# Python classes

PyO3 exposes a group of attributes powered by Rust's proc macro system for defining Python classes as Rust structs.

The main attribute is `#[pyclass]`, which is placed upon a Rust `struct` or a fieldless `enum` (a.k.a. C-like enum) to generate a Python type for it. They will usually also have *one* `#[pymethods]`-annotated `impl` block for the struct, which is used to define Python methods and constants for the generated Python type. (If the [`multiple-pymethods`] feature is enabled, each `#[pyclass]` is allowed to have multiple `#[pymethods]` blocks.) `#[pymethods]` may also have implementations for Python magic methods such as `__str__`.

This chapter will discuss the functionality and configuration these attributes offer. Below is a list of links to the relevant section of this chapter for each:

- [`#[pyclass]`](#defining-a-new-class)
  - [`#[pyo3(get, set)]`](#object-properties-using-pyo3get-set)
- [`#[pymethods]`](#instance-methods)
  - [`#[new]`](#constructor)
  - [`#[getter]`](#object-properties-using-getter-and-setter)
  - [`#[setter]`](#object-properties-using-getter-and-setter)
  - [`#[staticmethod]`](#static-methods)
  - [`#[classmethod]`](#class-methods)
  - [`#[classattr]`](#class-attributes)
  - [`#[args]`](#method-arguments)
- [Magic methods and slots](class/protocols.html)
- [Classes as function arguments](#classes-as-function-arguments)

## Defining a new class

To define a custom Python class, add the `#[pyclass]` attribute to a Rust struct or a fieldless enum.
```rust
# #![allow(dead_code)]
use pyo3::prelude::*;

#[pyclass]
struct Integer {
    inner: i32,
}

// A "tuple" struct
#[pyclass]
struct Number(i32);

// PyO3 supports custom discriminants in enums
#[pyclass]
enum HttpResponse {
    Ok = 200,
    NotFound = 404,
    Teapot = 418,
    // ...
}

#[pyclass]
enum MyEnum {
    Variant,
    OtherVariant = 30, // PyO3 supports custom discriminants.
}
```

The above example generates implementations for [`PyTypeInfo`] and [`PyClass`] for `MyClass` and `MyEnum`. To see these generated implementations, refer to the [implementation details](#implementation-details) at the end of this chapter.

### Restrictions

To integrate Rust types with Python, PyO3 needs to place some restrictions on the types which can be annotated with `#[pyclass]`. In particular, they must have no lifetime parameters, no generic parameters, and must implement `Send`. The reason for each of these is explained below.

#### No lifetime parameters

Rust lifetimes are used by the Rust compiler to reason about a program's memory safety. They are a compile-time only concept; there is no way to access Rust lifetimes at runtime from a dynamic language like Python.

As soon as Rust data is exposed to Python, there is no guarantee that the Rust compiler can make on how long the data will live. Python is a reference-counted language and those references can be held for an arbitrarily long time which is untraceable by the Rust compiler. The only possible way to express this correctly is to require that any `#[pyclass]` does not borrow data for any lifetime shorter than the `'static` lifetime, i.e. the `#[pyclass]` cannot have any lifetime parameters.

When you need to share ownership of data between Python and Rust, instead of using borrowed references with lifetimes consider using reference-counted smart pointers such as [`Arc`] or [`Py`].

#### No generic parameters

A Rust `struct Foo<T>` with a generic parameter `T` generates new compiled implementations each time it is used with a different concrete type for `T`. These new implementations are generated by the compiler at each usage site. This is incompatible with wrapping `Foo` in Python, where there needs to be a single compiled implementation of `Foo` which is integrated with the Python interpreter.

#### Must be Send

Because Python objects are freely shared between threads by the Python interpreter, there is no guarantee which thread will eventually drop the object. Therefore all types annotated with `#[pyclass]` must implement `Send` (unless annotated with [`#[pyclass(unsendable)]`](#customizing-the-class)).

## Constructor

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 method and annotate it with the `#[new]`
attribute. Only Python's `__new__` method can be specified, `__init__` is not available.

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    #[new]
    fn new(value: i32) -> Self {
        Number(value)
    }
}
```

Alternatively, if your `new` method may fail you can return `PyResult<Self>`.

```rust
# use pyo3::prelude::*;
# use pyo3::exceptions::PyValueError;
# #[pyclass]
# struct Nonzero(i32);
#
#[pymethods]
impl Nonzero {
    #[new]
    fn py_new(value: i32) -> PyResult<Self> {
        if value == 0 {
            Err(PyValueError::new_err("cannot be zero"))
        } else {
            Ok(Nonzero(value))
        }
    }
}
```

If you want to return an existing object (for example, because your `new`
method caches the values it returns), `new` can return `pyo3::Py<Self>`.

As you can see, the Rust method name is not important here; this way you can
still, use `new()` for a Rust-level constructor.

If no method marked with `#[new]` is declared, object instances can only be
created from Rust, but not from Python.

For arguments, see the [`Method arguments`](#method-arguments) section below.

## Adding the class to a module

The next step is to create the module initializer and add our class to it:

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct Number(i32);
#
#[pymodule]
fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<Number>()?;
    Ok(())
}
```

## PyCell and interior mutability

You sometimes need to convert your `pyclass` into a Python object and access it
from Rust code (e.g., for testing it).
[`PyCell`] is the primary interface for that.

