Why doesn't pandas reindex() operate in-place?

Question

From the reindex docs:

Conform DataFrame to new index with optional filling logic, placing NA/NaN in locations having no value in the previous index. A new object is produced unless the new index is equivalent to the current one and copy=False.

Therefore, I thought that I would get a reordered Dataframe by setting copy=False in place (!). It appears, however, that I do get a copy and need to assign it to the original object again. I don't want to assign it back, if I can avoid it (the reason comes from this other question).

This is what I am doing:

import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.rand(5, 5))

df.columns = [ 'a', 'b', 'c', 'd', 'e' ]

df.head()

Outs:

          a         b         c         d         e
0  0.234296  0.011235  0.664617  0.983243  0.177639
1  0.378308  0.659315  0.949093  0.872945  0.383024
2  0.976728  0.419274  0.993282  0.668539  0.970228
3  0.322936  0.555642  0.862659  0.134570  0.675897
4  0.167638  0.578831  0.141339  0.232592  0.976057

Reindex gives me the correct output, but I'd need to assign it back to the original object, which is what I wanted to avoid by using copy=False:

df.reindex( columns=['e', 'd', 'c', 'b', 'a'], copy=False )

The desired output after that line is:

          e         d         c         b         a
0  0.177639  0.983243  0.664617  0.011235  0.234296
1  0.383024  0.872945  0.949093  0.659315  0.378308
2  0.970228  0.668539  0.993282  0.419274  0.976728
3  0.675897  0.134570  0.862659  0.555642  0.322936
4  0.976057  0.232592  0.141339  0.578831  0.167638

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Why is copy=False not working in place?

Is it possible to do that at all?

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Working with python 3.5.3, pandas 0.23.3

Answer 1

reindex is a structural change, not a cosmetic or transformative one. As such, a copy is always returned because the operation cannot be done in-place (it would require allocating new memory for underlying arrays, etc). This means you have to assign the result back, there's no other choice.

df = df.reindex(['e', 'd', 'c', 'b', 'a'], axis=1)  

Also see the discussion on GH21598.

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The one corner case where copy=False is actually of any use is when the indices used to reindex df are identical to the ones it already has. You can check by comparing the ids:

id(df)
# 4839372504

id(df.reindex(df.index, copy=False)) # same object returned 
# 4839372504

id(df.reindex(df.index, copy=True))  # new object created - ids are different
# 4839371608