To define a custom python class, a rust struct needs to be annotated with the #[pyclass] attribute.

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

#[pyclass]
struct MyClass {
   num: i32,
   debug: bool,
}
#}

The above example generates implementations for PyTypeInfo and PyTypeObject for MyClass.

You can use pyclasses like normal rust structs.

However, if instantiated normally, you can't treat pyclasses as Python objects.

To get a Python object which includes pyclass, we have to use some special methods.

PyRef is a special reference, which ensures that the referred struct is a part of a Python object, and you are also holding the GIL.

You can get an instance of PyRef by PyRef::new, which does 3 things:

1. Allocate a Python object in the Python heap
2. Copies the rust struct into the Python object
3. Returns a reference of it

You can use PyRef just like &T, because it implements Deref<Target=T>.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# use pyo3::types::PyDict;
#[pyclass]
struct MyClass {
   num: i32,
   debug: bool,
}
let gil = Python::acquire_gil();
let py = gil.python();
let obj = PyRef::new(py, MyClass { num: 3, debug: true }).unwrap();
assert_eq!(obj.num, 3);
let dict = PyDict::new(py);
// You can treat a `PyRef` as a Python object
dict.set_item("obj", obj).unwrap();
#}

PyRefMut is a mutable version of PyRef.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
   num: i32,
   debug: bool,
}
let gil = Python::acquire_gil();
let py = gil.python();
let mut obj = PyRefMut::new(py, MyClass { num: 3, debug: true }).unwrap();
obj.num = 5;
#}

Py is a object wrapper which stores an object longer than the GIL lifetime.

You can use it to avoid lifetime problems.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
   num: i32,
}
fn return_myclass() -> Py<MyClass> {
    let gil = Python::acquire_gil();
    let py = gil.python();
    Py::new(py, MyClass { num: 1 }).unwrap()
}
let gil = Python::acquire_gil();
let obj = return_myclass();
assert_eq!(obj.as_ref(gil.python()).num, 1);
#}

The #[pyclass] macro accepts following parameters:

• name=XXX - Set the class name shown in python code. By default struct name is used as a class name.
• freelist=XXX - freelist parameter add support of free allocation list to custom class. The performance improvement applies to types that are often created and deleted in a row, so that they can benefit from a freelist. XXX is a number of items for free list.
• gc - Classes with the gc parameter participate in python garbage collector. If a custom class contains references to other python object that can be collected, the PyGCProtocol trait has to be implemented.
• weakref - adds support for python weak references
• extends=BaseType - use a custom base class. The base BaseType must implement PyTypeInfo.
• subclass - Allows Python classes to inherit from this class
• dict - adds __dict__ support, the instances of this type have a dictionary containing instance variables.

By default it is not possible to create an instance of a custom class from python code. To declare a constructor, you need to define a class method and annotate it with #[new] attribute. Only the python __new__ method can be specified, __init__ is not available.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# use pyo3::PyRawObject;
#[pyclass]
struct MyClass {
   num: i32,
}

#[pymethods]
impl MyClass {

     #[new]
     fn new(obj: &PyRawObject, num: i32) {
         obj.init({
             MyClass {
                 num,
             }
         });
     }
}
#}

Rules for the new method:

• If no method marked with #[new] is declared, object instances can only be created from Rust, but not from Python.
• The first parameter is the raw object and the custom new method must initialize the object with an instance of the struct using init method. The type of the object may be the type object of a derived class declared in Python.
• The first parameter implicitly has type &PyRawObject.
• For details on parameter-list, see the documentation of Method arguments section.
• The return type must be PyResult<T> for some T that implements IntoPyObject. Usually, T will be MyType.

By default PyObject is used as default base class. To override default base class base parameter for class needs to be used. Value is full path to base class. new method accepts PyRawObject object. obj instance must be initialized with value of custom class struct. Subclass must call parent's new method.

# extern crate pyo3;
# use pyo3::prelude::*;
# use pyo3::PyRawObject;
#[pyclass]
struct BaseClass {
   val1: usize,
}

#[pymethods]
impl BaseClass {
   #[new]
   fn new(obj: &PyRawObject) {
       obj.init(BaseClass{ val1: 10 });
   }

   pub fn method(&self) -> PyResult<()> {
      Ok(())
   }
}

#[pyclass(extends=BaseClass)]
struct SubClass {
   val2: usize,
}

#[pymethods]
impl SubClass {
   #[new]
   fn new(obj: &PyRawObject) {
       obj.init(SubClass{ val2: 10 });
       BaseClass::new(obj);
   }

   fn method2(&self) -> PyResult<()> {
      self.get_base().method()
   }
}

ObjectProtocol trait provides get_base() method. It returns reference to instance of base class.

Descriptor methods can be defined in #[pymethods] impl block only and has to be annotated with #[getter] or [setter] attributes. i.e.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
# }
#
#[pymethods]
impl MyClass {

     #[getter]
     fn num(&self) -> PyResult<i32> {
        Ok(self.num)
     }
}
#}

Getter or setter function's name is used as property name by default. There are several ways how to override name.

