Python 3 dictionary with known keys typing

Question

I'm using Python 3 typing feature for better autocomplete.

Many times I have functions that return key/value (dictionary) with specific keys. super simple example:

def get_info(name):
    name_first_letter = name[0]
    return {'my_name': name, 'first_letter': name_first_letter}

I want to add type hinting to this function to tell others who use this function what to expect.

I can do something like:

class NameInfo(object):
    def __init__(self, name, first_letter):
        self.name = name
        self.first_letter = first_letter

and then change the function signature to:

def get_info(name) -> NameInfo:

But it requires too much code for each dictionary.

What is the best practice in that case?

Have you considered using `collections.namedtuple` to create your custom types? It does make only immutable types, but depending on how you were using the dictionaries before, that might not be an issue. — Blckknght, May 28 '17 at 09:48
You can also try to use `Dict[str, str]` type hint from [typing](https://docs.python.org/3/library/typing.html) module. — Stanislav Ivanov, May 28 '17 at 10:08
@Blckknght thanks for remind me this option.Found exactly what i need. There is type named tuple https://docs.python.org/3/library/typing.html#typing.NamedTuple — Idok, May 28 '17 at 10:38
Related: [How to use static type checking using Dict with different value types in Python 3.6?](https://stackoverflow.com/q/48013561/7851470) — Georgy, Oct 10 '19 at 15:20
This is a good use case for @attrs : https://www.attrs.org/en/stable/ . Just define your object as an attrs-class and then `attr.asdict(name_info_pair)` will give you the desired dict — Peter, Jun 12 '20 at 12:29
..or, if you're on Python 3.7+ and don't want an external dependency on attrs, you can use [data classes](https://www.python.org/dev/peps/pep-0557/#abstract). — Nickolay, Jun 17 '21 at 11:52

score 103 · Answer 1 · edited Oct 10 '19 at 15:14

103

As pointed out by Blckknght, you and Stanislav Ivanov in the comments, you can use NamedTuple:

from typing import NamedTuple


class NameInfo(NamedTuple):
    name: str
    first_letter: str


def get_info(name: str) -> NameInfo:
    return NameInfo(name=name, first_letter=name[0])

Starting from Python 3.8 you can use TypedDict which is more similar to what you want:

from typing import TypedDict


class NameInfo(TypedDict):
    name: str
    first_letter: str


def get_info(name: str) -> NameInfo:
    return {'name': name, 'first_letter': name[0]}

edited Oct 10 '19 at 15:14

Georgy

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answered Jan 15 '19 at 11:46

Bernhard

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In this way you get some class-specific limitations, for example you can not pack your class using msgpack. – Prisacari Dmitrii Jul 04 '19 at 11:18
how to define, that NameInfo has key with service symbols, for example `last#name: str`? – Ivan Bryzzhin Jan 23 '20 at 10:25
@IvanBryzzhin You might try using [alternative syntax](https://stackoverflow.com/q/60003444/7851470). – Georgy Jan 31 '20 at 14:39
This only works for str keys though – Jerinaw Aug 26 '22 at 16:35

score 3 · Answer 2 · answered Jun 17 '23 at 02:04

TypedDict can also be created using a typename and a dictionary where the keys are the keys and the values are the type of the values, respectively, of the dict to be returned by get_info().

from typing import TypedDict

NameInfo = TypedDict('NameInfo', {'name': str, 'first-letter': str})

def get_info(name: str) -> NameInfo:
    return {'name': name, 'first-letter': name[0]}

x = get_info('cottontail')
print(x['first-letter'])   # c

Then on PyCharm, the key hint is there.

Python 3 dictionary with known keys typing

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