# Basic object customization

Recall the `Number` class from the previous chapter:

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

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    #[new]
    fn new(value: i32) -> Self {
        Self(value)
    }
}

#[pymodule]
fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<Number>()?;
    Ok(())
}
```

At this point Python code can import the module, access the class and create class instances - but
nothing else.

```python
from my_module import Number

n = Number(5)
print(n)
```

```text
<builtins.Number object at 0x000002B4D185D7D0>
```

### String representations

It can't even print an user-readable representation of itself! We can fix that by defining the
`__repr__` and `__str__` methods inside a `#[pymethods]` block. We do this by accessing the value
contained inside `Number`.

```rust
# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    // For `__repr__` we want to return a string that Python code could use to recreate
    // the `Number`, like `Number(5)` for example.
    fn __repr__(&self) -> String {
        // We use the `format!` macro to create a string. Its first argument is a
        // format string, followed by any number of parameters which replace the
        // `{}`'s in the format string.
        //
        //                       👇 Tuple field access in Rust uses a dot
        format!("Number({})", self.0)
    }
    // `__str__` is generally used to create an "informal" representation, so we
    // just forward to `i32`'s `ToString` trait implementation to print a bare number.
    fn __str__(&self) -> String {
        self.0.to_string()
    }
}
```

#### Accessing the class name

In the `__repr__`, we used a hard-coded class name. This is sometimes not ideal,
because if the class is subclassed in Python, we would like the repr to reflect
the subclass name. This is typically done in Python code by accessing 
`self.__class__.__name__`. In order to be able to access the Python type information
*and* the Rust struct, we need to use a `PyCell` as the `self` argument.

```rust
# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __repr__(slf: &PyCell<Self>) -> PyResult<String> {
        // This is the equivalent of `self.__class__.__name__` in Python.
        let class_name: &str = slf.get_type().name()?;
        // To access fields of the Rust struct, we need to borrow the `PyCell`.
        Ok(format!("{}({})", class_name, slf.borrow().0))
    }
}
```

### Hashing


Let's also implement hashing. We'll just hash the `i32`. For that we need a [`Hasher`]. The one
provided by `std` is [`DefaultHasher`], which uses the [SipHash] algorithm.

```rust
use std::collections::hash_map::DefaultHasher;

// Required to call the `.hash` and `.finish` methods, which are defined on traits.
use std::hash::{Hash, Hasher};

# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __hash__(&self) -> u64 {
        let mut hasher = DefaultHasher::new();
        self.0.hash(&mut hasher);
        hasher.finish()
    }
}
```

> **Note**: When implementing `__hash__` and comparisons, it is important that the following property holds:
>
> ```text
> k1 == k2 -> hash(k1) == hash(k2)
> ```
>
> In other words, if two keys are equal, their hashes must also be equal. In addition you must take
> care that your classes' hash doesn't change during its lifetime. In this tutorial we do that by not
> letting Python code change our `Number` class. In other words, it is immutable.
>
> 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:
>
> ```rust
> # use pyo3::prelude::*;
> #[pyclass]
> struct NotHashable {}
>
> #[pymethods]
> impl NotHashable {
>     #[classattr]
>     const __hash__: Option<Py<PyAny>> = None;
> }
> ```

### Comparisons

Unlike in Python, PyO3 does not provide the magic comparison methods you might expect like `__eq__`,
 `__lt__` and so on. Instead you have to implement all six operations at once with `__richcmp__`.
This method will be called with a value of `CompareOp` depending on the operation.

```rust
use pyo3::class::basic::CompareOp;

# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp) -> PyResult<bool> {
        match op {
            CompareOp::Lt => Ok(self.0 < other.0),
            CompareOp::Le => Ok(self.0 <= other.0),
            CompareOp::Eq => Ok(self.0 == other.0),
            CompareOp::Ne => Ok(self.0 != other.0),
            CompareOp::Gt => Ok(self.0 > other.0),
            CompareOp::Ge => Ok(self.0 >= other.0),
        }
    }
}
```

If you obtain the result by comparing two Rust values, as in this example, you
can take a shortcut using `CompareOp::matches`:

```rust
use pyo3::class::basic::CompareOp;

# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp) -> bool {
        op.matches(self.0.cmp(&other.0))
    }
}
```

It checks that the `std::cmp::Ordering` obtained from Rust's `Ord` matches
the given `CompareOp`.

Alternatively, if you want to leave some operations unimplemented, you can
return `py.NotImplemented()` for some of the operations:


```rust
use pyo3::class::basic::CompareOp;

# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp, py: Python<'_>) -> PyObject {
        match op {
            CompareOp::Eq => (self.0 == other.0).into_py(py),
            CompareOp::Ne => (self.0 != other.0).into_py(py),
            _ => py.NotImplemented(),
        }
    }
}
```

### Truthyness

We'll consider `Number` to be `True` if it is nonzero:

```rust
# use pyo3::prelude::*;
#
# #[pyclass]
# struct Number(i32);
#
#[pymethods]
impl Number {
    fn __bool__(&self) -> bool {
        self.0 != 0
    }
}
```

### Final code

```rust
use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};

use pyo3::prelude::*;
use pyo3::class::basic::CompareOp;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    #[new]
    fn new(value: i32) -> Self {
        Self(value)
    }

    fn __repr__(slf: &PyCell<Self>) -> PyResult<String> {
        let class_name: &str = slf.get_type().name()?;
        Ok(format!("{}({})", class_name, slf.borrow().0))
    }

    fn __str__(&self) -> String {
        self.0.to_string()
    }

    fn __hash__(&self) -> u64 {
        let mut hasher = DefaultHasher::new();
        self.0.hash(&mut hasher);
        hasher.finish()
    }

    fn __richcmp__(&self, other: &Self, op: CompareOp) -> PyResult<bool> {
        match op {
            CompareOp::Lt => Ok(self.0 < other.0),
            CompareOp::Le => Ok(self.0 <= other.0),
            CompareOp::Eq => Ok(self.0 == other.0),
            CompareOp::Ne => Ok(self.0 != other.0),
            CompareOp::Gt => Ok(self.0 > other.0),
            CompareOp::Ge => Ok(self.0 >= other.0),
        }
    }

    fn __bool__(&self) -> bool {
        self.0 != 0
    }
}

#[pymodule]
fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<Number>()?;
    Ok(())
}
```

[`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
[`Hasher`]: https://doc.rust-lang.org/std/hash/trait.Hasher.html
[`DefaultHasher`]: https://doc.rust-lang.org/std/collections/hash_map/struct.DefaultHasher.html
[SipHash]: https://en.wikipedia.org/wiki/SipHash
