DataFrame.interpolate() extrapolates over trailing missing data

[grahamjeffries]

See also the discussion at StackOverflow.

Linear interpolation on a series with missing data at the end of the array will overwrite trailing missing values with the last non-missing value. In effect, the function extrapolates rather than strictly interpolating.

Example:

import pandas as pd
import numpy as np

a = pd.Series([np.nan, 1, np.nan, 3, np.nan])
a.interpolate()

Yields (note the extrapolated 4):

0   NaN
1     1
2     2
3     3
4     4
5     4
dtype: float64

not

0   NaN
1     1
2     2
3     3
4     4
5     NaN
dtype: float64

I believe the fix is something along the lines of changing lines 1545:1546 in core/common.py from

result[firstIndex:][invalid] = np.interp(inds[invalid], inds[valid], yvalues[firstIndex:][valid])

to

result[firstIndex:][invalid] = np.interp(inds[invalid], inds[valid], yvalues[firstIndex:][valid], np.nan, np.nan)

[jreback]

pls show a complete reproducible example (e.g. copy-pastable code).

Then if you would like to do a pull-request would be great. These examples serve as the basis for a test, which should fail w/o a fix and pass after.

[TomAugspurger]

Traveling back today. I can take a look this weekend.

I'd like to see what the behavior was before I refactored this stuff.

[jreback]

@TomAugspurger can you circle back on this?

[TomAugspurger]

OK, so this is the same behavior as back in 0.11 before I refactored all the interpolate stuff.

>>> pd.__version__
>>> s = pd.Series([np.nan, 1, np.nan, 3, np.nan])
>>> s
0   NaN
1     1
2   NaN
3     3
4   NaN
dtype: float64
>>> s.interpolate()
0   NaN
1     1
2     2
3     3
4     3
dtype: float64

I'll look into adding an argument to handle the NaNs before and after. The default will have to stay the same for now, I think. Possibly switch to the "correct' default of not extrapolating later on.

[Jezzamonn]

is there a work around for now?

[jluttine]

@Jezzamonn One workaround solution: http://stackoverflow.com/questions/25255496/dataframe-interpolate-extrapolates-over-trailing-missing-data/33390872#33390872

[cancan101]

Any updates on this?

[jreback]

@cancan101 there is a closed PR (not merge) #8010 / #8013 which I believe was almost there. If you want to rebase and see where it is would be great.

[jorisvandenbossche]

Given that the filling of the trailing values does not follow the specified method, but just forward fills, I think we could consider this as a bug. However, of course, still a bug that people could rely upon, so not sure whether we should just change the behaviour.

[relonger]

This is definitely a bug. All new panda users will find this behaviour as confusing and error-prone (as I just did). If there is a code that rely on this bug - that's mean there is a bug in that code also. You should fix it.
Interpolate - means interpolate, not extrapolate in any way.

[jreback]

You should fix it.

@relonger welcome to have a PR for this.

this PR actually does provide for this option: #16513

welcome to have a look at it, seems stalled.

[willweil]

Just curious if there any updates on this issue? 'Cause as in pandas 0.20.3 this is still a puzzling question. See StackOverflow.

[jreback]

see
35812ea

might be able to close this issue

[willweil]

@jreback Thanks for the link. But I just tried one of the test examples in commit 35812ea and I didn't get the expected result as in the test:

>>> pd.__version__
 '0.20.3'
>>> s = pd.Series([nan, nan, 3, nan, nan, nan, 7, nan, nan])
>>> s
0    NaN
1    NaN
2    3.0
3    NaN
4    NaN
5    NaN
6    7.0
7    NaN
8    NaN
dtype: float64
>>> s.interpolate(method='linear', limit_area='inside')
0    NaN
1    NaN
2    3.0
3    4.0
4    5.0
5    6.0
6    7.0
7    7.0
8    7.0
dtype: float64

Any ideas? Should I try a newer version of pandas?

EDIT:
Also tried in a newer version of pandas '0.22.0' but still didn't get the expected results. The pandas document says the "limit_area" is new feature in version 0.21.0+. Any ideas?

>>> pd.__version__
'0.22.0'

[willweil]

@jreback UPDATE: limit_area works as expected in pandas 0.23.0+, but not in 0.21.0 or 0.22.0. Maybe the pandas document has a typo as it marks limit_area as "New in version 0.21.0."?

[jreback]

yeah it looks like a typo; this change is in 0.23

would love a PR to update!

[simonjayhawkins]

yeah it looks like a typo; this change is in 0.23

would love a PR to update!

xref #25418