docs/source/contributor-guide/pyo3-guidelines.md

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(pyo3_guidelines)=

# PyO3 binding guidelines

Conventions for the `#[pyclass]` bindings in `crates/`. These are review
policy for this repository, not part of the extension protocol — an extension
library is free to shape its own classes differently.

(ffi_pyclass_mutability)=

## Class mutability

PyO3 bindings should present immutable wrappers whenever a struct stores shared
or interior-mutable state. In practice this means that any `#[pyclass]`
containing an `Arc<RwLock<_>>` or similar synchronized primitive must opt into
`#[pyclass(frozen)]` unless there is a compelling reason not to.

The execution context illustrates the preferred pattern. `PySessionContext` in
{file}`src/context.rs` stays frozen even though it shares mutable state
internally via `SessionContext`. This ensures PyO3 tracks borrows correctly
while Python-facing APIs clone the inner `SessionContext` or return new
wrappers instead of mutating the existing instance in place:

```rust
#[pyclass(from_py_object, frozen, name = "SessionContext", module = "datafusion", subclass)]
#[derive(Clone)]
pub struct PySessionContext {
    pub ctx: SessionContext,
}
```

Occasionally a type must remain mutable—for example when PyO3 attribute setters
need to update fields directly. In these rare cases add an inline justification
so reviewers and future contributors understand why `frozen` is unsafe to
enable. `DataTypeMap` in {file}`src/common/data_type.rs` includes such a
comment because PyO3 still needs to track field updates:

```rust
// TODO: This looks like this needs pyo3 tracking so leaving unfrozen for now
#[derive(Debug, Clone)]
#[pyclass(from_py_object, name = "DataTypeMap", module = "datafusion.common", subclass)]
pub struct DataTypeMap {
    #[pyo3(get, set)]
    pub arrow_type: PyDataType,
    #[pyo3(get, set)]
    pub python_type: PythonType,
    #[pyo3(get, set)]
    pub sql_type: SqlType,
}
```

When reviewers encounter a mutable `#[pyclass]` without a comment, they should
request an explanation or ask that `frozen` be added. Keeping these wrappers
frozen by default helps avoid subtle bugs stemming from PyO3's interior
mutability tracking.