Replacing multiple columns (or just one) with iloc does not work

Code Sample, a copy-pastable example if possible

import pandas

columns = pandas.DataFrame({'a2': [11, 12, 13], 'b2': [14, 15, 16]})
inputs = pandas.DataFrame({'a1': [1, 2, 3], 'b1': [4, 5, 6], 'c1': [7, 8, 9]})

inputs.iloc[:, [1]] = columns.iloc[:, [0]]

print(inputs)

Problem description

I have a code which is replacing a set of columns with another set of columns, based on column indices. To make things done without a special case, I assumes I could just use iloc to both select and set columns in a DataFrame. But it seems that this not work and fails in strange ways.

Expected Output

   a1  b1  c1
0   1  11   7
1   2  12   8
2   3  13   9

But in reality, you get:

    a1  b1   c1
0  1.0 NaN  7.0
1  2.0 NaN  8.0
2  3.0 NaN  9.0

See how values converted to float and how column is NaNs?

But, if I do the following I get expected results:

inputs.iloc[:, [1]] = [[11], [12], [13]]

This also works:

inputs.iloc[:, [1]] = columns.iloc[:, [0]].values

So if it works with lists and ndarrays, one would assume it would also work with DataFrames themselves. But it does not.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None python: 3.6.3.final.0 python-bits: 64 OS: Linux OS-release: 4.13.0-46-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.23.3 pytest: None pip: 18.0 setuptools: 40.0.0 Cython: None numpy: 1.15.0 scipy: None pyarrow: None xarray: None IPython: None sphinx: None patsy: None dateutil: 2.7.3 pytz: 2018.5 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: None openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: None sqlalchemy: None pymysql: None psycopg2: None jinja2: None s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None

[jorisvandenbossche]

I can understand that this happens with .loc, but how can it happen with .iloc? .iloc should be position-based so it should not care about column names, no?

Yes, in principle iloc should not care about column names, so this certainly seems a bug to me.

[mitar]

Thanks.