TypeError: Object of type set is not JSON serializable

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Borislav Hadzhiev

Last updated: Apr 20, 2022

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TypeError: Object of type set is not JSON serializable #

The Python "TypeError: Object of type set is not JSON serializable" occurs when we try to convert a set object to a JSON string. To solve the error, convert the set to a list before serializing it to JSON, e.g. json.dumps(list(my_set)).

Here is an example of how the error occurs.

main.py
import json my_set = {'a', 'b', 'c', 'd'} # ⛔️ TypeError: Object of type set is not JSON serializable json_str = json.dumps(my_set)

We tried passing a set object to the json.dumps() method but the method doesn't handle set objects by default

To solve the error, use the built-in list() class to convert he set to a list before serializing it.

main.py
import json my_set = {'a', 'b', 'c', 'd'} json_str = json.dumps(list(my_set)) print(json_str) # '["b", "c", "a", "d"]' print(type(json_str)) # <class 'str'>

The default JSON encoder handles list values, so we can use a native Python list instead of a set when serializing to JSON.

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

Alternatively, you can extend from the JSONEncoder class and handle the conversions in a default method.

main.py
import json class SetEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, set): return list(obj) return json.JSONEncoder.default(self, obj) my_set = {'a', 'b', 'c', 'd'} json_str = json.dumps(my_set, cls=SetEncoder) print(json_str) # 👉️ '["b", "c", "a", "d"]' print(type(json_str)) # 👉️ <class 'str'>

We extended from the JSONEncoder class.

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

PythonJSON
dictobject
list, tuplearray
strstring
int, float, int and float derived Enumsnumber
Truetrue
Falsefalse
Nonenull

Notice that the JSONEncoder class doesn't support set to JSON conversion by default.

We can handle this by extending from the class and implementing a default() method that returns a serializable object.

main.py
import json class SetEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, set): return list(obj) return json.JSONEncoder.default(self, obj)

If the passed in value is a set, we convert it to a list and return the result.

The isinstance function returns True if the passed in object is an instance or a subclass of the passed in class.

In all other cases, we let the base classes' default method do the serialization.

To use a custom JSONEncoder, specify it with the cls keyword argument in your call to the json.dumps() method.

main.py
import json class SetEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, set): return list(obj) return json.JSONEncoder.default(self, obj) my_set = {'a', 'b', 'c', 'd'} # ✅ pass cls keyword argument json_str = json.dumps(my_set, cls=SetEncoder) print(json_str) # 👉️ '["b", "c", "a", "d"]' print(type(json_str)) # 👉️ <class 'str'>

If you don't provide the cls kwarg, the default JSONEncoder is used.

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