`PyCell<T: PyClass>` is always allocated in the Python heap, so Rust doesn't have ownership of it.
In other words, Rust code can only extract a `&PyCell<T>`, not a `PyCell<T>`.

Thus, to mutate data behind `&PyCell` safely, PyO3 employs the
[Interior Mutability Pattern](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html)
like [`RefCell`].

Users who are familiar with `RefCell` can use `PyCell` just like `RefCell`.

For users who are not very familiar with `RefCell`, here is a reminder of Rust's rules of borrowing:
- At any given time, you can have either (but not both of) one mutable reference or any number of immutable references.
- References must always be valid.

`PyCell`, like `RefCell`, ensures these borrowing rules by tracking references at runtime.

```rust
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
    #[pyo3(get)]
    num: i32,
}
Python::with_gil(|py| {
    let obj = PyCell::new(py, MyClass { num: 3 }).unwrap();
    {
        let obj_ref = obj.borrow(); // Get PyRef
        assert_eq!(obj_ref.num, 3);
        // You cannot get PyRefMut unless all PyRefs are dropped
        assert!(obj.try_borrow_mut().is_err());
    }
    {
        let mut obj_mut = obj.borrow_mut(); // Get PyRefMut
        obj_mut.num = 5;
        // You cannot get any other refs until the PyRefMut is dropped
        assert!(obj.try_borrow().is_err());
        assert!(obj.try_borrow_mut().is_err());
    }

    // You can convert `&PyCell` to a Python object
    pyo3::py_run!(py, obj, "assert obj.num == 5");
});
```

`&PyCell<T>` is bounded by the same lifetime as a [`GILGuard`].
To make the object longer lived (for example, to store it in a struct on the
Rust side), you can use `Py<T>`, which stores an object longer than the GIL
lifetime, and therefore needs a `Python<'_>` token to access.

```rust
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
    num: i32,
}

fn return_myclass() -> Py<MyClass> {
    Python::with_gil(|py| Py::new(py, MyClass { num: 1 }).unwrap())
}

let obj = return_myclass();

Python::with_gil(|py| {
    let cell = obj.as_ref(py); // Py<MyClass>::as_ref returns &PyCell<MyClass>
    let obj_ref = cell.borrow(); // Get PyRef<T>
    assert_eq!(obj_ref.num, 1);
});
```

### frozen classes: Opting out of interior mutability

As detailed above, runtime borrow checking is currently enabled by default. But a class can opt of out it by declaring itself `frozen`. It can still use interior mutability via standard Rust types like `RefCell` or `Mutex`, but it is not bound to the implementation provided by PyO3 and can choose the most appropriate strategy on field-by-field basis.

Classes which are `frozen` and also `Sync`, e.g. they do use `Mutex` but not `RefCell`, can be accessed without needing the Python GIL via the `PyCell::get` and `Py::get` methods:

```rust
use std::sync::atomic::{AtomicUsize, Ordering};
# use pyo3::prelude::*;

#[pyclass(frozen)]
struct FrozenCounter {
    value: AtomicUsize,
}

let py_counter: Py<FrozenCounter> = Python::with_gil(|py| {
    let counter = FrozenCounter {
        value: AtomicUsize::new(0),
    };

    Py::new(py, counter).unwrap()
});

py_counter.get().value.fetch_add(1, Ordering::Relaxed);
```

Frozen classes are likely to become the default thereby guiding the PyO3 ecosystem towards a more deliberate application of interior mutability. Eventually, this should enable further optimizations of PyO3's internals and avoid downstream code paying the cost of interior mutability when it is not actually required.

## Customizing the class

{{#include ../pyclass_parameters.md}}

These parameters are covered in various sections of this guide.

### Return type

Generally, `#[new]` methods have to return `T: Into<PyClassInitializer<Self>>` or
`PyResult<T> where T: Into<PyClassInitializer<Self>>`.

For constructors that may fail, you should wrap the return type in a PyResult as well.
Consult the table below to determine which type your constructor should return:

|                             | **Cannot fail**           | **May fail**                      |
|-----------------------------|---------------------------|-----------------------------------|
|**No inheritance**           | `T`                       | `PyResult<T>`                     |
|**Inheritance(T Inherits U)**| `(T, U)`                  | `PyResult<(T, U)>`                |
|**Inheritance(General Case)**| [`PyClassInitializer<T>`] | `PyResult<PyClassInitializer<T>>` |

## Inheritance

By default, `object`, i.e. `PyAny` is used as the base class. To override this default,
use the `extends` parameter for `pyclass` with the full path to the base class.
Currently, only classes defined in Rust and builtins provided by PyO3 can be inherited
from; inheriting from other classes defined in Python is not yet supported
([#991](https://github.com/PyO3/pyo3/issues/991)).


