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content/blog/python-dataclasses-derived-fields-validation.md

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+---
+title: "Python Dataclasses: Derived Fields and Validation"
+date: 2024-01-15T11:02:21-05:00
+draft: false
+tags: []
+math: false
+medium_enabled: false
+---
+
+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!
+