A Python data class is a class that has the @dataclass decorator and is specifically created to hold data. Since Python version 3.7, Python offers data classes through a built-in module called dataclass. There are several advantages over regular classes which we’ll explore in this article. We’ll also look at example code and a couple of common operations you might want to perform with data classes.
Advantages to using data classes
Why should you use a data class instead of a regular class? Let’s look at some of the advantages a Python data class has to offer.
Requires a minimal amount of code
The @dataclass decorator adds a lot of functionality to a class without adding any visible code. This allows your data class to be very compact while still offering many useful features. All you need to do is define the fields to hold your data. You don’t need to define any function.
Comparison
Two Python data classes can be compared with == because the so-called dunder method __eq__ is implemented automatically. Any Python object that implements this special method, can be compared to similar objects.
Printing a data class
Similarly, because __repr__ is implemented, you can print data classes and get a nice representation of it. This is especially useful for debugging.
Data classes require type hints
Data classes are build around the new(ish) type system Python offers. Using type hints reduces the chances of bugs and unexpected behavior in your code.
Python data class example
Here’s an example of a data class at work:
from dataclasses import dataclass
@dataclass
class Card:
rank: str
suit: str
card1 = Card("Q", "hearts")
card2 = Card("Q", "hearts")
print(card1 == card2)
# True
print(card1.rank)
# 'Q'
print(card1)
Card(rank='Q', suit='hearts')Default values
A Python data class can have default values. Assigning default values is as simple as assigning a value to a variable. For example, to make our Card class have a default value of Queen of hearts, we can do as follows:
from dataclasses import dataclass
@dataclass
class Card:
rank: str = 'Q'
suit: str = 'hearts'Converting a data class to JSON
The bad news: there’s no built-in way to convert a data class to JSON. Luckily, there is a Python package called dataclasses-json that simplifies the task. However, it requires an extra decorator. Also, the package doesn’t seem to be actively maintained. Something to keep in mind.
Here’s an example of how the package can be used:
from dataclasses import dataclass
from dataclasses_json import dataclass_json
@dataclass_json
@dataclass
class Card:
rank: str = 'Q'
suit: str = 'hearts'
card = Card()
print(card.to_json()){"rank": "Q", "suit": "hearts"}You’ll need to install the package with Pip or Pipenv, preferably inside a virtual environment.
Another method is to use Python inheritance and inherit from the JSONEncoder class to create your own custom encoder. The advantage here is that you don’t need to install an external package. How to do this is explained in a blog post.
Keep learning
- The official documentation on Python.org
- How to return multiple values in Python
