Convert numpy type to python

Question

I have a list of dicts in the following form that I generate from pandas. I want to convert it to a json format.

list_val = [{1.0: 685}, {2.0: 8}]
output = json.dumps(list_val)

However, json.dumps throws an error: TypeError: 685 is not JSON serializable

I am guessing it's a type conversion issue from numpy to python(?).

However, when I convert the values v of each dict in the array using np.int32(v) it still throws the error.

EDIT: Here's the full code

            new = df[df[label] == label_new] 
            ks_dict = json.loads(content)
            ks_list = ks_dict['variables']
            freq_counts = []

            for ks_var in ks_list:

                    freq_var = dict()
                    freq_var["name"] = ks_var["name"]
                    ks_series = new[ks_var["name"]]
                    temp_df = ks_series.value_counts().to_dict()
                    freq_var["new"] = [{u: np.int32(v)} for (u, v) in temp_df.iteritems()]            
                    freq_counts.append(freq_var)

           out = json.dumps(freq_counts)

Answer 1

It looks like you're correct:

>>> import numpy
>>> import json
>>> json.dumps(numpy.int32(685))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/json/__init__.py", line 243, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: 685 is not JSON serializable

The unfortunate thing here is that numpy numbers' __repr__ doesn't give you any hint about what type they are. They're running around masquerading as ints when they aren't (gasp). Ultimately, it looks like json is telling you that an int isn't serializable, but really, it's telling you that this particular np.int32 (or whatever type you actually have) isn't serializable. (No real surprise there -- No np.int32 is serializable). This is also why the dict that you inevitably printed before passing it to json.dumps looks like it just has integers in it as well.

The easiest workaround here is probably to write your own serializer¹:

class MyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, numpy.integer):
            return int(obj)
        elif isinstance(obj, numpy.floating):
            return float(obj)
        elif isinstance(obj, numpy.ndarray):
            return obj.tolist()
        else:
            return super(MyEncoder, self).default(obj)

You use it like this:

json.dumps(numpy.float32(1.2), cls=MyEncoder)
json.dumps(numpy.arange(12), cls=MyEncoder)
json.dumps({'a': numpy.int32(42)}, cls=MyEncoder)

etc.

^{1Or you could just write the default function and pass that as the defaut keyword argument to json.dumps. In this scenario, you'd replace the last line with raise TypeError, but ... meh. The class is more extensible :-)}

Answer 2

You could also convert the array to a python list (use the tolist method) and then convert the list to json.

Answer 3

You can use our fork of ujson to deal with NumPy int64. caiyunapp/ultrajson: Ultra fast JSON decoder and encoder written in C with Python bindings and NumPy bindings

pip install nujson

Then

>>> import numpy as np
>>> import nujson as ujson
>>> a = {"a": np.int64(100)}
>>> ujson.dumps(a)
'{"a":100}'
>>> a["b"] = np.float64(10.9)
>>> ujson.dumps(a)
'{"a":100,"b":10.9}'
>>> a["c"] = np.str_("12")
>>> ujson.dumps(a)
'{"a":100,"b":10.9,"c":"12"}'
>>> a["d"] = np.array(list(range(10)))
>>> ujson.dumps(a)
'{"a":100,"b":10.9,"c":"12","d":[0,1,2,3,4,5,6,7,8,9]}'
>>> a["e"] = np.repeat(3.9, 4)
>>> ujson.dumps(a)
'{"a":100,"b":10.9,"c":"12","d":[0,1,2,3,4,5,6,7,8,9],"e":[3.9,3.9,3.9,3.9]}'

Answer 4

In a simpler case when you only have numpy numbers to be converted the easiest is:

json.dumps(a, default=float)

Answer 5

If you leave the data in any of the pandas objects, the library supplies a to_json function on Series, DataFrame, and all of the other higher dimension cousins.

See Series.to_json()

Answer 6

In some cases simple json.dump(eval(str(a)), your_file) helps.

Answer 7

If you have dict consists of multiple numpy objects like ndarray or a float32 object you can manually convert an ndarray to a list using .tolist()

import numpy as np
import json

a = np.empty([2, 2], dtype=np.float32)
json.dumps(a.tolist()) # this should work

or save a float32 object using .item().

import numpy as np
import json

a = np.float32(1)
json.dumps(a.item()) # this should work

But if you have a complex dict with multiple dicts nested in lists which are further nested with numpy objects, navigating your code and manually updating each variable become cumbersome and you might not want to do that. Instead you can define a NumpyEncoder class which handles this for you during the json.dumps() reference

class NumpyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, np.float32):
            return obj.item()
        if isinstance(obj, np.ndarray):
            return obj.tolist()
        return json.JSONEncoder.default(self, obj)

with open('output.json', 'w') as outfile: 
    json.dump(json_dict, outfile, sort_keys=True, indent=4, cls=NumpyEncoder) # indent and sort_keys are just for cleaner output

This worked perfectly for me, this even allows us to handle any other data types when saving to JSON example, formatting the decimal places when saving.

class NumpyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, float):
            return "{:.2f}".format(obj)
        return json.JSONEncoder.default(self, obj)