TypeError: Object of type map is not JSON serializable

TypeError: Object of type map is not JSON serializable #

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

Here is an example of how the error occurs.

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import json

my_list = [1, 2, 3, 4]


result = map(lambda x: x + 1, my_list)

# ⛔️ TypeError: Object of type map is not JSON serializable
json_str = json.dumps(result)

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

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

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import json

my_list = [1, 2, 3, 4]


result = map(lambda x: x + 1, my_list)

# ✅ convert to list
json_str = json.dumps(list(result))

print(json_str)  # 👉️ '[2, 3, 4, 5]'
print(type(json_str))  # 👉️ <class 'str'>

The default JSON encoder handles list values, so we can use a native Python list instead of a map 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.

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import json


class MapEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, map):
            return list(obj)
        return json.JSONEncoder.default(self, obj)


my_list = [1, 2, 3, 4]


result = map(lambda x: x + 1, my_list)


json_str = json.dumps(result, cls=MapEncoder)


print(json_str)  # 👉️ [2, 3, 4, 5]
print(type(json_str))  # 👉️ <class 'str'>

We extended from the JSONEncoder class.

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

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

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

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import json


class MapEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, map):
            return list(obj)
        return json.JSONEncoder.default(self, obj)

If the passed in value is a map, 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.

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

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import json


class MapEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, map):
            return list(obj)
        return json.JSONEncoder.default(self, obj)


my_list = [1, 2, 3, 4]


result = map(lambda x: x + 1, my_list)

# ✅ pass cls keyword argument
json_str = json.dumps(result, cls=MapEncoder)


print(json_str)  # 👉️ [2, 3, 4, 5]
print(type(json_str))  # 👉️ <class 'str'>

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

Conclusion #

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