JSON serialize a dictionary with tuples as key

This solution:

• Avoids the security risk of eval().
• Is short.
• Is copy-pastable as save and load functions.
• Keeps the structure of tuple as the key, in case you are editing the JSON by hand.
• Adds ugly \" to the tuple representation, which is worse than the other str()/eval() methods here.
• Can only handle tuples as keys at the first level for nested dicts (as of this writing no other solution here can do better)

def json_dumps_tuple_keys(mapping):
    string_keys = {json.dumps(k): v for k, v in mapping.items()}
    return json.dumps(string_keys)

def json_loads_tuple_keys(string):
    mapping = json.loads(string)
    return {tuple(json.loads(k)): v for k, v in mapping.items()}

m = {(0,"a"): "first", (1, "b"): [9, 8, 7]}
print(m)      # {(0, 'a'): 'first', (1, 'b'): [9, 8, 7]}
s = json_dumps_tuple_keys(m)
print(s)      # {"[0, \"a\"]": "first", "[1, \"b\"]": [9, 8, 7]}
m2 = json_loads_tuple_keys(s)
print(m2)     # {(0, 'a'): 'first', (1, 'b'): [9, 8, 7]}
print(m==m2)  # True

You can actually not serialize tuples as key to json, but you can convert the tuple to a string and recover it, after you have deserialized the file.

with_tuple = {(0.1, 0.1): 3.14} ## this will work in python but is not serializable in json
{(0.1, 0.1): 3.14}

But you cannot serialize it with json. However, you can use

with_string = {str((0.1, 0.1))[1:-1]: 3.14} ## the expression [1,-1] removes the parenthesis surrounding the tuples in python. 

{'0.1, 0.1': 3.14} # This is serializable

With a bit of cheating, you will recover the original tuple (after having deserialized the whole file) by treating each key (as str) separately

tuple(json.loads("["+'0.1, 0.1'+"]")) ## will recover the tuple from string
(0.1, 0.1)

It is a bit of overload to convert a string to a tuple using json.loads, but it will work. Encapsulate it and you are done.

Peace out and happy coding!

Nicolas

Here's a complete example to encode/decode nested dictionaries with tuple keys and values into/from json. tuple key will be a string in JSON.

values of types tuple or set will be converted to list

def JSdecoded(item:dict, dict_key=False):
    if isinstance(item, list):
        return [ JSdecoded(e) for e in item ]
    elif isinstance(item, dict):
        return { literal_eval(key) : value for key, value in item.items() }
    return item

def JSencoded(item, dict_key=False):
    if isinstance(item, tuple):
        if dict_key:
            return str(item)
        else:
            return list(item)
    elif isinstance(item, list):
        return [JSencoded(e) for e in item]
    elif isinstance(item, dict):
        return { JSencoded(key, True) : JSencoded(value) for key, value in item.items() }
    elif isinstance(item, set):
        return list(item)
    return item

usage

import json
pydata = [
    { ('Apple','Green') : "Tree",
      ('Orange','Yellow'):"Orchard",
      ('John Doe', 1945) : "New York" }
    ]
jsstr= json.dumps(JSencoded(pydata), indent='\t')
print(jsstr)
#[
#   {
#       "('Apple', 'Green')": "Tree",
#       "('Orange', 'Yellow')": "Orchard",
#       "('John Doe', 1945)": "New York"
#   }
#]
data = json.loads(jsstr) #string keys
newdata = JSdecoded(data) #tuple keys
print(newdata)
#[{('Apple', 'Green'): 'Tree', ('Orange', 'Yellow'): 'Orchard', ('John Doe', 1945): 'New York'}]

Here are two functions you could use to convert a dict_having_tuple_as_key into a json_array_having_key_and_value_as_keys and then de-convert it the way back

import json

def json_dumps_dict_having_tuple_as_key(dict_having_tuple_as_key):
    if not isinstance(dict_having_tuple_as_key, dict):
        raise Exception('Error using json_dumps_dict_having_tuple_as_key: The input variable is not a dictionary.')  
    list_of_dicts_having_key_and_value_as_keys = [{'key': k, 'value': v} for k, v in dict_having_tuple_as_key.items()]
    json_array_having_key_and_value_as_keys = json.dumps(list_of_dicts_having_key_and_value_as_keys)
    return json_array_having_key_and_value_as_keys

def json_loads_dictionary_split_into_key_and_value_as_keys_and_underwent_json_dumps(json_array_having_key_and_value_as_keys):
    list_of_dicts_having_key_and_value_as_keys = json.loads(json_array_having_key_and_value_as_keys)
    if not all(['key' in diz for diz in list_of_dicts_having_key_and_value_as_keys]) and all(['value' in diz for diz in list_of_dicts_having_key_and_value_as_keys]):
        raise Exception('Error using json_loads_dictionary_split_into_key_and_value_as_keys_and_underwent_json_dumps: at least one dictionary in list_of_dicts_having_key_and_value_as_keys ismissing key "key" or key "value".')
    dict_having_tuple_as_key = {}
    for dict_having_key_and_value_as_keys in list_of_dicts_having_key_and_value_as_keys:
        dict_having_tuple_as_key[ tuple(dict_having_key_and_value_as_keys['key']) ] = dict_having_key_and_value_as_keys['value']
    return dict_having_tuple_as_key

usage example:

my_dict = {
    ('1', '1001', '2021-12-21', '1', '484'): {"name": "Carl", "surname": "Black", "score": 0},
    ('1', '1001', '2021-12-22', '1', '485'): {"name": "Joe", "id_number": 134, "percentage": 11}
}

my_json = json_dumps_dict_having_tuple_as_key(my_dict)
print(my_json)

[{'key': ['1', '1001', '2021-12-21', '1', '484'], 'value': {'name': 'Carl', 'surname': 'Black', 'score': 0}}, 
 {'key': ['1', '1001', '2021-12-22', '1', '485'],  'value': {'name': 'Joe', 'id_number': 134, 'percentage': 11}}]

my_dict_reconverted = json_loads_dictionary_split_into_key_and_value_as_keys_and_underwent_json_dumps(my_json)
print(my_dict_reconverted)

{('1', '1001', '2021-12-21', '1', '484'): {'name': 'Carl', 'surname': 'Black', 'score': 0}, 
 ('1', '1001', '2021-12-22', '1', '485'): {'name': 'Joe', 'id_number': 134, 'percentage': 11}}

# proof of working 1

my_dict == my_dict_reconverted

True

# proof of working 2

my_dict == json_loads_dictionary_split_into_key_and_value_as_keys_and_underwent_json_dumps(
json_dumps_dict_having_tuple_as_key(my_dict)
)

True

(Using concepts expressed by @SingleNegationElimination to answer @Kvothe comment)