Enum type becomes a Categorical type when file is read with scan_parquet

[mrphilroth]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

df = pl.DataFrame(pl.Series("d", ["foo", "bar"]), schema=pl.Schema({"d": pl.Enum(["foo", "bar", "ham"])}))
df.write_parquet("test.parquet")
print(pl.read_parquet("test.parquet").schema)
print(pl.scan_parquet("test.parquet").collect_schema())

Log output

Schema([('d', Enum(categories=['foo', 'bar', 'ham']))])
Schema([('d', Categorical(ordering='physical'))])

Issue description

Enum type should be preserved when file is read with scan_parquet

Expected behavior

Output from the example should be:

Schema([('d', Enum(categories=['foo', 'bar', 'ham']))])
Schema([('d', Enum(categories=['foo', 'bar', 'ham']))])

Installed versions

--------Version info---------
Polars:              1.10.0
Index type:          UInt32
Platform:            macOS-14.7-arm64-arm-64bit
Python:              3.10.6 (main, Aug  2 2022, 20:27:59) [Clang 14.0.3 ]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.9.0
gevent               <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         1.6.0
numpy                <not installed>
openpyxl             <not installed>
pandas               <not installed>
pyarrow              <not installed>
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[eitsupi]

I don't think the types Categorical and Enum exist in Parquet.

You should use the Arrow file format (Arrow IPC file) to store these types.

>>> import polars as pl
>>> df = pl.DataFrame(pl.Series("d", ["foo", "bar"]), schema=pl.Schema({"d": pl.Enum(["foo", "bar", "ham"])}))
>>> df.write_ipc("test.arrow")
>>> pl.read_ipc("test.arrow")
shape: (2, 1)
┌──────┐
│ d    │
│ ---  │
│ enum │
╞══════╡
│ foo  │
│ bar  │
└──────┘

[coastalwhite]

This can definitely be preserved either through the arrow schema or through a POLARS_SCHEMA metadata argument.