Setting values with multiindex in pandas

Question

There are already a couple of questions on SO relating to this, most notably this one, however none of the answers seem to work for me and quite a few links to docs (especially on lexsorting) are broken, so I'll ask another one.

I'm trying do to something (seemingly) very simple. Consider the following MultiIndexed Dataframe:

import pandas as pd; import random
arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'],
      ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]

tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
df = pd.concat([pd.Series(np.random.randn(8), index=index), pd.Series(np.random.randn(8), index=index)], axis=1)

Now I want to set all values in column 0 to some value (say np.NaN) for the observations in category one. I've failed with:

df.loc(axis=0)[:, "one"][0] = 1 # setting with copy warning

and

df.loc(axis=0)[:, "one", 0] = 1

which either yields a warning about length of keys exceeding length of index, or one about a lack of lexsorting to sufficient depth.

What is the correct way to do this?

jezrael · Accepted Answer · 2016-03-14 14:07:32Z

I think you can use loc with tuple for selecting MultiIndex and 0 for selecting column:

import pandas as pd; 
import random
arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'],
      ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]

#add for testing
np.random.seed(0)
tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
df = pd.concat([pd.Series(np.random.randn(8), index=index), pd.Series(np.random.randn(8), index=index)], axis=1)

print df
                     0         1
first second                    
bar   one     1.764052 -0.103219
      two     0.400157  0.410599
baz   one     0.978738  0.144044
      two     2.240893  1.454274
foo   one     1.867558  0.761038
      two    -0.977278  0.121675
qux   one     0.950088  0.443863
      two    -0.151357  0.333674

df.loc[('bar', "one"), 0] = 1
print df
                     0         1
first second                    
bar   one     1.000000 -0.103219
      two     0.400157  0.410599
baz   one     0.978738  0.144044
      two     2.240893  1.454274
foo   one     1.867558  0.761038
      two    -0.977278  0.121675
qux   one     0.950088  0.443863
      two    -0.151357  0.333674

If you need set all rows in level second with value one use slice(None):

df.loc[(slice(None), "one"), 0] = 1
print df
                     0         1
first second                    
bar   one     1.000000 -0.103219
      two     0.400157  0.410599
baz   one     1.000000  0.144044
      two     2.240893  1.454274
foo   one     1.000000  0.761038
      two    -0.977278  0.121675
qux   one     1.000000  0.443863
      two    -0.151357  0.333674

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