LookupError

In Python, LookupError is a built-in exception that serves as the base class for exceptions raised when a key or index used on a mapping or sequence is invalid.

This exception isn’t meant to be directly raised in your code. Instead, it provides a foundation for more specific lookup-related exceptions, such as IndexError and KeyError. The standard library does raise it directly in one case: codecs.lookup() and related functions like codecs.getencoder() raise LookupError when the requested encoding doesn’t exist.

As a developer, you should use LookupError when you want to catch all exceptions related to invalid lookups in mappings or sequences, allowing you to handle them in a unified way.

LookupError Can Be Used When

  • Catching all lookup-related exceptions, such as IndexError and KeyError, in a single try … except block
  • Handling errors when accessing elements in lists, tuples, or dictionaries
  • Creating custom exceptions that need to indicate lookup failures by subclassing LookupError
An Introduction to Python Exceptions

Tutorial

Python Exceptions: An Introduction

In this beginner tutorial, you'll learn what exceptions are good for in Python. You'll see how to raise exceptions and how to handle them with try ... except blocks.

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By Leodanis Pozo Ramos • Updated Aug. 15, 2026
Python’s Built-in Exceptions Share Feedback