Magic methods and slots

Python's object model defines several protocols for different object behavior, such as the sequence, mapping, and number protocols. You may be familiar with implementing these protocols in Python classes by "magic" methods, such as __str__ or __repr__. Because of the double-underscores surrounding their name, these are also known as "dunder" methods.

In the Python C-API which PyO3 is implemented upon, many of these magic methods have to be placed into special "slots" on the class type object, as covered in the previous section.

If a function name in #[pymethods] is a recognised magic method, it will be automatically placed into the correct slot in the Python type object. The function name is taken from the usual rules for naming #[pymethods]: the #[pyo3(name = "...")] attribute is used if present, otherwise the Rust function name is used.

The magic methods handled by PyO3 are very similar to the standard Python ones on this page - in particular they are the the subset which have slots as defined here. Some of the slots do not have a magic method in Python, which leads to a few additional magic methods defined only in PyO3:

  • Magic methods for garbage collection
  • Magic methods for the buffer protocol

When PyO3 handles a magic method, a couple of changes apply compared to other #[pymethods]:

  • The Rust function signature is restricted to match the magic method.
  • The #[pyo3(signature = (...)] and #[pyo3(text_signature = "...")] attributes are not allowed.

The following sections list of all magic methods PyO3 currently handles. The given signatures should be interpreted as follows:

  • All methods take a receiver as first argument, shown as <self>. It can be &self, &mut self or a PyCell reference like self_: PyRef<'_, Self> and self_: PyRefMut<'_, Self>, as described here.
  • An optional Python<'py> argument is always allowed as the first argument.
  • Return values can be optionally wrapped in PyResult.
  • object means that any type is allowed that can be extracted from a Python object (if argument) or converted to a Python object (if return value).
  • Other types must match what's given, e.g. pyo3::basic::CompareOp for __richcmp__'s second argument.
  • For the comparison and arithmetic methods, extraction errors are not propagated as exceptions, but lead to a return of NotImplemented.
  • For some magic methods, the return values are not restricted by PyO3, but checked by the Python interpreter. For example, __str__ needs to return a string object. This is indicated by object (Python type).

Basic object customization

  • __str__(<self>) -> object (str)

  • __repr__(<self>) -> object (str)

  • __hash__(<self>) -> isize

    Objects that compare equal must have the same hash value. Any type up to 64 bits may be returned instead of isize, PyO3 will convert to an isize automatically (wrapping unsigned types like u64 and usize).

    Disabling Python's default hash By default, all `#[pyclass]` types have a default hash implementation from Python. Types which should not be hashable can override this by setting `__hash__` to `None`. This is the same mechanism as for a pure-Python class. This is done like so: 
    #![allow(unused)]
fn main() {
use pyo3::prelude::*;

#[pyclass]
struct NotHashable {}

#[pymethods]
impl NotHashable {
    #[classattr]
    const __hash__: Option<PyObject> = None;
}
}

  • __richcmp__(<self>, object, pyo3::basic::CompareOp) -> object

    Overloads Python comparison operations (==, !=, <, <=, >, and >=). The CompareOp argument indicates the comparison operation being performed.

    Note that implementing __richcmp__ will cause Python not to generate a default __hash__ implementation, so consider implementing __hash__ when implementing __richcmp__.

    Return type The return type will normally be `PyResult`, but any Python object can be returned. If the second argument `object` is not of the type specified in the signature, the generated code will automatically `return NotImplemented`.

    You can use CompareOp::matches to adapt a Rust std::cmp::Ordering result to the requested comparison.

  • __getattr__(<self>, object) -> object

  • __getattribute__(<self>, object) -> object

    Differences between `__getattr__` and `__getattribute__` As in Python, `__getattr__` is only called if the attribute is not found by normal attribute lookup. `__getattribute__`, on the other hand, is called for *every* attribute access. If it wants to access existing attributes on `self`, it needs to be very careful not to introduce infinite recursion, and use `baseclass.__getattribute__()`.

  • __setattr__(<self>, value: object) -> ()

  • __delattr__(<self>, object) -> ()

    Overrides attribute access.

  • __bool__(<self>) -> bool

    Determines the "truthyness" of an object.

  • __call__(<self>, ...) -> object - here, any argument list can be defined as for normal pymethods

Iterable objects

Iterators can be defined using these methods:

  • __iter__(<self>) -> object
  • __next__(<self>) -> Option<object> or IterNextOutput (see details)

Returning None from __next__ indicates that that there are no further items.

