Type hints for custom column types using TypeDecorator

First, these stubs are amazing. Thank you!

I am trying to get my custom column types to work like a ArrowType using TypeDecorator from https://docs.sqlalchemy.org/en/latest/core/custom_types.html.

I tried following how you’re using the generic TypeEngine for native types like Column(Boolean), Column(Integer) but am coming up short.

Are custom columns working already? If not i figured i could add it using what you already have for the native types.

[ilevkivskyi]

They should work. For example you can write:

class FancyInteger(TypeDecorator[int]):
    ...

There are two caveats:

• Type decorators are currently only generic in Python type, implementation type is not exposed, if it will be really necessary, we can add a parameter for implementation type (see TODO comment in the stub)
• The above code works fine with mypy (and probably other type checkers), but not at runtime, so you need a workaround. For example see mypy docs:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    IntEngine = TypeDecorator[int]
else:
    IntEngine = TypeDecorator

class FancyInteger(IntEngine):
    ...

or similar.

[NotarySigningAgent]

i’m lost at decorator all yours

On Wed, Jun 17, 2020 at 1:38 AM Michel Albert [e-mail] wrote:

Sorry for hijacking this old issue, but I think it is related enough to stick this here:

If I have a TypeDecorator which is guaranteed to never store None in the DB, I would like to be able to type-hint this without the use of Optional. Is that possible? Currently I get the following message:

Argument 1 of “process_bind_param” is incompatible with supertype “TypeDecorator”; supertype defines the argument type as “Optional[MyType]”

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