read_csv with Ray engine fails with some combinations of `Column and Index Locations and Names` parameters

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Ubuntu 18.04
• Modin version (modin.__version__): 0.8.1.1+34.ga571e10
• Python version: 3.8.6
• Code we can use to reproduce:

import os

os.environ["MODIN_ENGINE"] = "ray"

import pandas
import modin.pandas as pd
from modin.pandas.test.utils import df_equals

test_filename = "test.csv"
kwargs = {
    "filepath_or_buffer": test_filename,
    "index_col": "col1",
    "usecols": ["col1"],
}
str_two_cols = """col1,col2
0,1
2,3
"""

try :
    with open(test_filename, "w") as f:
        f.write(str_two_cols)

    df_pandas = pandas.read_csv(**kwargs)
    print(df_pandas)
    df_pd = pd.read_csv(**kwargs)
    print(df_pd)
    df_equals(df_pd, df_pandas)
finally:
    os.remove(test_filename)

Describe the problem

Source code / logs

Empty DataFrame
Columns: []
Index: [0, 2]
Traceback (most recent call last):
  File "test.py", line 26, in <module>
    df_pd = pd.read_csv(**kwargs)
  File "/localdisk/amyskov/modin/modin/pandas/io.py", line 109, in parser_func
    return _read(**kwargs)
  File "/localdisk/amyskov/modin/modin/pandas/io.py", line 127, in _read
    pd_obj = EngineDispatcher.read_csv(**kwargs)
  File "/localdisk/amyskov/modin/modin/data_management/factories/dispatcher.py", line 104, in read_csv
    return cls.__engine._read_csv(**kwargs)
  File "/localdisk/amyskov/modin/modin/data_management/factories/factories.py", line 87, in _read_csv
    return cls.io_cls.read_csv(**kwargs)
  File "/localdisk/amyskov/modin/modin/engines/base/io/file_reader.py", line 29, in read
    query_compiler = cls._read(*args, **kwargs)
  File "/localdisk/amyskov/modin/modin/engines/base/io/text/csv_reader.py", line 142, in _read
    column_chunksize = compute_chunksize(empty_pd_df, num_splits, axis=1)
  File "/localdisk/amyskov/modin/modin/data_management/utils.py", line 54, in compute_chunksize
    col_chunksize = get_default_chunksize(len(df.columns), num_splits)
  File "/localdisk/amyskov/modin/modin/data_management/utils.py", line 29, in get_default_chunksize
    length // num_splits if length % num_splits == 0 else length // num_splits + 1
ZeroDivisionError: integer division or modulo by zero

[amyskov]

@ymoslem, temporally this bug can be avoided by adding index_col=False option to the read_csv function, see example below:

import modin.pandas as pd
import pandas
import ray
import os
os.environ["MODIN_ENGINE"] = "ray"
ray.init()

from modin.pandas.test.utils import df_equals

file_name = "ELRA-W0309.en-fr.en"

df_pandas = pandas.read_csv(file_name, names=['English'], sep="\n")
df_pd = pd.read_csv(file_name, names=['English'], sep="\n", index_col=False)
df_pd.reset_index(drop=True, inplace=True) # indexes aligning between pandas and Modin

df_equals(df_pandas, df_pd) # df_pd and df_pandas are equal!

Hope this helps!

[pyrito]

I am not able to reproduce this bug on master, so I’ll go ahead and close this issue. Please re-open if needed!