Jim Crist created ARROW-2444:
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Summary: Better handle reading empty parquet files
Key: ARROW-2444
URL: https://issues.apache.org/jira/browse/ARROW-2444
Project: Apache Arrow
Issue Type: Improvement
Reporter: Jim Crist
>From [https://github.com/dask/dask/pull/3387#issuecomment-380140003]
Currently pyarrow reads empty parts as float64, even if the underlying columns
have other dtypes. This can cause problems for pandas downstream, as certain
operations are only valid on certain dtypes, even if the columns are empty.
Copying the comment Uwe over:
bq. {quote}This is the expected behaviour as an empty string column in Pandas
is simply an empty column of type object. Sadly object does not tell us much
about the type of the column at all. We return numpy.float64 in this case as
it's the most efficient type to store nulls in Pandas.{quote}
{quote}This seems unintuitive at best to me. An empty object column in pandas
is treated differently in many operations than an empty float64 column (str
accessor is available, excluded from numeric operations, etc..). Having an
empty file read in as a different dtype than was written could lead to errors
in processing code downstream. Would arrow be willing to change this
behavior?{quote}
We should probably use another method than `field.type.to_pandas_dtype()` in
this case. The column saved in Parquet should be saved with `NA` as type which
sadly does not provide enough information.
We also store the original dtype in the Pandas metadata that is used for the
actual DataFrame reconstruction later on. If we would also pick up the metadata
when it was written, we should be able to correctly reconstruct the dtype.
{quote}
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