TypeError: Object of type float32 is not JSON serializable

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Borislav Hadzhiev

Last updated: Apr 20, 2022

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TypeError: Object of type float32 is not JSON serializable #

The Python "TypeError: Object of type float32 is not JSON serializable" occurs when we try to convert a numpy float32 object to a JSON string. To solve the error, convert the numpy float to a Python float before converting it to JSON, e.g. float(my_numpy_float).

Here is an example of how the error occurs.

main.py
import json import numpy as np salary = np.power(100, 2.75, dtype=np.float32) # ⛔️ TypeError: Object of type float32 is not JSON serializable json_str = json.dumps({'salary': salary})

We tried passing a numpy float32 object to the json.dumps() method but the method doesn't handle numpy floats by default.

To solve the error, use the built-in float() (or str()) constructor to convert the numpy float to a native Python float before serializing it.

main.py
import json import numpy as np salary = np.power(100, 2.75, dtype=np.float32) print(salary) # 👉️ 316227.78 # ✅ convert to float json_str = json.dumps({'salary': float(salary)}) print(json_str) # 👉️ '{"salary": 316227.78125}' print(type(json_str)) # 👉️ <class 'str'>

We used the built-in float() constructor to convert the numpy float32 value to a native Python float.

Note that if you are worried about losing precision, you can use the str() constructor to convert the value to a string instead.

The default JSON encoder handles float and str values, so we can use a native Python float instead of a numpy float32 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 import numpy as np class NpEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, np.integer): return int(obj) if isinstance(obj, np.floating): # 👇️ alternatively use str() return float(obj) if isinstance(obj, np.ndarray): return obj.tolist() return json.JSONEncoder.default(self, obj) salary = np.power(100, 2.75, dtype=np.float32) json_str = json.dumps({'salary': salary}, cls=NpEncoder) print(json_str) # 👉️ {"salary": 316227} 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 numpy float32 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 import numpy as np class NpEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, np.integer): return int(obj) if isinstance(obj, np.floating): return float(obj) if isinstance(obj, np.ndarray): return obj.tolist() return json.JSONEncoder.default(self, obj)

If the passed in object is an instance of np.integer, we convert the object to a Python int and return the result.

If the passed in object is an instance of np.floating, we convert it to a Python float and return the result.

Note that you can use the str() constructor instead of float() if you worry about losing precision.

If the object is an instance of np.ndarray, we convert it to a Python list and return the result.

In all other cases, we let the base class's 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 import numpy as np class NpEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, np.integer): return int(obj) if isinstance(obj, np.floating): return float(obj) if isinstance(obj, np.ndarray): return obj.tolist() return json.JSONEncoder.default(self, obj) salary = np.power(100, 2.75, dtype=np.float32) # ✅ pass cls kwarg json_str = json.dumps({'salary': salary}, cls=NpEncoder) print(json_str) # 👉️ {"salary": 316227} 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 float32 is not JSON serializable" occurs when we try to convert a numpy float32 object to a JSON string. To solve the error, convert the numpy float to a Python float before converting it to JSON, e.g. float(my_numpy_float).

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