[bug]: `class_definition` isn't used for generic classes

[Antyos]

Describe the bug

Writing a class that uses generics, either in the old style of class Foo(Generic[T]) ... or new style class Foo[T]: ... are not saved with the @app.class_definition decorator.

While this makes some sense for the old style, where T would have to be defined in a different cell were the class definition to be the only thing in the cell, but I don't think it makes sense for the new style.

Will you submit a PR?

• Yes

Environment

Details

{
  "marimo": "0.15.3",
  "editable": false,
  "OS": "Windows",
  "OS Version": "11",
  "Processor": "Intel64 Family 6 Model 198 Stepping 2, GenuineIntel",
  "Python Version": "3.13.6",
  "Binaries": {
    "Browser": "--",
    "Node": "v22.18.0"
  },
  "Dependencies": {
    "click": "8.2.1",
    "docutils": "0.22",
    "itsdangerous": "2.2.0",
    "jedi": "0.19.2",
    "markdown": "3.8.2",
    "narwhals": "2.2.0",
    "packaging": "25.0",
    "psutil": "7.0.0",
    "pygments": "2.19.2",
    "pymdown-extensions": "10.16.1",
    "pyyaml": "6.0.2",
    "starlette": "0.47.3",
    "tomlkit": "0.13.3",
    "typing-extensions": "4.15.0",
    "uvicorn": "0.35.0",
    "websockets": "15.0.1"
  },
  "Optional Dependencies": {
    "altair": "5.5.0",
    "duckdb": "1.3.2",
    "nbformat": "5.10.4",
    "openai": "1.102.0",
    "pandas": "2.3.2",
    "polars": "1.32.3",
    "pyarrow": "21.0.0",
    "loro": "1.5.4",
    "pytest": "8.4.1",
    "python-lsp-ruff": "2.2.2",
    "python-lsp-server": "1.13.1",
    "ruff": "0.12.11",
    "sqlglot": "27.9.0"
  },
  "Experimental Flags": {}
}

Code to reproduce

import marimo

__generated_with = "0.15.3"
app = marimo.App(width="medium")

with app.setup:
    from typing import Generic, TypeVar


@app.cell
def _():
    class GenericTypeParameter[T]:
        def __init__(self, value: T):
            self.value = value
    return


@app.cell
def _():
    T = TypeVar("T")
    return (T,)


@app.cell
def _(T):
    class GenericTypeVar(Generic[T]):
        def __init__(self, value: T):
            self.value = value
    return


@app.class_definition
class NoGeneric:
    def __init__(self, value):
        self.value = value


if __name__ == "__main__":
    app.run()

[mscolnick]

Doesn't T = TypeVar("T") need to be in the setup cell? so it had no dependencies

[Antyos]

I am dumb, yes it does. When T = TypeVar("T") is in the setup cell, then both generic class definitions behave as expected.

import marimo

__generated_with = "0.15.3"
app = marimo.App(width="medium")

with app.setup:
    from typing import Generic, TypeVar

    T = TypeVar("T")


@app.class_definition
class GenericTypeParameter[T]:
    def __init__(self, value: T):
        self.value = value


@app.class_definition
class GenericTypeVar(Generic[T]):
    def __init__(self, value: T):
        self.value = value


@app.class_definition
class NoGeneric:
    def __init__(self, value):
        self.value = value


if __name__ == "__main__":
    app.run()

However, I still think that class GenericTypeParameter[T] should work without having to explicitly define T = TypeVar("T"), because that is valid Python syntax.

[mscolnick]

@Antyos - ah yea that makes sense, should be doable, thanks for the report

[dmadisetti]

Hmm. Am leaning won't fix. This will work if you put "T" in a string

It's a difficult line to determine when we should promote statements to top level, so putting it in the setup cell is the answer.

But you should also get a hint for why it's not promoted (in the right corner)

[Antyos]

I didn't notice the hint in the bottom right--that makes sense.

Also, even with the generic defined, it seems like the @app.class_definition decorator doesn't pass class typing with or without generics.

type_parameter: GenericTypeParameter[str] = GenericTypeParameter("foo")  # type: Unknown
# Even without generics
no_generic: NoGeneric = NoGeneric()  # type: Unknown
no_generic2 = NoGeneric()  # type: Any

[mscolnick]

We won't close this as "Won't fix", since this is valid python (below). We do want to fix this but may not get to it right away, so the best workaround is wrapping "T" in a string

import marimo

__generated_with = "0.15.3"
app = marimo.App(width="medium")

with app.setup:
    from typing import Generic, TypeVar

@app.class_definition
class GenericTypeParameter[T]:
    def __init__(self, value: T):
        self.value = value


if __name__ == "__main__":
    app.run()

[mscolnick]

@Antyos if you'd like to jump into the code, we'd appreciate your help on improving the types (for your second issue you raised)

[Antyos]

Sure, I just submitted a PR. It was a simple fix.