✨ Add support for Literal types

[nsaccente]

This PR addresses issue 57. The original issue explains that attempting to use a Literal type annotation in a SQLModel raises a type error from issubclass. This is because Literal is not a class, but a typing._SpecialForm, so static funcs like isinstance and issubclass don't work with it.

The fix was pretty straightforward, I just added a check before an isinstance or issubclass would have been called that checks if type_ is Literal.

[nsaccente]

I have no idea how to add the proper labels to this PR, it looks like the permissions aren't set up to allow me to.

oh, wait...

[YuriiMotov]

@nsaccente, thanks for working on this!
This works, but could you please add a test?

[nsaccente]

@nsaccente, thanks for working on this! This works, but could you please add a test?

Sure. I'll try to get that done sometime this week.

[nsaccente]

I've been fighting with getting the tests to work. I've been able to get things to pass in some versions of python, but not others. Any guidance on how to run these CI tests locally?

[svlandeg]

@nsaccente: there were some tricky linting errors that I've now fixed.

@YuriiMotov: do you have time to give this another review?

[YuriiMotov]

It seems to me that we can simplify things by just importing Literal from typing_extensions directly. Why do we need that trick with if sys.version_info >= (3, 9):?

[svlandeg]

@YuriiMotov: you're right - it's more clean overall. I'm happy to defer the Literal import to either another PR or to just wait for 3.8 deprecation, at which point we shouldn't forget to clean this up. Let me clean up this PR, addressing your comments.

[tiangolo]

I'm seeing that this assumes all Literal values are strings, so a model like this:

classHero(SQLModel, table=True): id: Optional[int] =Field(default=None, primary_key=True) name: str=Field(unique=True) weakness: Literal[1, 2, 3]

...would still create the column in the DB as a string, not as an integer. 🤔

[YuriiMotov]

Things are getting complex.. We can easily add special cases for Literal[1, 2, 3] or Literal[True, False] to create column with proper type.

The question is, do we expect any kind of validation for these values?
Our database column type is just CHAR or INTEGER, so, database will not enforce the value to be correct against Literal[1, 2, 3] type. If we have the value 42 in database, it can break user's code in a very tricky way one day

Need to decide about data validation first