How to convert Dataclass to JSON in Python [5 Ways]

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# How to convert Dataclass to JSON in Python

To convert a dataclass to JSON in Python:

Use the dataclasses.asdict() method to convert the dataclass to a dictionary.
Pass the dictionary to the json.dumps() method.

main.py

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import json
from dataclasses import dataclass, asdict


@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)

to_json = json.dumps(asdict(item1))
print(to_json)

print('-' * 50)

print(type(to_json))

The code for this article is available on GitHub

Running the code sample produces the following output.

shell

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InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}
--------------------------------------------------
<class 'str'>

convert dataclass to json in python

The dataclasses.asdict() method converts the supplied dataclass instance to a dictionary.

main.py

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from dataclasses import dataclass, asdict


@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)

print(asdict(item1))

Running the code sample produces the following output.

shell

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InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{'name': 'A', 'unit_price': 100, 'quantity_available': 1}

convert dataclass to dictionary

The dictionary contains the name: value pairs of the dataclass object.

The last step is to pass the dictionary to the json.dumps() method.

main.py

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import json
from dataclasses import dataclass, asdict


@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)

print(json.dumps(asdict(item1)))

The code for this article is available on GitHub

Running the code sample produces the following output.

shell

Copied!
InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}

The json.dumps() method converts a Python object to a JSON formatted string.

The json.dumps() method handles the conversion of a dictionary to a JSON string without any issues.

# Converting a Dataclass to JSON with a custom `JSONEncoder`

You can also extend the built-in JSONEncoder class to convert a dataclass object to a JSON string.

main.py

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import json
from dataclasses import dataclass, asdict, is_dataclass


@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)


class DCJSONEncoder(json.JSONEncoder):
    def default(self, o):
        if is_dataclass(o):
            return asdict(o)
        return super().default(o)


json_str = json.dumps(item1, cls=DCJSONEncoder)
print(json_str)

The code for this article is available on GitHub

Running the code sample produces the following output.

shell

Copied!
InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}

We extended the default JSONEncoder class to be able to handle the conversion of dataclass objects to JSON.

The JSONEncoder class supports the following objects and types by default.

Python	JSON
dict	object
list, tuple	array
str	string
int, float, int and float derived Enums	number
True	true
False	false
None	null

By default, the JSONEncoder class doesn't support conversion of dataclass objects to JSON.

You can change the default behavior by implementing a default method.

main.py

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import json
from dataclasses import dataclass, asdict, is_dataclass

class DCJSONEncoder(json.JSONEncoder):
    def default(self, o):
        if is_dataclass(o):
            return asdict(o)
        return super().default(o)


json_str = json.dumps(item1, cls=DCJSONEncoder)
print(json_str)

The code for this article is available on GitHub

If the supplied object is a dataclass object, we return the result of calling asdict() with the object.

If the object is any other type, we use the default behavior.

Notice that we have to set the cls argument to the custom JSONEncoder class when calling json.dumps().

main.py

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json_str = json.dumps(item1, cls=DCJSONEncoder)

If you forget to pass the cls argument, the default JSONEncoder is used and you'd get an error.

# Convert a Dataclass to JSON with the `dataclasses_json` package

You can also use the dataclasses-json module to convert a dataclass object to JSON.

First, open your terminal in your project's root directory and install the package.

shell

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pip install dataclasses-json

# or with pip3
pip3 install dataclasses-json

install dataclasses json

Now import the module and use the dataclass_json method as a decorator.

main.py

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from dataclasses import dataclass
from dataclasses_json import dataclass_json


@dataclass_json
@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)

json_str = item1.to_json()
print(json_str)

The code for this article is available on GitHub

Running the code sample produces the following output.

shell

Copied!
InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}

convert dataclass to json with dataclasses json module

Notice that we used the dataclass_json method to decorate the class.

main.py

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from dataclasses import dataclass
from dataclasses_json import dataclass_json


@dataclass_json
@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0

The next step is to call the to_json() method on the dataclass object.

main.py

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json_str = item1.to_json()
print(json_str)

If you need to convert the JSON string back to a dataclass object, use the InventoryItem.from_json() method.

main.py

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from dataclasses import dataclass
from dataclasses_json import dataclass_json


@dataclass_json
@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)

json_str = item1.to_json()
print(json_str)

print('-' * 50)

dataclass_obj = InventoryItem.from_json(json_str)
print(dataclass_obj)

Running the code sample produces the following output.

shell

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InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}
--------------------------------------------------
InventoryItem(name='A', unit_price=100.0, quantity_available=1)

Notice that I called the from_json() method on the decorated class.

main.py

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dataclass_obj = InventoryItem.from_json(json_str)
print(dataclass_obj)

convert dataclass to json and back

# Converting a `dataclass` object to a JSON string with the `default` argument

You can also specify the default argument when calling json.dumps() to convert a dataclass object to a JSON string.

main.py

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import json
from dataclasses import dataclass


@dataclass
class InventoryItem:
    name: str
    unit_price: float
    quantity_available: int = 0


item1 = InventoryItem('A', 100, 1)
print(item1)

print('-' * 50)


json_str = json.dumps(item1, default=lambda dc: dc.__dict__)
print(json_str)

using default argument

The code for this article is available on GitHub

Running the code sample produces the following output.

shell

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InventoryItem(name='A', unit_price=100, quantity_available=1)
--------------------------------------------------
{"name": "A", "unit_price": 100, "quantity_available": 1}

The default argument should be set to a function that gets invoked for objects that cannot be otherwise deserialized.

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How to convert Dataclass to JSON in Python [5 Ways]

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