pydantic_string_stuff.py

from pydantic import BaseModel, ConstrainedStr, StrictStr

class MyStr(ConstrainedStr):
    to_upper = True

class Foo(BaseModel):
    x: str

class Bar(BaseModel):
    x: MyStr

class Baz(BaseModel):
    x: StrictStr


obj1 = Foo(x=MyStr("hello, world!"))
print(obj1.x.to_upper)  # works, despite objections of IDE

obj2 = Bar(x="hello, world!")
print(obj2.x.to_upper)  # fails, despite IDE not telling you

obj3 = Baz(x="hello, world!")
print(obj3.x.to_upper)  # fails, despite IDE not telling you

obj4 = Baz(x=StrictStr("hello, world!"))
print(obj4.x.to_upper)  # works + type is StrictStr

obj5 = Baz(x=MyStr("hello, world!"))
print(obj5.x.to_upper)  # works, but type is MyStr