Type Safety: is silent truncation a bug?

[dhirschfeld]

Happy to move to JIRA if this is confirmed as a bug

In [8]: import pandas as pd
   ...: import pyarrow as arw

In [9]: df = pd.DataFrame({'A': list('abc'), 'B': np.arange(3)})
   ...: df
Out[9]:
   A  B
0  a  0
1  b  1
2  c  2

In [10]: schema = arw.schema([
    ...:     arw.field('A', arw.string()),
    ...:     arw.field('B', arw.int32()),
    ...: ])

In [11]: tbl = arw.Table.from_pandas(df, preserve_index=False, schema=schema)
    ...: tbl
Out[11]:
pyarrow.Table
A: string
B: int32
metadata
--------
{b'pandas': b'{"index_columns": [], "column_indexes": [], "columns": [{"name":'
            b' "A", "field_name": "A", "pandas_type": "unicode", "numpy_type":'
            b' "object", "metadata": null}, {"name": "B", "field_name": "B", "'
            b'pandas_type": "int32", "numpy_type": "int32", "metadata": null}]'
            b', "pandas_version": "0.23.1"}'}

In [12]: tbl.to_pandas().equals(df)
Out[12]: True

...so if the schema matches the pandas datatypes all is well - we can roundtrip the DataFrame.

Now, say we have some bad data such that column 'B' is now of type float64. The datatypes of the DataFrame don't match the explicitly supplied schema object but rather than raising a TypeError the data is silently truncated and the roundtrip DataFrame doesn't match our input DataFame without even a warning raised!

In [13]: df['B'].iloc[0] = 1.23
    ...: df
Out[13]:
   A     B
0  a  1.23
1  b  1.00
2  c  2.00

In [14]: # I would expect/want this to raise a TypeError since the schema doesn't match the pandas datatypes
    ...: tbl = arw.Table.from_pandas(df, preserve_index=False, schema=schema)
    ...: tbl
Out[14]:
pyarrow.Table
A: string
B: int32
metadata
--------
{b'pandas': b'{"index_columns": [], "column_indexes": [], "columns": [{"name":'
            b' "A", "field_name": "A", "pandas_type": "unicode", "numpy_type":'
            b' "object", "metadata": null}, {"name": "B", "field_name": "B", "'
            b'pandas_type": "int32", "numpy_type": "float64", "metadata": null'
            b'}], "pandas_version": "0.23.1"}'}

In [15]: tbl.to_pandas()  # <-- SILENT TRUNCATION!!!
Out[15]:
   A  B
0  a  1
1  b  1
2  c  2

To be clear, I would really like Table.from_pandas to raise a TypeError if the DataFrame types don't match an explicitly supplied schema and would hope this current behaviour would be considered a bug.

win64/py36
arrow 0.9.0

[wesm]

hi @dhirschfeld -- yes I would expect the silent truncation to not happen unless the user passes safe=False to be consistent with Array.cast. Could you open a JIRA to add a safe/unsafe option to Table.from_pandas?

Thanks!

[dhirschfeld]

Moved to ARROW-2799