If function name starts with get_ or set_ for getter or setter respectively. Descriptor name becomes function name with prefix removed. This is useful in case of rust's special keywords like type.

# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
# }
#
#[pymethods]
impl MyClass {

     #[getter]
     fn get_num(&self) -> PyResult<i32> {
        Ok(self.num)
     }

     #[setter]
     fn set_num(&mut self, value: i32) -> PyResult<()> {
        self.num = value;
        Ok(())
     }
}

In this case property num is defined. And it is available from python code as self.num.

Also both #[getter] and #[setter] attributes accepts one parameter. If this parameter is specified, it is used as a property name. i.e.

# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
# }
#
#[pymethods]
impl MyClass {

     #[getter(number)]
     fn num(&self) -> PyResult<i32> {
        Ok(self.num)
     }

     #[setter(number)]
     fn set_num(&mut self, value: i32) -> PyResult<()> {
        self.num = value;
        Ok(())
     }
}

In this case the property number is defined and is available from python code as self.number.

For simple cases you can also define getters and setters in your Rust struct field definition, for example:

# extern crate pyo3;
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
  #[pyo3(get, set)]
  num: i32
}

Then it is available from Python code as self.num.

To define a python compatible method, impl block for struct has to be annotated with the #[pymethods] attribute. PyO3 generates python compatible wrappers for all functions in this block with some variations, like descriptors, class method static methods, etc.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
# }
#
#[pymethods]
impl MyClass {

     fn method1(&self) -> PyResult<i32> {
        Ok(10)
     }

     fn set_method(&mut self, value: i32) -> PyResult<()> {
        self.num = value;
        Ok(())
     }
}
#}

Calls to this methods protected by GIL, &self or &mut self can be used. The return type must be PyResult<T> for some T that implements IntoPyObject.

Python parameter can be specified as part of method signature, in this case py argument get injected by method wrapper. i.e

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
#    debug: bool,
# }

#[pymethods]
impl MyClass {
     fn method2(&self, py: Python) -> PyResult<i32> {
        Ok(10)
     }
}
#}

From python perspective method2, in above example, does not accept any arguments.

To specify a class method for a custom class, the method needs to be annotated with the #[classmethod] attribute.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# use pyo3::types::PyType;
# #[pyclass]
# struct MyClass {
#    num: i32,
#    debug: bool,
# }

#[pymethods]
impl MyClass {
     #[classmethod]
     fn cls_method(cls: &PyType) -> PyResult<i32> {
        Ok(10)
     }
}
#}

Declares a class method callable from Python.

• The first parameter is the type object of the class on which the method is called. This may be the type object of a derived class.
• The first parameter implicitly has type &PyType.
• For details on parameter-list, see the documentation of Method arguments section.
• The return type must be PyResult<T> for some T that implements IntoPyObject.

To specify a static method for a custom class, method needs to be annotated with #[staticmethod] attribute. The return type must be PyResult<T> for some T that implements IntoPyObject.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#    num: i32,
#    debug: bool,
# }

#[pymethods]
impl MyClass {
     #[staticmethod]
     fn static_method(param1: i32, param2: &str) -> PyResult<i32> {
        Ok(10)
     }
}
#}

To specify a custom __call__ method for a custom class, call methods need to be annotated with the #[call] attribute. Arguments of the method are specified same as for instance method.

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
use pyo3::types::PyTuple;
# #[pyclass]
# struct MyClass {
#    num: i32,
#    debug: bool,
# }

#[pymethods]
impl MyClass {
     #[call]
     #[args(args="*")]
     fn __call__(&self, args: &PyTuple) -> PyResult<i32> {
        println!("MyClass has been called");
        Ok(self.num)
     }
}
#}

By default PyO3 uses function signatures to determine which arguments are required. Then it scans incoming args parameter and then incoming kwargs parameter. If it can not find all required parameters, it raises a TypeError exception. It is possible to override the default behavior with #[args(...)] attribute. args attribute accepts a comma separated list of parameters in form of attr_name="default value". Each parameter has to match the method parameter by name.

Each parameter could be one of following type:

• "*": var arguments separator, each parameter defined after "*" is keyword only parameters. corresponds to python's def meth(*, arg1.., arg2=..)
• args="*": "args" is var args, corresponds to python's def meth(*args). Type of args parameter has to be &PyTuple.
• kwargs="**": "kwargs" is keyword arguments, corresponds to python's def meth(**kwargs). Type of kwargs parameter has to be Option<&PyDict>.
• arg="Value": arguments with default value. corresponds to python's def meth(arg=Value). if arg argument is defined after var arguments it is treated as keyword argument. Note that Value has to be valid rust code, PyO3 just inserts it into generated code unmodified.

