LBYL

LBYL, or look before you leap, is a coding style that emphasizes checking for conditions before executing a particular action. In Python, this often means performing explicit checks to ensure that an operation can be safely executed.

LBYL can be useful when you want to avoid exceptions and ensure that your code runs smoothly by validating conditions upfront. However, it can sometimes lead to more verbose and less Pythonic code. It can also lead to issues in contexts where race conditions can occur between the check and the action.

When using LBYL, you typically use conditional statements to verify that the conditions are suitable for an operation. This approach is often used when dealing with situations where exceptions are costly, or where conditions can be easily checked without performance penalties.

Example

Here’s an example of the LBYL approach in action:

Python 
>>> likes = {"pet": "dog", "color": "blue", "fruit": "apple"}
>>> key = "pet"

>>> if key in likes:
...     print(f"Your favorite {key} is {likes[key]}")
... else:
...     print(f"You don't have a favorite {key}")
...
Your favorite pet is dog

In this example, we check if the key exists in the dictionary before attempting to access it. This prevents KeyError exceptions and allows you to handle the situation gracefully.

LBYL vs EAFP: Preventing or Handling Errors in Python

Tutorial

LBYL vs EAFP: Preventing or Handling Errors in Python

In this tutorial, you'll learn about two popular coding styles in Python: look before you leap (LBYL) and easier to ask forgiveness than permission (EAFP). You can use these styles to deal with errors and exceptional situations in your code. You'll dive into the LBYL vs EAFP discussion in Python.

intermediate best-practices python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated Dec. 19, 2024 • Reviewed by Dan Bader
Python Glossary Share Feedback