Data Classes in Python | An Introduction

dataclass module is introduced in Python 3.7 as a utility tool to make structured classes specially for storing data. These classes hold certain properties and functions to deal specifically with the data and its representation.
DataClasses in widely used Python3.6
Although the module was introduced in Python3.7, one can also use it in Python3.6 by installing dataclasses library. 

pip install dataclasses

The DataClasses are implemented by using decorators with classes. Attributes are declared using Type Hints in Python which is essentially, specifying data type for variables in python.

Python3

# A basic Data Class

# Importing dataclass module
from dataclasses import dataclass

@dataclass
class GfgArticle():
    """A class for holding an article content"""

    # Attributes Declaration
    # using Type Hints

    title: str
    author: str
    language: str
    upvotes: int

# A DataClass object
article = GfgArticle("DataClasses",
                     "vibhu4agarwal",
                     "Python", 0)
print(article)

Output:

GfgArticle(title=’DataClasses’, author=’vibhu4agarwal’, language=’Python’, upvotes=0) 

The two noticeable points in above code. 

• Without a __init__() constructor, the class accepted values and assigned it to appropriate variables.
• The output of printing object is a neat representation of the data present in it, without any explicit function coded to do this. That means it has a modified __repr__() function.

The dataclass provides an in built __init__() constructor to classes which handle the data and object creation for them. 

Python3

article = GfgArticle()

TypeError: __init__() missing 4 required positional arguments: ‘title’, ‘author’, ‘language’, and ‘upvotes’ 

We can also modify the functioning of in-built constructor by passing certain arguments or using special functions which will be discussed in further articles.
Equality of DataClasses
Since the classes store data, checking two objects if they have the same data is a very common task that’s needed with dataclasses. This is accomplished by using the == operator. 
Below is the code for an equivalent class for storing an article without a dataclass decorator. 

Python3

class NormalArticle():
    """A class for holding an article content"""

    # Equivalent Constructor
    def __init__(self, title, author, language, upvotes):
        self.title = title
        self.author = author
        self.language = language
        self.upvotes = upvotes

# Two DataClass objects
dClassArticle1 = GfgArticle("DataClasses",
                            "vibhu4agarwal",
                            "Python", 0)
dClassArticle2 = GfgArticle("DataClasses",
                            "vibhu4agarwal",
                            "Python", 0)

# Two objects of a normal class
article1 = NormalArticle("DataClasses",
                         "vibhu4agarwal",
                         "Python", 0)
article2 = NormalArticle("DataClasses",
                         "vibhu4agarwal",
                         "Python", 0)

Python3

print("DataClass Equal:", dClassArticle1 == dClassArticle2)
print("Normal Class Equal:", article1 == article2)

Output:

DataClass Equal: True
Normal Class Equal: False

Equality between two objects using == operator in python checks for the same memory location. Since two objects take different memory locations on creation, the output for equality is False. Equality between DataClass objects checks for the equality of data present in it. This accounts for True as output for equality check between two DataClass objects which contain same data.