For convenience, `(T, U)` implements `Into<PyClassInitializer<T>>` where `U` is the
base class of `T`.
But for a more deeply nested inheritance, you have to return `PyClassInitializer<T>`
explicitly.

To get a parent class from a child, use [`PyRef`] instead of `&self` for methods,
or [`PyRefMut`] instead of `&mut self`.
Then you can access a parent class by `self_.as_ref()` as `&Self::BaseClass`,
or by `self_.into_super()` as `PyRef<Self::BaseClass>`.

```rust
# use pyo3::prelude::*;

#[pyclass(subclass)]
struct BaseClass {
    val1: usize,
}

#[pymethods]
impl BaseClass {
    #[new]
    fn new() -> Self {
        BaseClass { val1: 10 }
    }

    pub fn method(&self) -> PyResult<usize> {
        Ok(self.val1)
    }
}

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

#[pymethods]
impl SubClass {
    #[new]
    fn new() -> (Self, BaseClass) {
        (SubClass { val2: 15 }, BaseClass::new())
    }

    fn method2(self_: PyRef<'_, Self>) -> PyResult<usize> {
        let super_ = self_.as_ref(); // Get &BaseClass
        super_.method().map(|x| x * self_.val2)
    }
}

#[pyclass(extends=SubClass)]
struct SubSubClass {
    val3: usize,
}

#[pymethods]
impl SubSubClass {
    #[new]
    fn new() -> PyClassInitializer<Self> {
        PyClassInitializer::from(SubClass::new()).add_subclass(SubSubClass { val3: 20 })
    }

    fn method3(self_: PyRef<'_, Self>) -> PyResult<usize> {
        let v = self_.val3;
        let super_ = self_.into_super(); // Get PyRef<'_, SubClass>
        SubClass::method2(super_).map(|x| x * v)
    }

    #[staticmethod]
    fn factory_method(py: Python<'_>, val: usize) -> PyResult<PyObject> {
        let base = PyClassInitializer::from(BaseClass::new());
        let sub = base.add_subclass(SubClass { val2: val });
        if val % 2 == 0 {
            Ok(Py::new(py, sub)?.to_object(py))
        } else {
            let sub_sub = sub.add_subclass(SubSubClass { val3: val });
            Ok(Py::new(py, sub_sub)?.to_object(py))
        }
    }
}
# Python::with_gil(|py| {
#     let subsub = pyo3::PyCell::new(py, SubSubClass::new()).unwrap();
#     pyo3::py_run!(py, subsub, "assert subsub.method3() == 3000");
#     let subsub = SubSubClass::factory_method(py, 2).unwrap();
#     let subsubsub = SubSubClass::factory_method(py, 3).unwrap();
#     let cls = py.get_type::<SubSubClass>();
#     pyo3::py_run!(py, subsub cls, "assert not isinstance(subsub, cls)");
#     pyo3::py_run!(py, subsubsub cls, "assert isinstance(subsubsub, cls)");
# });
```

You can inherit native types such as `PyDict`, if they implement
[`PySizedLayout`]({{#PYO3_DOCS_URL}}/pyo3/type_object/trait.PySizedLayout.html).
This is not supported when building for the Python limited API (aka the `abi3` feature of PyO3).

However, because of some technical problems, we don't currently provide safe upcasting methods for types
that inherit native types. Even in such cases, you can unsafely get a base class by raw pointer conversion.

```rust
# #[cfg(not(Py_LIMITED_API))] {
# use pyo3::prelude::*;
use pyo3::types::PyDict;
use std::collections::HashMap;

#[pyclass(extends=PyDict)]
#[derive(Default)]
struct DictWithCounter {
    counter: HashMap<String, usize>,
}

#[pymethods]
impl DictWithCounter {
    #[new]
    fn new() -> Self {
        Self::default()
    }

    fn set(mut self_: PyRefMut<'_, Self>, key: String, value: &PyAny) -> PyResult<()> {
        self_.counter.entry(key.clone()).or_insert(0);
        let py = self_.py();
        let dict: &PyDict = unsafe { py.from_borrowed_ptr_or_err(self_.as_ptr())? };
        dict.set_item(key, value)
    }
}
# Python::with_gil(|py| {
#     let cnt = pyo3::PyCell::new(py, DictWithCounter::new()).unwrap();
#     pyo3::py_run!(py, cnt, "cnt.set('abc', 10); assert cnt['abc'] == 10")
# });
# }
```