Example:

# #![allow(unused_variables)]
#fn main() {
# extern crate pyo3;
# use pyo3::prelude::*;
use pyo3::types::{PyDict, PyTuple};
#
# #[pyclass]
# struct MyClass {
#    num: i32,
#    debug: bool,
# }
#
#[pymethods]
impl MyClass {
    #[args(arg1=true, args="*", arg2=10, kwargs="**")]
    fn method(&self, arg1: bool, args: &PyTuple, arg2: i32, kwargs: Option<&PyDict>) -> PyResult<i32> {
        Ok(1)
    }
}
#}

Python's object model defines several protocols for different object behavior, like sequence, mapping or number protocols. PyO3 defines separate traits for each of them. To provide specific python object behavior you need to implement the specific trait for your struct. Important note, each protocol implementation block has to be annotated with #[pyproto] attribute.

PyObjectProtocol trait provide several basic customizations.

To customize object attribute access define following methods:

• fn __getattr__(&self, name: FromPyObject) -> PyResult<impl IntoPyObject>
• fn __setattr__(&mut self, name: FromPyObject, value: FromPyObject) -> PyResult<()>
• fn __delattr__(&mut self, name: FromPyObject) -> PyResult<()>

Each methods corresponds to python's self.attr, self.attr = value and del self.attr code.

• fn __repr__(&self) -> PyResult<impl ToPyObject<ObjectType=PyString>>

• fn __str__(&self) -> PyResult<impl ToPyObject<ObjectType=PyString>>

  Possible return types for __str__ and __repr__ are PyResult<String> or PyResult<PyString>. In Python 2.7, Unicode strings returned by __str__ and __repr__ will be converted to byte strings by the Python runtime, which results in an exception if the string contains non-ASCII characters.

• fn __bytes__(&self) -> PyResult<PyBytes>

  On Python 3.x, provides the conversion to bytes. On Python 2.7, __bytes__ is allowed but has no effect.

• fn __unicode__(&self) -> PyResult<PyUnicode>

  On Python 2.7, provides the conversion to unicode. On Python 3.x, __unicode__ is allowed but has no effect.

• fn __format__(&self, format_spec: &str) -> PyResult<impl ToPyObject<ObjectType=PyString>>

  Special method that is used by the format() builtin and the str.format() method. Possible return types are PyResult<String> or PyResult<PyString>.

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

  Overloads Python comparison operations (==, !=, <, <=, >, and >=). The op argument indicates the comparison operation being performed. The return type will normally be PyResult<bool>, but any Python object can be returned. If other is not of the type specified in the signature, the generated code will automatically return NotImplemented.

• fn __hash__(&self) -> PyResult<impl PrimInt>

  Objects that compare equal must have the same hash value. The return type must be PyResult<T> where T is one of Rust's primitive integer types.

• fn __bool__(&self) -> PyResult<bool>

  Determines the "truthiness" of the object. This method works for both python 3 and python 2, even on Python 2.7 where the Python spelling was __nonzero__.

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 PyGCProtocol trait for your struct. It includes 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. Example:

# #![allow(unused_variables)]
#fn main() {
extern crate pyo3;

use pyo3::prelude::*;
use pyo3::PyTraverseError;
use pyo3::gc::{PyGCProtocol, PyVisit};

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

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

    fn __clear__(&mut self) {
        if let Some(obj) = self.obj.take() {
          // Release reference, this decrements ref counter.
          let gil = GILGuard::acquire();
          let py = gil.python();
          py.release(obj);
        }
    }
}
#}

Special protocol trait implementations have to be annotated with the #[pyproto] attribute.

It is also possible to enable GC for custom class using gc parameter for class annotation. i.e. #[pyclass(gc)]. In that case instances of custom class participate in python garbage collector, and it is possible to track them with gc module methods.

Iterators can be defined using the PyIterProtocol trait. It includes two methods __iter__ and __next__:

• fn __iter__(slf: PyRefMut<Self>) -> PyResult<impl IntoPyObject>
• fn __next__(slf: PyRefMut<Self>) -> PyResult<Option<impl IntoPyObject>>

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

Example:

# #![allow(unused_variables)]
#fn main() {
extern crate pyo3;

use pyo3::prelude::*;
use pyo3::PyIterProtocol;

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

#[pyproto]
impl PyIterProtocol for MyIterator {
    fn __iter__(slf: PyRefMut<Self>) -> PyResult<Py<MyIterator>> {
        Ok(slf.into())
    }
    fn __next__(mut slf: PyRefMut<Self>) -> PyResult<Option<PyObject>> {
        Ok(slf.iter.next())
    }
}
#}

TODO: Which traits to implement (basically PyTypeCreate: PyObjectAlloc + PyTypeInfo + PyMethodsProtocol + Sized) and what they mean.

Users should be able to define a #[pyclass] with or without #[pymethods], while PyO3 needs a trait with a function that returns all methods. Since it's impossible to make the code generation in pyclass dependent on whether there is an impl block, we'd need to implement the trait on #[pyclass] and override the implementation in #[pymethods], which is to the best of my knowledge only possible with the specialization feature, which can't be used on stable.

To escape this we use inventory, which allows us to collect impls from arbitrary source code by exploiting some binary trick. See inventory: how it works and pyo3_derive_backend::py_class::impl_inventory for more details.