How to select columns from groupby object in pandas?

Question

I grouped my dataframe by the two columns below

df = pd.DataFrame({'a': [1, 1, 3],
                   'b': [4.0, 5.5, 6.0],
                   'c': [7L, 8L, 9L],
                   'name': ['hello', 'hello', 'foo']})
df.groupby(['a', 'name']).median()

and the result is:

            b    c
a name            
1 hello  4.75  7.5
3 foo    6.00  9.0

How can I access the name field of the resulting median (in this case hello, foo)? This fails:

df.groupby(['a', 'name']).median().name

Asked By: user248237

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Source

Answer 1

You need to get the index values, they are not columns. In this case level 1

df.groupby(["a", "name"]).median().index.get_level_values(1)

Out[2]:

Index([u'hello', u'foo'], dtype=object)

You can also pass the index name

df.groupby(["a", "name"]).median().index.get_level_values('name')

as this will be more intuitive than passing integer values.

You can convert the index values to a list by calling tolist()

df.groupby(["a", "name"]).median().index.get_level_values(1).tolist()

Out[5]:

['hello', 'foo']

Answered By: EdChum

Answer 2

You can also reset_index() on your groupby result to get back a dataframe with the name column now accessible.

import pandas as pd
df = pd.DataFrame({"a":[1,1,3], "b":[4,5.5,6], "c":[7,8,9], "name":["hello","hello","foo"]})
df_grouped = df.groupby(["a", "name"]).median().reset_index()
df_grouped.name
 0    hello
 1      foo
 Name: name, dtype: object

If you perform an operation on a single column the return will be a series with multiindex and you can simply apply pd.DataFrame to it and then reset_index.

Answered By: cwharland

Answer 3

Set as_index = False during groupby

df = pandas.DataFrame({"a":[1,1,3], "b":[4,5.5,6], "c":[7,8,9], "name":["hello","hello","foo"]})
df.groupby(["a", "name"] , as_index = False).median()

Answered By: proutray

Answer 4

Using reset_index() after the group by will do the trick:

df = pd.DataFrame({'a': [1, 1, 3],
                   'b': [4.0, 5.5, 6.0],
                   'c': ['7L', '8L', '9L'],
                   'name': ['hello', 'hello', 'foo']})
df.groupby(['a', 'name']).median().reset_index().name

here is the result:

 0    hello
 1      foo
 Name: name, dtype: object

and if you want the list of the values, you can simply:

df = pd.DataFrame({'a': [1, 1, 3],
                   'b': [4.0, 5.5, 6.0],
                   'c': ['7L', '8L', '9L'],
                   'name': ['hello', 'hello', 'foo']})

df.groupby(['a', 'name']).median().reset_index().name.values

The result of using values will be a list containing the values for the name column. The code above returns the following list as the results:

array(['hello', 'foo'], dtype=object)

Answered By: Mina

How to select columns from groupby object in pandas?

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