Python dataclasses provide a simplified way of creating simple classes that hold data.
```python
from dataclasses import dataclass
@dataclass
class Person:
name: str
birth_year: int
```
The above code is equivalent to:
```python
class A:
def __init__(name: str, birth_year: int):
self.name = name
self.birth_year = birth_year
self.__post__init__()
```
Notice the call to `__post__init__` at the end. We can override that method to do whatever we'd like. I have found two great use cases for this.
## Use Case 1: Derived Fields
Straight from the [Python documentation](https://docs.python.org/3/library/dataclasses.html#dataclasses.__post_init__), this use case is for when we want to use some variables to create a new variable.
For example, to compute a new field `age` from a person's `birth_year`:
```python
class Person:
name: str
birth_year: int
age: int = field(init=False)
def __post_init__(self):
# Assuming the current year is 2024 and their birthday already passed
self.age = 2024 - self.birth_year
```
## Use Case 2: Validation
Another use case is to make sure
that the user instantiates the fields
of a data class in a way we expect.
```python
class Person:
name: str
birth_year: int
def __post__init__(self):
assert self.birth_year > 0
assert isinstance(self.name, str)
```
Nothing is stopping us from combining both of these use cases within the `__post_init__` method!