Example:

#![allow(unused)]
fn main() {
use pyo3::prelude::*;

#[pyclass]
struct MyIterator {
    iter: Box<dyn Iterator<Item = PyObject> + Send>,
}

#[pymethods]
impl MyIterator {
    fn __iter__(slf: PyRef<'_, Self>) -> PyRef<'_, Self> {
        slf
    }
    fn __next__(mut slf: PyRefMut<'_, Self>) -> Option<PyObject> {
        slf.iter.next()
    }
}
}

In many cases you'll have a distinction between the type being iterated over (i.e. the iterable) and the iterator it provides. In this case, the iterable only needs to implement __iter__() while the iterator must implement both __iter__() and __next__(). For example:

#![allow(unused)]
fn main() {
use pyo3::prelude::*;

#[pyclass]
struct Iter {
    inner: std::vec::IntoIter<usize>,
}

#[pymethods]
impl Iter {
    fn __iter__(slf: PyRef<'_, Self>) -> PyRef<'_, Self> {
        slf
    }

    fn __next__(mut slf: PyRefMut<'_, Self>) -> Option<usize> {
        slf.inner.next()
    }
}

#[pyclass]
struct Container {
    iter: Vec<usize>,
}

#[pymethods]
impl Container {
    fn __iter__(slf: PyRef<'_, Self>) -> PyResult<Py<Iter>> {
        let iter = Iter {
            inner: slf.iter.clone().into_iter(),
        };
        Py::new(slf.py(), iter)
    }
}

Python::with_gil(|py| {
    let container = Container { iter: vec![1, 2, 3, 4] };
    let inst = pyo3::PyCell::new(py, container).unwrap();
    pyo3::py_run!(py, inst, "assert list(inst) == [1, 2, 3, 4]");
    pyo3::py_run!(py, inst, "assert list(iter(iter(inst))) == [1, 2, 3, 4]");
});
}

For more details on Python's iteration protocols, check out the "Iterator Types" section of the library documentation.

Returning a value from iteration

This guide has so far shown how to use Option<T> to implement yielding values during iteration. In Python a generator can also return a value. To express this in Rust, PyO3 provides the IterNextOutput enum to both Yield values and Return a final value - see its docs for further details and an example.

Awaitable objects

  • __await__(<self>) -> object
  • __aiter__(<self>) -> object
  • __anext__(<self>) -> Option<object> or IterANextOutput

Mapping & Sequence types

The magic methods in this section can be used to implement Python container types. They are two main categories of container in Python: "mappings" such as dict, with arbitrary keys, and "sequences" such as list and tuple, with integer keys.

The Python C-API which PyO3 is built upon has separate "slots" for sequences and mappings. When writing a class in pure Python, there is no such distinction in the implementation - a __getitem__ implementation will fill the slots for both the mapping and sequence forms, for example.

By default PyO3 reproduces the Python behaviour of filling both mapping and sequence slots. This makes sense for the "simple" case which matches Python, and also for sequences, where the mapping slot is used anyway to implement slice indexing.

Mapping types usually will not want the sequence slots filled. Having them filled will lead to outcomes which may be unwanted, such as:

  • The mapping type will successfully cast to PySequence. This may lead to consumers of the type handling it incorrectly.
  • Python provides a default implementation of __iter__ for sequences, which calls __getitem__ with consecutive positive integers starting from 0 until an IndexError is returned. Unless the mapping only contains consecutive positive integer keys, this __iter__ implementation will likely not be the intended behavior.

Use the #[pyclass(mapping)] annotation to instruct PyO3 to only fill the mapping slots, leaving the sequence ones empty. This will apply to __getitem__, __setitem__, and __delitem__.

Use the #[pyclass(sequence)] annotation to instruct PyO3 to fill the sq_length slot instead of the mp_length slot for __len__. This will help libraries such as numpy recognise the class as a sequence, however will also cause CPython to automatically add the sequence length to any negative indices before passing them to __getitem__. (__getitem__, __setitem__ and __delitem__ mapping slots are still used for sequences, for slice operations.)

  • __len__(<self>) -> usize

    Implements the built-in function len().

  • __contains__(<self>, object) -> bool

    Implements membership test operators. Should return true if item is in self, false otherwise. For objects that don’t define __contains__(), the membership test simply traverses the sequence until it finds a match.

    Disabling Python's default contains

    By default, all #[pyclass] types with an __iter__ method support a default implementation of the in operator. Types which do not want this can override this by setting __contains__ to None. This is the same mechanism as for a pure-Python class. This is done like so:

    #![allow(unused)]
fn main() {
use pyo3::prelude::*;

#[pyclass]
struct NoContains {}

#[pymethods]
impl NoContains {
    #[classattr]
    const __contains__: Option<PyObject> = None;
}
}

  • __getitem__(<self>, object) -> object

    Implements retrieval of the self[a] element.

    Note: Negative integer indexes are not handled specially by PyO3. However, for classes with #[pyclass(sequence)], when a negative index is accessed via PySequence::get_item, the underlying C API already adjusts the index to be positive.

  • __setitem__(<self>, object, object) -> ()

    Implements assignment to the self[a] element. Should only be implemented if elements can be replaced.

    Same behavior regarding negative indices as for __getitem__.

  • __delitem__(<self>, object) -> ()

    Implements deletion of the self[a] element. Should only be implemented if elements can be deleted.

    Same behavior regarding negative indices as for __getitem__.

  • fn __concat__(&self, other: impl FromPyObject) -> PyResult<impl ToPyObject>

    Concatenates two sequences. Used by the + operator, after trying the numeric addition via the __add__ and __radd__ methods.