If `SubClass` does not provide a base class initialization, the compilation fails.
```rust,compile_fail
# use pyo3::prelude::*;

#[pyclass]
struct BaseClass {
    val1: usize,
}

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

#[pymethods]
impl SubClass {
    #[new]
    fn new() -> Self {
        SubClass { val2: 15 }
    }
}
```

The `__new__` constructor of a native base class is called implicitly when
creating a new instance from Python.  Be sure to accept arguments in the
`#[new]` method that you want the base class to get, even if they are not used
in that `fn`:

```rust
# #[allow(dead_code)]
# #[cfg(not(Py_LIMITED_API))] {
# use pyo3::prelude::*;
use pyo3::types::PyDict;

#[pyclass(extends=PyDict)]
struct MyDict {
    private: i32,
}

#[pymethods]
impl MyDict {
    #[new]
    #[pyo3(signature = (*args, **kwargs))]
    fn new(args: &PyAny, kwargs: Option<&PyAny>) -> Self {
        Self { private: 0 }
    }

    // some custom methods that use `private` here...
}
# Python::with_gil(|py| {
#     let cls = py.get_type::<MyDict>();
#     pyo3::py_run!(py, cls, "cls(a=1, b=2)")
# });
# }
```

Here, the `args` and `kwargs` allow creating instances of the subclass passing
initial items, such as `MyDict(item_sequence)` or `MyDict(a=1, b=2)`.

## Object properties

PyO3 supports two ways to add properties to your `#[pyclass]`:
- For simple struct fields with no side effects, a `#[pyo3(get, set)]` attribute can be added directly to the field definition in the `#[pyclass]`.
- For properties which require computation you can define `#[getter]` and `#[setter]` functions in the [`#[pymethods]`](#instance-methods) block.

We'll cover each of these in the following sections.

### Object properties using `#[pyo3(get, set)]`

For simple cases where a member variable is just read and written with no side effects, you can declare getters and setters in your `#[pyclass]` field definition using the `pyo3` attribute, like in the example below:

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

The above would make the `num` field available for reading and writing as a `self.num` Python property. To expose the property with a different name to the field, specify this alongside the rest of the options, e.g. `#[pyo3(get, set, name = "custom_name")]`.

Properties can be readonly or writeonly by using just `#[pyo3(get)]` or `#[pyo3(set)]` respectively.

To use these annotations, your field type must implement some conversion traits:
- For `get` the field type must implement both `IntoPy<PyObject>` and `Clone`.
- For `set` the field type must implement `FromPyObject`.

For example, implementations of those traits are provided for the `Cell` type, if the inner type also implements the trait. This means you can use `#[pyo3(get, set)]` on fields wrapped in a `Cell`.

### Object properties using `#[getter]` and `#[setter]`

For cases which don't satisfy the `#[pyo3(get, set)]` trait requirements, or need side effects, descriptor methods can be defined in a `#[pymethods]` `impl` block.

This is done using the `#[getter]` and `#[setter]` attributes, like in the example below:

```rust
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
    num: i32,
}

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

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

If a function name starts with `get_` or `set_` for getter or setter respectively,
the descriptor name becomes the function name with this prefix removed. This is also useful in case of
Rust keywords like `type`
([raw identifiers](https://doc.rust-lang.org/edition-guide/rust-2018/module-system/raw-identifiers.html)
can be used since Rust 2018).

```rust
# 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, a property `num` is defined and available from Python code as `self.num`.

Both the `#[getter]` and `#[setter]` attributes accept one parameter.
If this parameter is specified, it is used as the property name, i.e.

```rust
# 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 available from Python code as `self.number`.

Attributes defined by `#[setter]` or `#[pyo3(set)]` will always raise `AttributeError` on `del`
operations. Support for defining custom `del` behavior is tracked in
[#1778](https://github.com/PyO3/pyo3/issues/1778).

## Instance methods

To define a Python compatible method, an `impl` block for your 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.

Since Rust allows any number of `impl` blocks, you can easily split methods
between those accessible to Python (and Rust) and those accessible only to Rust. However to have multiple
`#[pymethods]`-annotated `impl` blocks for the same struct you must enable the [`multiple-pymethods`] feature of PyO3.

```rust
# 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 these methods are protected by the GIL, so both `&self` and `&mut self` can be used.
The return type must be `PyResult<T>` or `T` for some `T` that implements `IntoPy<PyObject>`;
the latter is allowed if the method cannot raise Python exceptions.

A `Python` parameter can be specified as part of method signature, in this case the `py` argument
gets injected by the method wrapper, e.g.

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
# #[allow(dead_code)]
#     num: i32,
# }
#[pymethods]
impl MyClass {
    fn method2(&self, py: Python<'_>) -> PyResult<i32> {
        Ok(10)
    }
}
```

From the Python perspective, the `method2` in this example does not accept any arguments.

## Class methods

To create a class method for a custom class, the method needs to be annotated
with the `#[classmethod]` attribute.
This is the equivalent of the Python decorator `@classmethod`.

