects.