pd.Series.ffill() raise the error: AttributeError: 'numpy.ndarray' object has no attribute 'ffill'

System information

• OS Platform and Distribution - WIndows10:
• Modin version (0.15.0+7.g4ec7f634):
• Python version 3.9.12:
• Code we can use to reproduce:

  import modin
  import modin.pandas as pd
  from distributed import Client
  
  import numpy as np
  
  
  if __name__ == '__main__':
      if pd.__name__ == 'modin.pandas':
          client = Client(n_workers=3)
          print(modin.__version__)

    df = pd.DataFrame(
        dict(
            a=[1, 2, None, None, None, ],
            b=(1, None, 3, 4, 5,),
        )
    )

    df.a.ffill(inplace=True)
    print(df)

    df['tr_id'] = 0

    df.tr_id = np.where(
        (df.b <= 4),
        3,
        None,
    )

    df.tr_id.ffill(inplace=True)

Describe the problem

I have the output:

0.15.0+7.g4ec7f634
UserWarning: Ray execution environment not yet initialized. Initializing...
To remove this warning, run the following python code before doing dataframe operations:

    import ray
    ray.init()

UserWarning: Distributing <class 'dict'> object. This may take some time.
     a    b
0  1.0  1.0
1  2.0  NaN
2  2.0  3.0
3  2.0  4.0
4  2.0  5.0
Traceback (most recent call last):
  File "d:\OD\OneDrive\Projects\Chud_Amaz\Soft_in_dev\moduled_way_OOP\modin_test_ffill.py", line 33, in <module>
    df.tr_id.ffill(inplace=True)
AttributeError: 'numpy.ndarray' object has no attribute 'ffill'

Problem

When new column was filled with np.where - method ffill does not work

[mvashishtha]

May beput resulting numpy in Series on right hand?

@VasilijKolomiets unfortunately it turns out that that won’t work in Modin either 😢 Modin assigns the right-hand side as-is to the attribute instead of pointing the attribute to the the new column.

import modin.pandas as pd

df = pd.DataFrame([[1]], columns=['col0'])
df.col0 = pd.Series([3])
df.iloc[0, 0] = 4
# BUG: col0 is unchanged!!!
assert df.col0.equals(df['col0'])

What you can do instead is use __setitem__ instead of __setattr__, e.g. df['tr_id'] = np.where(... instead of df.tr_id = np.where(.... This works:

import modin.pandas as pd

df = pd.DataFrame([[1]], columns=['col0'])
df['col0'] = pd.Series([3])
df.iloc[0, 0] = 4
assert df.col0.equals(df['col0'])

Meanwhile, @pyrito will work on a PR that should fix all the bugs identified here.

[mvashishtha]

In the snippet I posted above, the Modin dataframe’s __setattr__ calls __setitem__ to modify the col0 column in place. It then calls object.__setattr__(self, key, value), which re-assigns the col0 property to exactly the value that was passed in, i.e. the list. I think a fairly simple fix would be to call object.__setattr__(self, key, self.__getitem__(key)) in the case here where we call __setitem__.

I can’t take this on right now, so I’ll leave it unassigned.