```rust
# use pyo3::prelude::*;
# use pyo3::types::PyType;
# #[pyclass]
# struct MyClass {
#     #[allow(dead_code)]
#     num: i32,
# }
#[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>` or `T` for some `T` that implements `IntoPy<PyObject>`.

### Constructors which accept a class argument

To create a constructor which takes a positional class argument, you can combine the `#[classmethod]` and `#[new]` modifiers:
```rust
# use pyo3::prelude::*;
# use pyo3::types::PyType;
# #[pyclass]
# struct BaseClass(PyObject);
#
#[pymethods]
impl BaseClass {
    #[new]
    #[classmethod]
    fn py_new<'p>(cls: &'p PyType, py: Python<'p>) -> PyResult<Self> {
        // Get an abstract attribute (presumably) declared on a subclass of this class.
        let subclass_attr = cls.getattr("a_class_attr")?;
        Ok(Self(subclass_attr.to_object(py)))
    }
}
```

## Static methods

To create a static method for a custom class, the method needs to be annotated with the
`#[staticmethod]` attribute. The return type must be `T` or `PyResult<T>` for some `T` that implements
`IntoPy<PyObject>`.

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {
#     #[allow(dead_code)]
#     num: i32,
# }
#[pymethods]
impl MyClass {
    #[staticmethod]
    fn static_method(param1: i32, param2: &str) -> PyResult<i32> {
        Ok(10)
    }
}
```

## Class attributes

To create a class attribute (also called [class variable][classattr]), a method without
any arguments can be annotated with the `#[classattr]` attribute.

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {}
#[pymethods]
impl MyClass {
    #[classattr]
    fn my_attribute() -> String {
        "hello".to_string()
    }
}

Python::with_gil(|py| {
    let my_class = py.get_type::<MyClass>();
    pyo3::py_run!(py, my_class, "assert my_class.my_attribute == 'hello'")
});
```

> Note: if the method has a `Result` return type and returns an `Err`, PyO3 will panic during
class creation.

If the class attribute is defined with `const` code only, one can also annotate associated
constants:

```rust
# use pyo3::prelude::*;
# #[pyclass]
# struct MyClass {}
#[pymethods]
impl MyClass {
    #[classattr]
    const MY_CONST_ATTRIBUTE: &'static str = "foobar";
}
```

## Classes as function arguments

Free functions defined using `#[pyfunction]` interact with classes through the same mechanisms as the self parameters of instance methods, i.e. they can take GIL-bound references, GIL-bound reference wrappers or GIL-indepedent references:

```rust
# #![allow(dead_code)]
# use pyo3::prelude::*;
#[pyclass]
struct MyClass {
    my_field: i32,
}

// Take a GIL-bound reference when the underlying `PyCell` is irrelevant.
#[pyfunction]
fn increment_field(my_class: &mut MyClass) {
    my_class.my_field += 1;
}

// Take a GIL-bound reference wrapper when borrowing should be automatic,
// but interaction with the underlying `PyCell` is desired.
#[pyfunction]
fn print_field(my_class: PyRef<'_, MyClass>) {
    println!("{}", my_class.my_field);
}

// Take a GIL-bound reference to the underlying cell
// when borrowing needs to be managed manually.
#[pyfunction]
fn increment_then_print_field(my_class: &PyCell<MyClass>) {
    my_class.borrow_mut().my_field += 1;

    println!("{}", my_class.borrow().my_field);
}

// Take a GIL-indepedent reference when you want to store the reference elsewhere.
#[pyfunction]
fn print_refcnt(my_class: Py<MyClass>, py: Python<'_>) {
    println!("{}", my_class.get_refcnt(py));
}
```

Classes can also be passed by value if they can be cloned, i.e. they automatically implement `FromPyObject` if they implement `Clone`, e.g. via `#[derive(Clone)]`:

```rust
# #![allow(dead_code)]
# use pyo3::prelude::*;
#[pyclass]
#[derive(Clone)]
struct MyClass {
    my_field: Box<i32>,
}

#[pyfunction]
fn dissamble_clone(my_class: MyClass) {
    let MyClass { mut my_field } = my_class;
    *my_field += 1;
}
```

Note that `#[derive(FromPyObject)]` on a class is usually not useful as it tries to construct a new Rust value by filling in the fields by looking up attributes of any given Python value.

## Method arguments

Similar to `#[pyfunction]`, the `#[pyo3(signature = (...))]` attribute can be used to specify the way that `#[pymethods]` accept arguments. Consult the documentation for [`function signatures`](./function/signature.md) to see the parameters this attribute accepts.

The following example defines a class `MyClass` with a method `method`. This method has a signature that sets default values for `num` and `name`, and indicates that `py_args` should collect all extra positional arguments and `py_kwargs` all extra keyword arguments:

```rust
# use pyo3::prelude::*;
use pyo3::types::{PyDict, PyTuple};
#
# #[pyclass]
# struct MyClass {
#     num: i32,
# }
#[pymethods]
impl MyClass {
    #[new]
    #[pyo3(signature = (num=-1))]
    fn new(num: i32) -> Self {
        MyClass { num }
    }

    #[pyo3(signature = (num=10, *py_args, name="Hello", **py_kwargs))]
    fn method(
        &mut self,
        num: i32,
        py_args: &PyTuple,
        name: &str,
        py_kwargs: Option<&PyDict>,
    ) -> String {
        let num_before = self.num;
        self.num = num;
        format!(
            "num={} (was previously={}), py_args={:?}, name={}, py_kwargs={:?} ",
            num, num_before, py_args, name, py_kwargs,
        )
    }
}
```

In Python, this might be used like:

```python
>>> import mymodule
>>> mc = mymodule.MyClass()
>>> print(mc.method(44, False, "World", 666, x=44, y=55))
py_args=('World', 666), py_kwargs=Some({'x': 44, 'y': 55}), name=Hello, num=44, num_before=-1
>>> print(mc.method(num=-1, name="World"))
py_args=(), py_kwargs=None, name=World, num=-1, num_before=44
```

## Making class method signatures available to Python

The [`text_signature = "..."`](./function.md#text_signature) option for `#[pyfunction]` also works for `#[pymethods]`:

```rust
# #![allow(dead_code)]
use pyo3::prelude::*;
use pyo3::types::PyType;

#[pyclass]
struct MyClass {}

#[pymethods]
impl MyClass {
    #[new]
    #[pyo3(text_signature = "(c, d)")]
    fn new(c: i32, d: &str) -> Self {
        Self {}
    }
    // the self argument should be written $self
    #[pyo3(text_signature = "($self, e, f)")]
    fn my_method(&self, e: i32, f: i32) -> i32 {
        e + f
    }
    // similarly for classmethod arguments, use $cls
    #[classmethod]
    #[pyo3(text_signature = "($cls, e, f)")]
    fn my_class_method(cls: &PyType, e: i32, f: i32) -> i32 {
        e + f
    }
    #[staticmethod]
    #[pyo3(text_signature = "(e, f)")]
    fn my_static_method(e: i32, f: i32) -> i32 {
        e + f
    }
}
#
# fn main() -> PyResult<()> {
#     Python::with_gil(|py| {
#         let inspect = PyModule::import(py, "inspect")?.getattr("signature")?;
#         let module = PyModule::new(py, "my_module")?;
#         module.add_class::<MyClass>()?;
#         let class = module.getattr("MyClass")?;
#
#         if cfg!(not(Py_LIMITED_API)) || py.version_info() >= (3, 10)  {
#             let doc: String = class.getattr("__doc__")?.extract()?;
#             assert_eq!(doc, "");
#
#             let sig: String = inspect
#                 .call1((class,))?
#                 .call_method0("__str__")?
#                 .extract()?;
#             assert_eq!(sig, "(c, d)");
#         } else {
#             let doc: String = class.getattr("__doc__")?.extract()?;
#             assert_eq!(doc, "");
#
#             inspect.call1((class,)).expect_err("`text_signature` on classes is not compatible with compilation in `abi3` mode until Python 3.10 or greater");
#          }
#
#         {
#             let method = class.getattr("my_method")?;
#
#             assert!(method.getattr("__doc__")?.is_none());
#
#             let sig: String = inspect
#                 .call1((method,))?
#                 .call_method0("__str__")?
#                 .extract()?;
#             assert_eq!(sig, "(self, /, e, f)");
#         }
#
#         {
#             let method = class.getattr("my_class_method")?;
#
#             assert!(method.getattr("__doc__")?.is_none());
#
#             let sig: String = inspect
#                 .call1((method,))?
#                 .call_method0("__str__")?
#                 .extract()?;
#             assert_eq!(sig, "(e, f)");  // inspect.signature skips the $cls arg
#         }
#
#         {
#             let method = class.getattr("my_static_method")?;
#
#             assert!(method.getattr("__doc__")?.is_none());
#
#             let sig: String = inspect
#                 .call1((method,))?
#                 .call_method0("__str__")?
#                 .extract()?;
#             assert_eq!(sig, "(e, f)");
#         }
#
#         Ok(())
#     })
# }
```

Note that `text_signature` on `#[new]` is not compatible with compilation in
`abi3` mode until Python 3.10 or greater.

