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70 lines
1.6 KiB
Markdown
70 lines
1.6 KiB
Markdown
---
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title: "Python Dataclasses: Derived Fields and Validation"
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date: 2024-01-15T11:02:21-05:00
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draft: false
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tags:
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- Python
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math: false
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medium_enabled: false
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---
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Python dataclasses provide a simplified way of creating simple classes that hold data.
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```python
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from dataclasses import dataclass
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@dataclass
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class Person:
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name: str
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birth_year: int
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```
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The above code is equivalent to:
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```python
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class A:
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def __init__(name: str, birth_year: int):
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self.name = name
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self.birth_year = birth_year
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self.__post__init__()
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```
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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.
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## Use Case 1: Derived Fields
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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.
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For example, to compute a new field `age` from a person's `birth_year`:
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```python
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class Person:
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name: str
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birth_year: int
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age: int = field(init=False)
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def __post_init__(self):
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# Assuming the current year is 2024 and their birthday already passed
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self.age = 2024 - self.birth_year
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```
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## Use Case 2: Validation
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Another use case is to make sure
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that the user instantiates the fields
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of a data class in a way we expect.
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```python
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class Person:
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name: str
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birth_year: int
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def __post__init__(self):
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assert self.birth_year > 0
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assert isinstance(self.name, str)
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```
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Nothing is stopping us from combining both of these use cases within the `__post_init__` method!
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