Why is there a KEY ERROR in one place but no KEY ERROR in another similar statement?

Question

I made a PANDAS Series in the first line of the code below. But I can't understand why there is an error in the 3rd line but no error in the 2nd line. Please run the code in Jupyter in 3 separate blocks and help me if you understood the cause of error in 3rd line.

data = pd.Series([1,2,3,4,5], index=['a', 'b', 'c', 7, 8])
data['g']=5 # NO ERROR.
data[9] = 7 # WHY ERROR ??

Answer 1

You need to use the pandas loc() function to avoid this error:

import pandas as pd

data = pd.Series([1,2,3,4,5], index=['a', 'b', 'c', 7, 8])
data['g'] = 5
data.loc[9] = 7
print(data)

Output:

a    1
b    2
c    3
7    4
8    5
g    5
9    7
dtype: int64