FromPyObject

FromPyObject is a trait in PyO3, the Rust binding for Python, that defines how to convert a Python object into a Rust value. The trait is parameterized with a lifetime (FromPyObject<'source>) and requires an extract function that receives a reference to a Python object (typically a PyAny) and returns a PyResult<Self>. Implementations exist for common Rust types such as integers, floats, booleans, and strings, as well as compound types like Vec<T> and Option<T> when T: FromPyObject.

In practice, PyO3 provides a convenient method on Python objects called extract that uses the FromPyObject trait to perform the conversion. For example, given a PyAny representing a Python integer, one can obtain a Rust i64 by calling value.extract::<i64>()?; similarly, a Python list can be extracted to Vec<T> if the elements can be converted. The trait also supports custom types: developers can implement FromPyObject for their own Rust structs to specify how to construct the value from a Python mapping or sequence.

Key considerations include the lifetime aspect: the extracted value borrows from the original Python object, so it must not outlive the PyObject it was derived from. PyO3 uses PyResult to handle conversion errors, enabling robust error handling in interop code. FromPyObject is a central component of PyO3’s type conversion system, complementing ToPyObject and IntoPyObject, and it facilitates ergonomic extraction of Python data into Rust during cross-language calls.