There is no error when str is passed in when the field type is int?

Question

I am verifying the data in the dictionary. I specify that the age field is of type int, but in practice, the age I pass in is of type str. Why is there no error?

from marshmallow import Schema, fields


class UserSchema(Schema):
    name = fields.Str()
    age = fields.Int()


user_data = {
    "name": "Ken",
    "age": "12"
}

try:
    UserSchema().load(user_data)
except Exception as e:
    print(e)

Answer 1

By default, marshmallow casts "12" into an int.

If you want to ensure it is an int and not a string, use the strict parameter.

https://marshmallow.readthedocs.io/en/stable/api_reference.html#marshmallow.fields.Integer

strict – If True, only integer types are valid. Otherwise, any value castable to int is valid.

class UserSchema(Schema):
    name = fields.Str()
    age = fields.Int(strict=True)