What happens if the specified index is not present in the series Python Pandas?

When accessing pandas Series elements using custom index values, you use the syntax series_name['index_value']. If the specified index exists, the corresponding data is returned. However, if the index is not present in the series, pandas raises a KeyError.

Example: Accessing Non-existent Index

Let's see what happens when we try to access an index that doesn't exist ?

import pandas as pd

my_data = [34, 56, 78, 90, 123, 45]
my_index = ['ab', 'mn', 'gh', 'kl', 'wq', 'az']
my_series = pd.Series(my_data, index=my_index)

print("The series contains following elements:")
print(my_series)
print("\nAttempting to access non-existent index 'mm':")
print(my_series['mm'])

The output shows the series data and then raises a KeyError ?

The series contains following elements:
ab     34
mn     56
gh     78
kl     90
wq    123
az     45
dtype: int64

Attempting to access non-existent index 'mm':
Traceback (most recent call last):
KeyError: 'mm'

Safe Methods to Handle Missing Index

Using get() Method

The get() method returns None for missing keys instead of raising an error ?

import pandas as pd

my_data = [34, 56, 78, 90, 123, 45]
my_index = ['ab', 'mn', 'gh', 'kl', 'wq', 'az']
my_series = pd.Series(my_data, index=my_index)

# Safe access using get()
result = my_series.get('mm')
print(f"Value for 'mm': {result}")

# With default value
result = my_series.get('mm', 'Index not found')
print(f"Value for 'mm' with default: {result}")

Value for 'mm': None
Value for 'mm' with default: Index not found

Using try-except Block

Handle the KeyError explicitly using exception handling ?

import pandas as pd

my_data = [34, 56, 78, 90, 123, 45]
my_index = ['ab', 'mn', 'gh', 'kl', 'wq', 'az']
my_series = pd.Series(my_data, index=my_index)

try:
    value = my_series['mm']
    print(f"Value found: {value}")
except KeyError:
    print("Index 'mm' not found in the series")

Index 'mm' not found in the series

Checking Index Existence

You can check if an index exists before accessing it ?

import pandas as pd

my_data = [34, 56, 78, 90, 123, 45]
my_index = ['ab', 'mn', 'gh', 'kl', 'wq', 'az']
my_series = pd.Series(my_data, index=my_index)

# Check if index exists
if 'mm' in my_series.index:
    print(f"Value: {my_series['mm']}")
else:
    print("Index 'mm' does not exist")
    
# Check existing index
if 'ab' in my_series.index:
    print(f"Value for 'ab': {my_series['ab']}")

Index 'mm' does not exist
Value for 'ab': 34

Comparison of Methods

Conclusion

When a specified index is not present in a pandas Series, accessing it directly raises a KeyError. Use get() method for safe access or check index existence with in operator to handle missing indices gracefully.