Print warning in summary when comparing data frames with no rows

[MoritzPotthoffQC]

If you compare data frames that both have no rows, the summary correctly shows a perfect match.

In exploratory work, I sometimes run into cases where I get a perfect match but it is actually just caused by both input data frames being empty (e.g., because of a faulty previous join). I think it would be nice to add a warning to the summary in case both data frames have no rows.

Example:

import polars as pl
from diffly import compare_frames

left = pl.DataFrame({"id": [], "value": [], "name": []}).cast(
    {"id": pl.Int64, "value": pl.Float64, "name": pl.Utf8}
)
right = pl.DataFrame({"id": [], "value": [], "name": []}).cast(
    {"id": pl.Int64, "value": pl.Float64, "name": pl.Utf8}
)

comparison = compare_frames(left, right, primary_key="id")
print(comparison.summary())

prints

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃                                     Diffly Summary                                     ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
                            --- Data frames match exactly! ---

[MariusMerkleQC]

Makes sense, what do you think about changing the message from "Data frames match exactly!" to "Data frames are empty, but their schema matches exactly!"? What do you think? EgeKaraismailogluQC Oliver Borchert (@borchero)

[MariusMerkleQC]

Solved by #20

[MoritzPotthoffQC]

Thanks Marius Merkle (@MariusMerkleQC) for the quick fix 🚀 🫶