DataFrame.ffill behaves different than DataFrame.interpolate(method='ffill') along axes

[EVaisman]

It looks like df.ffill(axis=0) has the same behavior as test_df.interpolate(method='ffill', axis=1).

from pandas.util.testing import assert_frame_equal
import numpy as np
import pandas as pd

n = np.nan
test_df = pd.DataFrame([[0, 2, n, n],
                        [1, n, 4, 6],
                        [n, 3, 5, n]])

assert_frame_equal(
    test_df.interpolate(method='ffill', axis=1),
    test_df.ffill(axis=0),
)

Is this the desired behavior?

In [2]: pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 20.3.1
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.2
pytz: 2016.3
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5
matplotlib: 1.4.3
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.7.3
boto: 2.39.0

[jorisvandenbossche]

I suppose you mean the axis=1 vs axis=0 to obtain the same result? (or is there something else you didn't expect?).
At first sight, that seems like a bug to me.

For other methods (eg the default 'linear'), the direction is as expected (axis=0 -> filling per column), but for method='ffill' it is swapped.

In [77]: test_df
Out[77]:
     0    1    2    3
0  0.0  2.0  NaN  NaN
1  1.0  NaN  4.0  6.0
2  NaN  3.0  5.0  NaN

In [78]: test_df.interpolate()
Out[78]:
     0    1    2    3
0  0.0  2.0  NaN  NaN
1  1.0  2.5  4.0  6.0
2  1.0  3.0  5.0  6.0

In [81]: test_df.interpolate(axis=0, method='linear')
Out[81]:
     0    1    2    3
0  0.0  2.0  NaN  NaN
1  1.0  2.5  4.0  6.0
2  1.0  3.0  5.0  6.0

In [82]: test_df.interpolate(axis=0, method='ffill')
Out[82]:
     0    1    2    3
0  0.0  2.0  2.0  2.0
1  1.0  1.0  4.0  6.0
2  NaN  3.0  5.0  5.0

Also have to note that this behaviour is already there since the beginning (tested 0.13), but the ability to specify filling methods in interpolate is also not documented, so that is maybe the reason there weren't any bug reports.

@TomAugspurger Is there a reason for this behaviour, or is it just a bug?

[EVaisman]

You are correct Joris, that's what I was referring to. 

[edit: remove email history]

[TomAugspurger]

interpolate shares a code path with fillna here, where we try one of the fillna methods, and if the method isn't valid e.g. linear we try an interpolate method.

Might need to flip the axis argument here.
@EVaisman interested in submitting a fix and some tests?

[EVaisman]

Yes! But will probably have to wait until this weekend.

[jreback]

hmm, we should not be accepting the fill methods directly in .interpolate I don't think. (or if we do, then they should be tested / listed). So let's make this an error.