Set a string as index of pandas DataFrame

Question

Given the dataframe df

df = pd.DataFrame([1,2,3,4])
print(df)
   0
0  1
1  2
2  3
3  4

I would like to modify it as

print(df)
   0
A  1
A  2
A  3
A  4

Why would you want an index that is not unique? This seems redundant to me. Additionally indexing will be problematic — EdChum
– EdChum, Commented Oct 4, 2017 at 14:31
On the face of it, this is a bad idea (See EdChum's comment). Giving you the benefit of the doubt, maybe this is a part in a larger process that you feel you need to accomplish in order to get the larger task completed. If that is True then there is certainly a way to better accomplish that task without doing this. Otherwise, we are just trying to accomplish parlor tricks. — piRSquared
– piRSquared, Commented Oct 4, 2017 at 14:35

Brad Solomon · Accepted Answer · 2017-10-04 14:29:42Z

11

In this specific case you can use:

df.index = ['A'] * len(df)

answered Oct 4, 2017 at 14:29

Brad Solomon

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Comments

Zero · Accepted Answer · 2017-10-04 14:34:49Z

6

Use set_index

In [797]: df.set_index([['A']*len(df)], inplace=True)

In [798]: df
Out[798]:
   0
A  1
A  2
A  3
A  4

answered Oct 4, 2017 at 14:29

Zero

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Don't use inplace=True, always be explicit and reassign. inplace does a copy anyway.

BENY · Accepted Answer · 2017-10-04 14:32:56Z

2

When you create the df, you can add it.

df = pd.DataFrame([1,2,3,4],index=['A']*4)
df
Out[325]: 
   0
A  1
A  2
A  3
A  4

answered Oct 4, 2017 at 14:32

BENY

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