DataFrameMapper changes columns types when default=None.

[leonardommarques]

When I use DataFrameMapper and set up default=None to transform a column, all other columns
types are changed to object. But this does not happen when I have only float and/or int columns

import pandas as pd
import numpy as np
from sklearn_pandas import DataFrameMapper
from sklearn.impute import SimpleImputer


# all numerical columns lead to no error
da = pd.DataFrame({
    'a':[1,3,np.nan],
    'b': [1.2,2,3]})
print(da.dtypes)

aux_imp = DataFrameMapper([
    (['a'], SimpleImputer(strategy='mean'))], 
    df_out=True, default=None)

da = aux_imp.fit_transform(da)
print(da.dtypes)

# if a column is of str it leads to errors
da = pd.DataFrame({
    'a':[1,3,np.nan],
    'b': [1.2,2,3],
    'c':['c', 'c', 'a']
})
print(da.dtypes)

aux_imp = DataFrameMapper(
    [(['a'], SimpleImputer(strategy='mean'))], 
    df_out=True, default=None)

da = aux_imp.fit_transform(da)
print(da.dtypes)

[dukebody]

I believe this is because the dataframe mapper uses the same "empty transformer" selecting all not explicitly selected columns, therefore if their types are mixed, the best type for the extracted numpy array is "object", to be able to cover strings, ints, floats, etc.

I don't know if this can be worked around by "copying" the default columns one by one, keeping the dtype.

[monda00]

Hi, I'm new to open source contribution.
Is is okay for me to work on this issue?

[pradumna123]

Hello, I would like to work on it
Can you please assign it to me

[ajayverma90]

is this issue resolved ?
I am facing the same issue, I have a DataFrame containing columns of float and str dtypes.
using default=None converts the dtype of all the columns to object
which is causing my Pipelines to fail.