  • fn __repeat__(&self, count: isize) -> PyResult<impl ToPyObject>

    Repeats the sequence count times. Used by the * operator, after trying the numeric multiplication via the __mul__ and __rmul__ methods.

  • fn __inplace_concat__(&self, other: impl FromPyObject) -> PyResult<impl ToPyObject>

    Concatenates two sequences. Used by the += operator, after trying the numeric addition via the __iadd__ method.

  • fn __inplace_repeat__(&self, count: isize) -> PyResult<impl ToPyObject>

    Concatenates two sequences. Used by the *= operator, after trying the numeric multiplication via the __imul__ method.

Descriptors

  • __get__(<self>, object, object) -> object
  • __set__(<self>, object, object) -> ()
  • __delete__(<self>, object) -> ()

Numeric types

Binary arithmetic operations (+, -, *, @, /, //, %, divmod(), pow() and **, <<, >>, &, ^, and |) and their reflected versions:

(If the object is not of the type specified in the signature, the generated code will automatically return NotImplemented.)

  • __add__(<self>, object) -> object
  • __radd__(<self>, object) -> object
  • __sub__(<self>, object) -> object
  • __rsub__(<self>, object) -> object
  • __mul__(<self>, object) -> object
  • __rmul__(<self>, object) -> object
  • __matmul__(<self>, object) -> object
  • __rmatmul__(<self>, object) -> object
  • __floordiv__(<self>, object) -> object
  • __rfloordiv__(<self>, object) -> object
  • __truediv__(<self>, object) -> object
  • __rtruediv__(<self>, object) -> object
  • __divmod__(<self>, object) -> object
  • __rdivmod__(<self>, object) -> object
  • __mod__(<self>, object) -> object
  • __rmod__(<self>, object) -> object
  • __lshift__(<self>, object) -> object
  • __rlshift__(<self>, object) -> object
  • __rshift__(<self>, object) -> object
  • __rrshift__(<self>, object) -> object
  • __and__(<self>, object) -> object
  • __rand__(<self>, object) -> object
  • __xor__(<self>, object) -> object
  • __rxor__(<self>, object) -> object
  • __or__(<self>, object) -> object
  • __ror__(<self>, object) -> object
  • __pow__(<self>, object, object) -> object
  • __rpow__(<self>, object, object) -> object

In-place assignment operations (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=):

  • __iadd__(<self>, object) -> ()
  • __isub__(<self>, object) -> ()
  • __imul__(<self>, object) -> ()
  • __imatmul__(<self>, object) -> ()
  • __itruediv__(<self>, object) -> ()
  • __ifloordiv__(<self>, object) -> ()
  • __imod__(<self>, object) -> ()
  • __ipow__(<self>, object, object) -> ()
  • __ilshift__(<self>, object) -> ()
  • __irshift__(<self>, object) -> ()
  • __iand__(<self>, object) -> ()
  • __ixor__(<self>, object) -> ()
  • __ior__(<self>, object) -> ()

Unary operations (-, +, abs() and ~):

  • __pos__(<self>) -> object
  • __neg__(<self>) -> object
  • __abs__(<self>) -> object
  • __invert__(<self>) -> object

Coercions:

  • __index__(<self>) -> object (int)
  • __int__(<self>) -> object (int)
  • __float__(<self>) -> object (float)

Buffer objects

  • __getbuffer__(<self>, *mut ffi::Py_buffer, flags) -> ()
  • __releasebuffer__(<self>, *mut ffi::Py_buffer) -> () Errors returned from __releasebuffer__ will be sent to sys.unraiseablehook. It is strongly advised to never return an error from __releasebuffer__, and if it really is necessary, to make best effort to perform any required freeing operations before returning. __releasebuffer__ will not be called a second time; anything not freed will be leaked.

Garbage Collector Integration

If your type owns references to other Python objects, you will need to integrate with Python's garbage collector so that the GC is aware of those references. To do this, implement the two methods __traverse__ and __clear__. These correspond to the slots tp_traverse and tp_clear in the Python C API. __traverse__ must call visit.call() for each reference to another Python object. __clear__ must clear out any mutable references to other Python objects (thus breaking reference cycles). Immutable references do not have to be cleared, as every cycle must contain at least one mutable reference.

  • __traverse__(<self>, pyo3::class::gc::PyVisit<'_>) -> Result<(), pyo3::class::gc::PyTraverseError>
  • __clear__(<self>) -> ()

Example:

#![allow(unused)]
fn main() {
use pyo3::prelude::*;
use pyo3::PyTraverseError;
use pyo3::gc::PyVisit;

#[pyclass]
struct ClassWithGCSupport {
    obj: Option<PyObject>,
}

#[pymethods]
impl ClassWithGCSupport {
    fn __traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
        if let Some(obj) = &self.obj {
            visit.call(obj)?
        }
        Ok(())
    }

    fn __clear__(&mut self) {
        // Clear reference, this decrements ref counter.
        self.obj = None;
    }
}
}

Note: these methods are part of the C API, PyPy does not necessarily honor them. If you are building for PyPy you should measure memory consumption to make sure you do not have runaway memory growth. See this issue on the PyPy bug tracker.