ENH: allow multiple rolling quantiles in one pass

[StephenKappel]

Currently, rolling_quantile() accepts only a single float for the quantile argument. I find myself wanting to compute multiple quantiles over the same data. Instead of doing three calls to rolling_quantile(), I'd like to be able to call rolling_quantile() once with a sequence of floats as the quantile argument and get back a list of results. This has benefits both in terms of code conciseness and efficiency.

This suggested behavior would be analogous to how np.percentile works. http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html

Currently:

>>> import pandas as pd
>>> ser = pd.Series(np.array([1,2,3,4,5,6,7,8,9]))
>>> ser.rolling(window=3).quantile(quantile=0.5)
0   NaN
1   NaN
2     2
3     3
4     4
5     5
6     6
7     7
8     8
dtype: float64

Desired enhancement:

>>> ser.rolling(window=3).quantile(quantile=[0.25,0.5,1])
[0   NaN
1   NaN
2     1
3     2
4     3
5     4
6     5
7     6
8     7
dtype: float64, 0   NaN
1   NaN
2     2
3     3
4     4
5     5
6     6
7     7
8     8
dtype: float64, 0   NaN
1   NaN
2     3
3     4
4     5
5     6
6     7
7     8
8     9
dtype: float64]

[jreback]

Dataframe/Series.quantile has this behavior so not w/o precedent

[jreback]

would return a Dataframe not a list of Series

[max-sixty]

My humbled view is unless this is a vectorized routine that is materially faster than a python list comprehension, it's muddies the API without that much benefit.

A list comprehension really isn't that verbose:
pd.DataFrame({q:ser.rolling(window=3).quantile(quantile=q) for q in [0.25,0.5,1])}

[kawochen]

yes this would only need one pass of the data, so much faster than list comp

[max-sixty]

yes this would only need one pass of the data, so much faster than list comp

Then 👍. That's not what quantile does now FWIW: https://github.com/pydata/pandas/blob/723a147a4a197960d4a9da01dd5f107d59aafc46/pandas/core/series.py#L1313

[kawochen]

Yes in the rolling version the skip list construction can be shared. I will probably work on switching to the C implementation next, and see if I can squeeze this change in as well.

[jreback]

@kawochen ok I'll mark this as a perf enhancer, implementing directly in the cython would be best.