Type hint for nested dict

Question

The kind of data structure I parse in my Python script is a json file which, after json.load(file_handle) , is of type <class 'dict'> . So far so good. Now for a function using it as an input argument, I want a type hint for the parsed json. I read in the typing documentation , that for dict s as arguments, I should use Mapping[key_type, value_type] :

from typing import Mapping

def foo(json_data: Mapping[str, str]) -> None:
    ...

The json I parse has str -type keys and str -type values, but more often than not, its structure is highly recursive. Hence a value is more likely to be a dict with str keys and even such dict s as values. It is very nested, until, at the deepest level, the last dict finally has str keys and str values.

So how do I represent this data structure more precisely? I was thinking something, along the lines of this question , that it might be:

Union[Mapping[str, str], Mapping[str, Mapping]]

But it does seem to represent only one level of recursion. Is there a better way to type-hint this?

Answer 1

simple dicts

you can simply use Mapping[K, V] as the value for another Mapping . let's say your json looks like this:

{
   "person_1": {"money": 1000},
   "person_2": {"money": 1000}
}

in this case you can use a type hint that looks like this:

Mapping[str, Mapping[str, int]]

harder dicts

but let's say you have something more complex, like this:

{
    "person_1": {
        "money": 1000,
        "job": "coder",
    }
}

how could you type hint this then? you can use Union (from typing ) to do something like this:

Mapping[str, Mapping[str, Union[str, int]]]

but now we would run into a problem with tools like mypy thinking that our_dict["person_1"]["job"] has the type Union[str, int]

well it is not wrong, that is what we told it after all. something that could help here is TypedDict (link to pep explaining it) from [typing_extensions]( https://pypi.org/project/typing-extensions/ .

from typing_extensions import TypedDict

class Person(TypedDict):
    money: int
    job: str

# type hint you would use in your function:
Mapping[str, Person]

NOTE: in python 3.8 TypedDict is part of typing