## #[pyclass] enums

Currently PyO3 only supports fieldless enums. PyO3 adds a class attribute for each variant, so you can access them in Python without defining `#[new]`. PyO3 also provides default implementations of `__richcmp__` and `__int__`, so they can be compared using `==`:

```rust
# use pyo3::prelude::*;
#[pyclass]
enum MyEnum {
    Variant,
    OtherVariant,
}

Python::with_gil(|py| {
    let x = Py::new(py, MyEnum::Variant).unwrap();
    let y = Py::new(py, MyEnum::OtherVariant).unwrap();
    let cls = py.get_type::<MyEnum>();
    pyo3::py_run!(py, x y cls, r#"
        assert x == cls.Variant
        assert y == cls.OtherVariant
        assert x != y
    "#)
})
```

You can also convert your enums into `int`:

```rust
# use pyo3::prelude::*;
#[pyclass]
enum MyEnum {
    Variant,
    OtherVariant = 10,
}

Python::with_gil(|py| {
    let cls = py.get_type::<MyEnum>();
    let x = MyEnum::Variant as i32; // The exact value is assigned by the compiler.
    pyo3::py_run!(py, cls x, r#"
        assert int(cls.Variant) == x
        assert int(cls.OtherVariant) == 10
        assert cls.OtherVariant == 10  # You can also compare against int.
        assert 10 == cls.OtherVariant
    "#)
})
```

PyO3 also provides `__repr__` for enums:

```rust
# use pyo3::prelude::*;
#[pyclass]
enum MyEnum{
    Variant,
    OtherVariant,
}

Python::with_gil(|py| {
    let cls = py.get_type::<MyEnum>();
    let x = Py::new(py, MyEnum::Variant).unwrap();
    pyo3::py_run!(py, cls x, r#"
        assert repr(x) == 'MyEnum.Variant'
        assert repr(cls.OtherVariant) == 'MyEnum.OtherVariant'
    "#)
})
```

All methods defined by PyO3 can be overridden. For example here's how you override `__repr__`:

```rust
# use pyo3::prelude::*;
#[pyclass]
enum MyEnum {
    Answer = 42,
}

#[pymethods]
impl MyEnum {
    fn __repr__(&self) -> &'static str {
        "42"
    }
}

Python::with_gil(|py| {
    let cls = py.get_type::<MyEnum>();
    pyo3::py_run!(py, cls, "assert repr(cls.Answer) == '42'")
})
```

Enums and their variants can also be renamed using `#[pyo3(name)]`.

```rust
# use pyo3::prelude::*;
#[pyclass(name = "RenamedEnum")]
enum MyEnum {
    #[pyo3(name = "UPPERCASE")]
    Variant,
}

Python::with_gil(|py| {
    let x = Py::new(py, MyEnum::Variant).unwrap();
    let cls = py.get_type::<MyEnum>();
    pyo3::py_run!(py, x cls, r#"
        assert repr(x) == 'RenamedEnum.UPPERCASE'
        assert x == cls.UPPERCASE
    "#)
})
```

You may not use enums as a base class or let enums inherit from other classes.

```rust,compile_fail
# use pyo3::prelude::*;
#[pyclass(subclass)]
enum BadBase {
    Var1,
}
```

```rust,compile_fail
# use pyo3::prelude::*;

#[pyclass(subclass)]
struct Base;

#[pyclass(extends=Base)]
enum BadSubclass {
    Var1,
}
```

`#[pyclass]` enums are currently not interoperable with `IntEnum` in Python.

## Implementation details

The `#[pyclass]` macros rely on a lot of conditional code generation: each `#[pyclass]` can optionally have a `#[pymethods]` block.

To support this flexibility the `#[pyclass]` macro expands to a blob of boilerplate code which sets up the structure for ["dtolnay specialization"](https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md). This implementation pattern enables the Rust compiler to use `#[pymethods]` implementations when they are present, and fall back to default (empty) definitions when they are not.

This simple technique works for the case when there is zero or one implementations. To support multiple `#[pymethods]` for a `#[pyclass]` (in the [`multiple-pymethods`] feature), a registry mechanism provided by the [`inventory`](https://github.com/dtolnay/inventory) crate is used instead. This collects `impl`s at library load time, but isn't supported on all platforms. See [inventory: how it works](https://github.com/dtolnay/inventory#how-it-works) for more details.

