mypy type checking doesn't seem to work

[gjcarneiro]

Bug

For bugs/questions:

• OS: ubuntu 18.10
• Python version import sys; print(sys.version): 3.7
• Pydantic version import pydantic; print(pydantic.VERSION): master

Where possible please include a self contained code snippet describing your
bug, question, or where applicable feature request:

from datetime import datetime
from typing import List, Optional
from pydantic import BaseModel, NoneStr

class Model(BaseModel):
    age: int
    first_name = 'John'
    last_name: NoneStr = None
    signup_ts: Optional[datetime] = None
    list_of_ints: List[int]

m = Model(age=42, list_of_ints=[1, '2', b'3'])
print(m.age)
# > 42

Model()
# will raise a validation error for age and list_of_ints

The above is the example from the docs: https://pydantic-docs.helpmanual.io/#usage-with-mypy

With mypy 0.650, doesn't raise any validation error.

Moreover, changing print(m.age) to print(m.agexxx), it still doesn't give any error saying that the field name is wrong.

[samuelcolvin]

We should make the docs clearer, but while mypy won't give a false negative when it parses a pydantic model, it obviously can't check if model validation is going to pass or fail.

Also as per #245 mypy can't check whether an attribute exists.

This is a (completely understandable) limitation of mypy. You might want to use dataclasses which mypy should support better.

[gjcarneiro]

Yes, please make the docs clearer. Essentially I can't use pydantic if I want static type checking. The docs say:

because pydantic data structures are just instances of classes you define; auto-completion, linting, mypy and your intuition should all work properly with your validated data.

Well, I would claim that, no, mypy does not work correctly with pydantic. The docs are misleading.

[samuelcolvin]

That's fair, I'll update docs.

Some things do work however

from pydantic import BaseModel

class Test(BaseModel):
    test_int: int
    test_str: str

m = Test(test_int=42, test_str='foobar')
print(123 + m.test_str)

Returns:

(env) pydantic 0 1.19s ➤  mypy --ignore-missing-imports --follow-imports=skip --strict-optional test.py 
test.py:11: error: Unsupported operand types for + ("int" and "str")

While the same with print(123 + m.test_int) would pass.

So I would say mypy and pydantic work together partially, "Essentially I can't use pydantic if I want static type checking." is a bit too far the other way.

[samuelcolvin]

see #372.