How to add type inference to Python Flask and Marshmallow

Question

I'm new to Python from a Typescript background. Putting together a Flask API server and using Marshmallow to validate input DTOs.

How can I get VSCode to infer types from the Marshmallow Schema load?

class PostInput:
    myString = fields.Str(
        required=True
    )

@app.route('/foo', methods=['POST'])
def post_foo():
    try:
        data = PostInput().load(request.json)
    except ValidationError as err:
        return jsonify(err.messages), 400
      
    myString = data.get("myString")

    myString <== I want this to be known as a str

Answer 1

Adding typing hint to the dict key:

from marshmallow import fields, Schema

class PostInput(Schema):
    myString = fields.Str(
        required=True
    )
    
json = '{"myString": "hello"}'
data = PostInput().loads(json, many=False)
myString:str = data.get("myString")

or serializing data into the dataclass:

from marshmallow import fields, Schema, post_load
from dataclasses import dataclass

@dataclass
class InputClass:
    myString: str


class PostInput(Schema):
    myString = fields.Str(
        required=True
    )
    @post_load
    def make_user(self, data, **kwargs) -> InputClass:
        return InputClass(**data)
        
    
json = '{"myString": "hello"}'
data: InputClass = PostInput().loads(json, many=False)
myString = data.myString

I would recommend to use pydantic for validation and loading the json. It does both validates the data and creates smth like a dataclass that inherits types of the attributes:

from pydantic import BaseModel, Field

class PostInput(BaseModel):
    myString: str = Field(required=True)    

json = '{"myString": "hello"}'
data = PostInput.model_validate_json(json)
myString = data.myString