basic level typechecking error in polars integration documentation/example

[Molier]

Location of the documentation

https://pandera.readthedocs.io/en/latest/polars.html

Documentation problem

Issues:

1. The initial example in the documentation shows validation of a LazyFrame but requires .collect(), which is confusing
2. The example code has type checking errors with modern type checkers, pylance at "basic"
3. Return types from validate() don't properly indicate Polars methods

Example of current type errors:

and example in my code: .

This would help new users better understand the intended usage pattern and avoid confusion with type checking.

I am running into a lot of basic level type checking errors using pandera. I'm quite new to type checking, but trying to write more robust code and thus use proper validation on my data when using polars.

Wording

The docs also say which we then use to validate a polars.LazyFrame object. Which is weird since the code beneath needs to 'collect' to validate, thus forgoing the lazy optimizations?

Suggested fix for documentation

I don't really know what the cause of this is or how to improve it, but i keep running into a lot of type errors when using pandera.

Suggested Improvements:

1. Update the initial example to clearly show the recommended validation pattern
2. Add proper type hints for Pandera's return types
3. Move the explanation of LazyFrame vs DataFrame validation earlier in the docs

[Molier]

core issue is that

at https://pandera.readthedocs.io/en/latest/polars.html#usage is giving type error for .collect()

[baldwinj30]

In order to fix that type error you could add the following imports:

from pandera.typing.polars import LazyFrame
import typing

And then change the last line to:

typing.cast(LazyFrame[Schema], Schema.validate(lf)).collect()

The call to typing.cast does not change the functionality of the code at all; it just tells the type checker to expect a LazyFrame that follows the Schema specification.

This is necessary when using the class-based API: https://pandera.readthedocs.io/en/latest/dataframe_models.html. The pandera.typing.polars module is the polars version of pandera.typing from the link. There is a reference to it in the docs on the polars side, but not until the next section.

I would be happy to submit a PR to tweak these docs; I had a little difficulty in sorting through this when switching from the pandas to the polars engine. My thought would be to make a small change to reference the later sections from the start, and to more explicitly discuss the pandera.polars.typing module.