Python dataclass decorator

last modified July 6, 2020

Python dataclass tutorial shows how to use dataclass decorators in Python in custom classes. The dataclass decorator helps reduce some boilerplate code.

Python dataclass decorator

The dataclass decorator is used to automatically generate special methods to classes, including __str__() and __repr__(). It helps reduce some boilerplate code. The dataclass decorator is located in the dataclasses module.

The dataclass decorator examines the class to find fields. A field is defined as class variable that has a type annotation.

@dataclass
class Test:
...

@dataclass()
class Test:
...

@dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False)
class Test:
...

These three declarations are equivalent. If no parameters are set in the decorator, the default ones are used. If the init parameter is set to True, the __init__() method will be generated. If the class already defines the __init__(), the parameter is ignored. If the repr parameter is set to True, the __repr__() method will be generated. If the class already defines the __repr__(), the parameter is ignored. If the eq parameter is set to True, the __eq__() method will be generated. If the class already defines __eq__(), this parameter is ignored.

If the order parameter is set to True, the __lt__(), __le__(), __gt__(), and __ge__() methods are generated. If the class already defines any of the methods, the ValueError is raised. If the unsafe_hash is defined to False, the __hash__() method is generated according to how eq and frozen are set. If the frozen parameter is set to True, the assignment to fields will generate an exception.

Python regular custom class

In a regular custom Python class, we provide a constructor and other methods such as __repr__() manually.

regular_class.py

#!/usr/bin/env python3

class Person:

    def __init__(self, name, age):

        self.name = name
        self.age = age

    def __repr__(self):

        return f'Person{{name: {self.name}, age: {self.age}}}'


p = Person('John Doe', 34)
print(p)

The example shows a Person class with a constructor and the __repr__() method, which gives a complete representation of the object.

$ ./regular_class.py
Person{name: John Doe, age: 34}

This is the output.

Python dataclass example

The following example shows a simple usage of the dataclass decorator.

simple_dataclass.py

#!/usr/bin/env python3

from dataclasses import dataclass

@dataclass
class Person:
    name: str
    age: int

p = Person('John Doe', 34)
print(p)

We have a class with two fields: name and str.

from dataclasses import dataclass

The dataclass decorator is located in the dataclasses module.

@dataclass
class Person:
    name: str
    age: int

We apply the dataclass decorator on the Person class.

p = Person('John Doe', 34)
print(p)

A new person object is created. Its __init__() method is called, which is auto-generated by the dataclass decorator.

$ ./simple_dataclass.py
Person(name='John Doe', age=34)

This is the output.

Python dataclass default values

It is possible to provide default values to the fields.

default_values.py

#!/usr/bin/env python3

from dataclasses import dataclass

@dataclass
class Person:
    name: str = 'unknown'
    age: int = 0

p = Person('John Doe', 34)
print(p)

p2 = Person()
print(p2)

In the example, the Person class has two fields; the fields have some default values.

@dataclass
class Person:
    name: str = 'unknown'
    age: int = 0

With the assignment operator (=), we give the fields default values.

p2 = Person()
print(p2)

When we do not provide values in the constructor, the fields will have default values.

$ ./default_values.py
Person(name='John Doe', age=34)
Person(name='unknown', age=0)

This is the output.

The dataclass frozen parameter

If the frozen parameter is set to True, we cannot assign values to fields.

frozen.py

#!/usr/bin/env python3

from dataclasses import dataclass

@dataclass(frozen=True)
class Person:
    name: str
    age: int

p = Person('John Doe', 34)
p.occupation = 'gardener'

print(p)
print(p.occupation)

In the example, the frozen parameter is set to True. The program fails with the following error message: dataclasses.FrozenInstanceError: cannot assign to field 'occupation'.

The dataclass asdict function

The asdict() function converts a dataclass instance to a dict of its fields.

as_dict_fun.py

#!/usr/bin/env python3

from dataclasses import dataclass, asdict

@dataclass
class Person:
    name: str
    occupation: str
    age: int

p = Person('John Doe', 'gardener', 34)
print(p)

print(asdict(p))

In the example, we print the fields of the Person class with the help of the asdict() function.

$ ./as_dict_fun.py
Person(name='John Doe', occupation='gardener', age=34)
{'name': 'John Doe', 'occupation': 'gardener', 'age': 34}

The first line is the output of the __repr__() method. The second line is the dictionary of the fields.

The dataclass field function

With the field() function, we can provide some additional per-field information.

fields.py

#!/usr/bin/env python3

from dataclasses import dataclass, field

@dataclass
class Person:
    name: str
    age: int
    occupation: str = field(init=False, repr=False)

p = Person('John Doe', 34)
print(p)

p.occupation = "Gardener"
print(f'{p.name} is a {p.occupation}')

In the example, we have an additional occupation field.

occupation: str = field(init=False, repr=False)

The occupation field is not included in the __init__() and __repr__() methods.

$ ./fields.py
Person(name='John Doe', age=34)
John Doe is a Gardener

This is the output.

In this tutorial, we have worked with Python dataclass decorators.

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Python dataclass decorator