TST: add test to read empty array

[nakatomotoi]

• closes BUG: New param [use_nullable_dtypes] of pd.read_parquet() can't handle empty parquet file #41241
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[jreback]

ping when updated and green

[mzeitlin11]

Can you add back the original test you add for all types? It doesn't need to include the nullable types since they are added in the other test that uses use_nullable_dtypes

[jreback]

can you add more types here, float, int, period[D], category, Float64, string

[nakatomotoi]

@jreback
I get "AssertionError" when I add "category", is "category" necessary as a test case?

self = <pandas.tests.io.test_parquet.TestParquetPyArrow object at 0x7f020ec01640>, pa = 'pyarrow', dtype = 'category'

    @pytest.mark.parametrize(
        "dtype",
        [
            "Int64",
            "UInt8",
            "boolean",
            "object",
            "datetime64[ns, UTC]",
            "float",
            "int",
            "period[D]",
            "category",
            "Float64",
            "string",
        ],
    )
    def test_read_empty_array(self, pa, dtype):
        # GH #41241
        df = pd.DataFrame(
            {
                "value": pd.array([], dtype=dtype),
            }
        )
>       check_round_trip(df, pa)

pandas/tests/io/test_parquet.py:957: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/tests/io/test_parquet.py:221: in check_round_trip
    compare(repeat)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

repeat = 2

    def compare(repeat):
        for _ in range(repeat):
            df.to_parquet(path, **write_kwargs)
            with catch_warnings(record=True):
                actual = read_parquet(path, **read_kwargs)
    
>           tm.assert_frame_equal(
                expected,
                actual,
                check_names=check_names,
                check_like=check_like,
                check_dtype=check_dtype,
            )
E           AssertionError: Attributes of DataFrame.iloc[:, 0] (column name="value") are different
E           
E           Attribute "dtype" are different
E           [left]:  CategoricalDtype(categories=[], ordered=False)
E           [right]: object

pandas/tests/io/test_parquet.py:211: AssertionError

[jreback]

need to add read_kwars={'use_nullable_types': True}

[jreback]

move this test near here:

     def test_use_nullable_dtypes(self, engine):
        import pyarrow.parquet as pq

        if engine == "fastparquet":
            # We are manually disabling fastparquet's
            # nullable dtype support pending discussion
            pytest.skip("Fastparquet nullable dtype support is disabled")

and in fact structure it similarly

[nakatomotoi]

@jreback
I get a timeout on the pandas-dev.pandas test, could you tell me why?
If you know the solution, I'd like to hear it.

[jreback]

@jreback I get a timeout on the pandas-dev.pandas test, could you tell me why? If you know the solution, I'd like to hear it.

you can merge master and see if that fixes, but sometimes this does timeout unrelated to your change

[jreback]

thanks @nakatomotoi very nice.

if you would like to do a PR with an empty category would be great as well (llikley just need to construct it properly)