October 13, 2024

Python Tutorials

Python KeyError

A KeyError in Python occurs when you attempt to access a dictionary key that does not exist. It is a common exception that indicates the key you’re trying to access is not found in the dictionary. This guide will help you understand the KeyError and how to handle it.

1. Understanding KeyError

A KeyError is raised when a dictionary is accessed with a key that is not present in the dictionary. Here’s an example:

my_dict = {'name': 'Alice', 'age': 25}

# Accessing an existing key
print(my_dict['name'])  # Output: Alice

# Accessing a non-existing key
print(my_dict['address'])  # Raises KeyError: 'address'

2. Handling KeyError with try-except

You can handle a KeyError using a try-except block to catch the exception and handle it gracefully:

my_dict = {'name': 'Alice', 'age': 25}

try:
    print(my_dict['address'])
except KeyError:
    print("Key not found in the dictionary.")

3. Using dict.get() Method

The dict.get() method allows you to access dictionary keys without raising a KeyError. If the key is not found, it returns a default value:

my_dict = {'name': 'Alice', 'age': 25}

# Accessing an existing key
print(my_dict.get('name'))  # Output: Alice

# Accessing a non-existing key with a default value
print(my_dict.get('address', 'Not Available'))  # Output: Not Available

4. Checking if a Key Exists

Before accessing a key, you can check if it exists in the dictionary using the in keyword:

my_dict = {'name': 'Alice', 'age': 25}

if 'address' in my_dict:
    print(my_dict['address'])
else:
    print("Key not found in the dictionary.")

5. Example: Safe Dictionary Access

Here’s an example demonstrating safe access to a dictionary:

def safe_access(dictionary, key):
    return dictionary.get(key, 'Key not found')

my_dict = {'name': 'Alice', 'age': 25}

print(safe_access(my_dict, 'name'))     # Output: Alice
print(safe_access(my_dict, 'address'))  # Output: Key not found

6. Summary

A KeyError occurs when trying to access a dictionary key that does not exist. You can handle this exception using try-except blocks, the dict.get() method, or by checking if the key is present before accessing it. Understanding and handling KeyError helps make your code more robust and user-friendly.

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