How to check that a column only contains uppercase letters?

[philiporlando]

I'm trying to ensure that a "County" column only contains uppercase characters.

import polars as pl
import pandera.polars as pa

df = pl.DataFrame(
    {
        "County": ["LOS ANGELES", "SAN FRANCISCO", "ALAMEDA"],
    }
)


schema = pa.DataFrameSchema(
    {
        "County": pa.Column(
            dtype=str,
            checks=[
                pa.Check(
                  lambda s: s.upper() == s,
                  error="County must be uppercase",
                  element_wise=True,
                ),
            ],
        )
    }
)

validated_df = schema.validate(df)
print(schema(df))

This appears to be working with this small reprex, but I'm also seeing a warning related to a missing return_dtype that I don't understand.

┌───────────────┐
│ County        │
│ ---           │
│ str           │
╞═══════════════╡
│ LOS ANGELES   │
│ SAN FRANCISCO │
│ ALAMEDA       │
└───────────────┘

# MapWithoutReturnDtypeWarning: Calling `map_elements` without specifying `return_dtype` can lead to unpredictable results. Specify `return_dtype` to silence this warning.
  passed = check_result.check_passed.collect().item()

However, when I try to apply the same check with my production data, I'm seeing the below error:

self = <pandera.api.base.error_handler.ErrorHandler object at 0x000001EC1A876510>
error_type = <ValidationScope.DATA: 'data'>
reason_code = <SchemaErrorReason.CHECK_ERROR: 'check_error'>
schema_error = SchemaError('ComputeError("custom python function failed: cannot unpack series of type `str` into `bool`")')
original_exc = ComputeError('custom python function failed: cannot unpack series of type `str` into `bool`')

    def collect_error(
        self,
        error_type: ErrorCategory,
        reason_code: SchemaErrorReason,
        schema_error: SchemaError,
        original_exc: Union[BaseException, None] = None,
    ):
        """Collect schema error, raising exception if lazy is False.
    
        :param error_type: type of error
        :param reason_code: string representing reason for error
        :param schema_error: ``SchemaError`` object.
        """
        if not self._lazy:
>           raise schema_error from original_exc
E           pandera.errors.SchemaError: ComputeError("custom python function failed: cannot unpack series of type `str` into `bool`")

.venv\Lib\site-packages\pandera\api\base\error_handler.py:54: SchemaError

I'm hoping to learn more about why I'm encountering this error and identify a more reliable way of checking for string casing within polars dataframes.

[philiporlando]

This error was due to a bug that was fixed in #1572.