问题
The pandas pivot tables documentation seems to recomend dealing with more than two dimensions of data by using multiindexing:
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: import pandas.util.testing as tm; tm.N = 3
In [4]: def unpivot(frame):
...: N, K = frame.shape
...: data = {'value' : frame.values.ravel('F'),
...: 'variable' : np.asarray(frame.columns).repeat(N),
...: 'date' : np.tile(np.asarray(frame.index), K)}
...: return pd.DataFrame(data, columns=['date', 'variable', 'value'])
...:
In [5]: df = unpivot(tm.makeTimeDataFrame())
In [6]: df
Out[6]:
date variable value value2
0 2000-01-03 A 0.462461 0.924921
1 2000-01-04 A -0.517911 -1.035823
2 2000-01-05 A 0.831014 1.662027
3 2000-01-03 B -0.492679 -0.985358
4 2000-01-04 B -1.234068 -2.468135
5 2000-01-05 B 1.725218 3.450437
6 2000-01-03 C 0.453859 0.907718
7 2000-01-04 C -0.763706 -1.527412
8 2000-01-05 C 0.839706 1.679413
9 2000-01-03 D -0.048108 -0.096216
10 2000-01-04 D 0.184461 0.368922
11 2000-01-05 D -0.349496 -0.698993
In [7]: df['value2'] = df['value'] * 2
In [8]: df.pivot('date', 'variable')
Out[8]:
value value2 \
variable A B C D A B
date
2000-01-03 -1.558856 -1.144732 -0.234630 -1.252482 -3.117712 -2.289463
2000-01-04 -1.351152 -0.173595 0.470253 -1.181006 -2.702304 -0.347191
2000-01-05 0.151067 -0.402517 -2.625085 1.275430 0.302135 -0.805035
variable C D
date
2000-01-03 -0.469259 -2.504964
2000-01-04 0.940506 -2.362012
2000-01-05 -5.250171 2.550861
I thought that xarray was made for handling multidimensional datasets like this:
In [9]: import xarray as xr
In [10]: xr.DataArray(dict([(var, df[df.variable==var].drop('variable', 1)) for var in np.unique(df.variable)]))
Out[10]:
<xarray.DataArray ()>
array({'A': date value value2
0 2000-01-03 0.462461 0.924921
1 2000-01-04 -0.517911 -1.035823
2 2000-01-05 0.831014 1.662027, 'C': date value value2
6 2000-01-03 0.453859 0.907718
7 2000-01-04 -0.763706 -1.527412
8 2000-01-05 0.839706 1.679413, 'B': date value value2
3 2000-01-03 -0.492679 -0.985358
4 2000-01-04 -1.234068 -2.468135
5 2000-01-05 1.725218 3.450437, 'D': date value value2
9 2000-01-03 -0.048108 -0.096216
10 2000-01-04 0.184461 0.368922
11 2000-01-05 -0.349496 -0.698993}, dtype=object)
Is one of these approaches better than the other? Why hasn't xarray completely replaced multiindexing?
回答1:
There does seem to be a transition to xarray for doing work on multi-dimensional arrays. Pandas will be depreciating the support for the 3D Panels data structure and in the documentation even suggest using xarray for working with multidemensional arrays:
'Oftentimes, one can simply use a MultiIndex DataFrame for easily working with higher dimensional data.
In addition, the xarray package was built from the ground up, specifically in order to support the multi-dimensional analysis that is one of Panel s main use cases. Here is a link to the xarray panel-transition documentation.'
From the xarray documentation they state their aims and goals:
xarray aims to provide a data analysis toolkit as powerful as pandas but designed for working with homogeneous N-dimensional arrays instead of tabular data...
...Our target audience is anyone who needs N-dimensional labelled arrays, but we are particularly focused on the data analysis needs of physical scientists – especially geoscientists who already know and love netCDF
The main advantage of xarray over using straight numpy is that it makes use of labels in the same way pandas does over multiple dimensions. If you are working with 3-dimensional data using multi-indexing or xarray might be interchangeable. As the number of dimensions grows in your data set xarray becomes much more manageable. I cannot comment on how each performs in terms of efficiency or speed.
来源：https://stackoverflow.com/questions/42876278/when-to-use-multiindexing-vs-xarray-in-pandas