Dask - How to concatenate Series into a DataFrame

Question

How do I return multiple values from a function applied on a Dask Series? I am trying to return a series from each iteration of dask.Series.apply and for the final result to be a dask.DataFrame.

The following code tells me that the meta is wrong. The all-pandas version however works. What's wrong here?

Update: I think that I am not specifying the meta/schema correctly. How do I do it correctly? Now it works when I drop the meta argument. However, it raises a warning. I would like to use dask "correctly".

import dask.dataframe as dd
import pandas as pd
import numpy as np
from sklearn import datasets

iris = datasets.load_iris()

def transformMyCol(x):
    #Minimal Example Function
    return(pd.Series(['Tom - ' + str(x),'Deskflip - ' + str(x / 8),'']))

#
## Pandas Version - Works as expected.
#
pandas_df = pd.DataFrame(data= np.c_[iris['data'], iris['target']], columns= iris['feature_names'] + ['target'])
pandas_df.target.apply(transformMyCol,1)

#
## Dask Version (second attempt) - Raises a warning
#
df = dd.from_pandas(pandas_df, npartitions=10)

unpacked = df.target.apply(transformMyCol)
unpacked.head()

#
## Dask Version (first attempt) - Raises an exception 
#
df = dd.from_pandas(pandas_df, npartitions=10)

unpacked_dask_schema = {"name" : str, "action" : str, "comments" : str}

unpacked = df.target.apply(transformMyCol, meta=unpacked_dask_schema)
unpacked.head()

This is the error that I get:

  File "/anaconda3/lib/python3.7/site-packages/dask/dataframe/core.py", line 3693, in apply_and_enforce
    raise ValueError("The columns in the computed data do not match"
ValueError: The columns in the computed data do not match the columns in the provided metadata

I have also trued the following and it also does not work.

meta_df = pd.DataFrame(dtype='str',columns=list(unpacked_dask_schema.keys()))


unpacked = df.FILEDATA.apply(transformMyCol, meta=meta_df)
unpacked.head()

Same error:

  File "/anaconda3/lib/python3.7/site-packages/dask/dataframe/core.py", line 3693, in apply_and_enforce
    raise ValueError("The columns in the computed data do not match"
ValueError: The columns in the computed data do not match the columns in the provided metadata

Answer 1

You're right, the problem is you're not specifying the meta correctly; more specifically and as the error message says, the metadata columns ("name", "action", "comments") do not match the columns in the computed data (0, 1, 2). You should either:

1. Change the metadata columns to 0, 1, 2:

   unpacked_dask_schema = dict.fromkeys(range(3), str)
   df.target.apply(transformMyCol, meta=unpacked_dask_schema)

or

2. Change transformMyCol to use the named columns:

    def transformMyCol(x):
        return pd.Series({
            'name': 'Tom - ' + str(x), 
            'action': 'Deskflip - ' + str(x / 8), 
            'comments': '',
        }))