update_forward_refs modifies __dict__ of a module

[Paul-Ilyin]

Bug

OS: macOS Catalina 10.15.3  
python: 3.7.3
pydantic version: 1.1

After creating a BaseModel subclass and calling update_forward_ref() classmethod, pydantic can modify dict of a module in which the model was created if the model was declared inside some function of this module.

from typing import Optional, ForwardRef
from pydantic import BaseModel

def first_func():
    class Mod(BaseModel):
        field: Optional[ForwardRef("Mod")]
    Mod.update_forward_refs()
    return Mod

def second_func():
    class Mod(BaseModel):
        another_field: Optional[ForwardRef("Mod")]

    Mod.update_forward_refs()
    
    return Mod

first_model = first_func()
second_model = second_func()

print(second_model.__fields__["another_field"].type_)
# Output: <class '__main__.first_func.<locals>.Mod'>
# Expected: <class '__main__.second_func.<locals>.Mod'>

It happens because of these two lines: https://github.com/samuelcolvin/pydantic/blob/645e5fe6a0c74c95c24410571e9bb804af3eb677/pydantic/main.py#L632-L633
They can be fixed that way:

        globalns = sys.modules[cls.__module__].__dict__.copy()
        globalns.setdefault(cls.__name__, cls)

This bug can lead to errors when two different models with the same names are defined in two different test cases or some other errors which can be hard to debug.

[samuelcolvin]

thanks for reporting, PR welcome.