Convert python dictionaries with Decimal keys to JSON using custom encoder

Question

JSON only allows strings as keys.

The code below uses a custom JSONEncoder to turn Decimal values into strings.

Is there a way to specify an encoder that will turn Decimal keys into strings?

import json
import decimal

class DecimalEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, decimal.Decimal):
            return str(obj)
        return json.JSONEncoder.default(self, obj)

d1 = {3: decimal.Decimal(50)}
print(json.dumps(d1, cls=DecimalEncoder))

d2 = {decimal.Decimal(50): 3}
json.dumps(d2, cls=DecimalEncoder)  # TypeError: keys must be a string

I am using python3.6.

Note: Obviously I could iterate through my dictionary and replace the Decimal types with string values, but I am hoping to find a more elegant solution perhaps by adding behaviour to the encoder.

Martijn Pieters · Accepted Answer · 2017-10-12 07:11:42Z

1

No, you can't hook the handling of keys, you'll have to convert them before you encode. You can do so with a recursive handler like:

from functools import singledispatch

@singledispatch
def string_keys(obj):
    return obj

@string_keys.register(dict)
def _(d):
    return {str(k): string_keys(v) for k, v in d.items()}

@string_keys.register(list)
def _(l):
    return [string_keys(v) for v in l]

All this does is convert a nested structure of lists and dicts recursively where all keys are forced to be strings.

Use this when converting to JSON:

json_encoded = json.dumps(string_keys(data))

You can extend this to handle Decimal objects (outside of keys) too by adding another registry:

@string_keys.register(Decimal)
def _(d):
    return str(d)

Going the other way is a little tricky, unless you explicitly mark up Decimal keys (with a prefix, say), you can't easily distinguish between keys that were strings to start with and Decimal values. You could use a try/except approach here:

from functools import singledispatch

@singledispatch
def keys_to_decimal(obj):
    return obj

@keys_to_decimal.register(dict)
def _(d):
    def try_decimal(k):
        try:
            return Decimal(k)
        except ValueError:
            return k
    return {try_decimal(k): keys_to_decimal(v) for k, v in d.items()}

@keys_to_decimal.register(list)
def _(l):
    return [keys_to_decimal(v) for v in l]

Demo:

>>> string_keys([{Decimal(0): 'foo'}])
[{'0': 'foo'}]
>>> keys_to_decimal(string_keys([{Decimal(0): 'foo'}]))
[{Decimal('0'): 'foo'}]

edited Oct 12, 2017 at 7:11

answered Oct 12, 2017 at 6:47

Martijn Pieters

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2 Comments

ChrisGuest Over a year ago

I am getting this error NameError: name 'k' is not defined in the line return {str(k): string_keys(v) for v in d} .

Martijn Pieters Over a year ago

@ChrisGuest: and another mistake in the keys_to_decimal handling; now actually tested. :-)

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