Add a value to the end of a pandas index object

Question

I have a pandas index object and I'd like to add a single value to the end of it. The .append() method doesn't seem to work like one would expect, and since I'm trying to add an element, I can't insert at the location of -1 because that puts the value in the second-to-last position. For example

import pandas as pd
ser = pd.Series([1,2,3,4,5], index=[11,12,13,14,15])
indx = ser.index

Say I want to add the value 20 to the end of the index. This throws an error:

indx.append(20)

This returns [11,12,13,14,20,15]:

indx.insert(-1, 20)

This works but seems like a work-around:

indx.insert(len(indx), 20)

Is there something I'm missing? This is on pandas 0.18.1. Thanks.

Answer 1

The method append takes another index as input, but union will work if you simply pass an array-like object:

indx.union([20])

Note that index objects in pandas are immutable, so any such operation will return a new index rather than modifying the existing one.

Answer 2

You need to pass a collection of index values as parameter while appending to the given index object.

indx.append(pd.Index([20]))   # Pass the values inside the list 
Int64Index([11, 12, 13, 14, 15, 20], dtype='int64')

Answer 3

You may want to try these two options:

import pandas as pd
import numpy as np
ser.append(pd.Series([np.nan], index = [20]))

# 11    1.0
# 12    2.0
# 13    3.0
# 14    4.0
# 15    5.0
# 20    NaN
# dtype: float64

ser.set_value(20, np.nan)

# 11    1.0
# 12    2.0
# 13    3.0
# 14    4.0
# 15    5.0
# 20    NaN
# dtype: float64