The `#[pyclass]` macro expands to roughly the code seen below. The `PyClassImplCollector` is the type used internally by PyO3 for dtolnay specialization:

```rust
# #[cfg(not(feature = "multiple-pymethods"))] {
# use pyo3::prelude::*;
// Note: the implementation differs slightly with the `multiple-pymethods` feature enabled.
struct MyClass {
    # #[allow(dead_code)]
    num: i32,
}
unsafe impl pyo3::type_object::PyTypeInfo for MyClass {
    type AsRefTarget = pyo3::PyCell<Self>;
    const NAME: &'static str = "MyClass";
    const MODULE: ::std::option::Option<&'static str> = ::std::option::Option::None;
    #[inline]
    fn type_object_raw(py: pyo3::Python<'_>) -> *mut pyo3::ffi::PyTypeObject {
        <Self as pyo3::impl_::pyclass::PyClassImpl>::lazy_type_object()
            .get_or_init(py)
            .as_type_ptr()
    }
}

impl pyo3::PyClass for MyClass {
    type Frozen = pyo3::pyclass::boolean_struct::False;
}

impl<'a, 'py> pyo3::impl_::extract_argument::PyFunctionArgument<'a, 'py> for &'a MyClass
{
    type Holder = ::std::option::Option<pyo3::PyRef<'py, MyClass>>;

    #[inline]
    fn extract(obj: &'py pyo3::PyAny, holder: &'a mut Self::Holder) -> pyo3::PyResult<Self> {
        pyo3::impl_::extract_argument::extract_pyclass_ref(obj, holder)
    }
}

impl<'a, 'py> pyo3::impl_::extract_argument::PyFunctionArgument<'a, 'py> for &'a mut MyClass
{
    type Holder = ::std::option::Option<pyo3::PyRefMut<'py, MyClass>>;

    #[inline]
    fn extract(obj: &'py pyo3::PyAny, holder: &'a mut Self::Holder) -> pyo3::PyResult<Self> {
        pyo3::impl_::extract_argument::extract_pyclass_ref_mut(obj, holder)
    }
}

impl pyo3::IntoPy<PyObject> for MyClass {
    fn into_py(self, py: pyo3::Python<'_>) -> pyo3::PyObject {
        pyo3::IntoPy::into_py(pyo3::Py::new(py, self).unwrap(), py)
    }
}

impl pyo3::impl_::pyclass::PyClassImpl for MyClass {
    const IS_BASETYPE: bool = false;
    const IS_SUBCLASS: bool = false;
    type BaseType = PyAny;
    type ThreadChecker = pyo3::impl_::pyclass::SendablePyClass<MyClass>;
    type PyClassMutability = <<pyo3::PyAny as pyo3::impl_::pyclass::PyClassBaseType>::PyClassMutability as pyo3::impl_::pycell::PyClassMutability>::MutableChild;
    type Dict = pyo3::impl_::pyclass::PyClassDummySlot;
    type WeakRef = pyo3::impl_::pyclass::PyClassDummySlot;
    type BaseNativeType = pyo3::PyAny;

    fn items_iter() -> pyo3::impl_::pyclass::PyClassItemsIter {
        use pyo3::impl_::pyclass::*;
        let collector = PyClassImplCollector::<MyClass>::new();
        static INTRINSIC_ITEMS: PyClassItems = PyClassItems { slots: &[], methods: &[] };
        PyClassItemsIter::new(&INTRINSIC_ITEMS, collector.py_methods())
    }

    fn lazy_type_object() -> &'static pyo3::impl_::pyclass::LazyTypeObject<MyClass> {
        use pyo3::impl_::pyclass::LazyTypeObject;
        static TYPE_OBJECT: LazyTypeObject<MyClass> = LazyTypeObject::new();
        &TYPE_OBJECT
    }

    fn doc(py: Python<'_>) -> pyo3::PyResult<&'static ::std::ffi::CStr> {
        use pyo3::impl_::pyclass::*;
        static DOC: pyo3::once_cell::GILOnceCell<::std::borrow::Cow<'static, ::std::ffi::CStr>> = pyo3::once_cell::GILOnceCell::new();
        DOC.get_or_try_init(py, || {
            let collector = PyClassImplCollector::<Self>::new();
            build_pyclass_doc(<MyClass as pyo3::PyTypeInfo>::NAME, "", None.or_else(|| collector.new_text_signature()))
        }).map(::std::ops::Deref::deref)
    }
}

# Python::with_gil(|py| {
#     let cls = py.get_type::<MyClass>();
#     pyo3::py_run!(py, cls, "assert cls.__name__ == 'MyClass'")
# });
# }
```


[`GILGuard`]: {{#PYO3_DOCS_URL}}/pyo3/struct.GILGuard.html
[`PyTypeInfo`]: {{#PYO3_DOCS_URL}}/pyo3/type_object/trait.PyTypeInfo.html

[`Py`]: {{#PYO3_DOCS_URL}}/pyo3/struct.Py.html
[`PyCell`]: {{#PYO3_DOCS_URL}}/pyo3/pycell/struct.PyCell.html
[`PyClass`]: {{#PYO3_DOCS_URL}}/pyo3/pyclass/trait.PyClass.html
[`PyRef`]: {{#PYO3_DOCS_URL}}/pyo3/pycell/struct.PyRef.html
[`PyRefMut`]: {{#PYO3_DOCS_URL}}/pyo3/pycell/struct.PyRefMut.html
[`PyClassInitializer<T>`]: {{#PYO3_DOCS_URL}}/pyo3/pyclass_init/struct.PyClassInitializer.html

[`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
[`RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html

[classattr]: https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables

[`multiple-pymethods`]: features.md#multiple-pymethods
