{'key' : '...$(expression)$...'}

from HyperDict import HyperDict
from json import dumps
dictionary = HyperDict({}) # {} - your dict

example = Hyperdict(
    {
        'a': 5,
        'b': '$(a)$'
    }
)

print(dumps(example))

    {
       'a': 5,
       'b': 5
    }

example = Hyperdict(
    {
        'a': 5,
        'b': '$(a + x)$ ' # here with a space
    }
)

print(dumps(example))

    {
       'a': 5,
       'b': '5 '
    }

example = Hyperdict(
    {
        'a': 5,
        'b': 100,
        'c': '$(b + d)$',
        'd': '$(b + a)$'
    }
)

print(dumps(example))

    {
        'a': 5,
        'b': 100,
        'c': 205,
        'd': 105
    }

example = Hyperdict(
    {
        'a': ['one_item'],
        'b': '$(a + ["another_item"])$'
    }
)

print(dumps(example))

    {
        'a': ['one_item'],
        'b': ['one_item', 'another_item']
    }

some_func = lambda arg: arg + 1

example = Hyperdict(
    {
        'a': 5,
        'b': '$(some_func(a))$'
    }
)

print(dumps(example))

    {
       'a': 5,
       'b': 5
    }

example = Hyperdict(
    {
        'a': 5,
        'b': '$(a)$'
    }
)
print(example) 
# >>> {'a': 5, 'b': 5}
example['a'] = 'another_val'
print(example)
# >>> {'a': 'another_val', 'b': 'another_val'}
example['a'] = '4'
print(example) 
# >>> {'a': 'another_val', 'b': 4}

# !!! 'b' links 'a', but 'a' doesn't link 'b'

# Keys of your dict must be only strings
{5: 'abc', True: []} # is prohibited
{'5': 'abc', 'True': []} # is allowed