Adventures in Machine Learning

Python Coding Styles: LBYL vs EAFP – Which Approach Should You Choose?

LBYL: Look Before You Leap

In programming, preventing errors is just as important as handling exceptional situations. One way to do this is by adopting the LBYL coding style.

This style is a defensive programming approach where you check for any possible errors or exceptions before executing a task. It involves conditional statements, primarily “if” statements, that check for errors before executing the code.

When using the LBYL style, you ensure that there are no surprises while running the code. Instead, you can predict the outcomes by checking the inputs before executing the code.

In Python programming, LBYL helps you to define a clear path of execution and avoid ambiguity.

For instance, consider a scenario where you need to perform a mathematical operation on a pair of values.

Before doing so, you would check if the input is an integer or not, to ensure the operation doesn’t break. Similarly, you can have conditional statements to verify if an input is not empty, a known value range, or format before proceeding with the execution.

EAFP: Easier to Ask Forgiveness Than Permission

On the other hand, the EAFP coding style is an alternate approach that emphasizes handling exceptional situations rather than preventing errors. This style is based on the concept of “try…except” statements in Python, where you first execute the code and then handle the exception if any.

The EAFP style is an efficient way of handling exceptional situations in Python programming. It enables you to create a flexible code that can handle unanticipated cases.

This coding style is the opposite of LBYL since it assumes that unexpected things can happen while executing the code. The EAFP style is ideal for handling file or network operations where the inputs are often unpredictable.

In such scenarios, setting up conditional statements for all possible inputs might be impractical or impossible. In such cases, the “try…except” statement comes in handy and allows you to perform the operation, keep track of the success/failure, and then handle the exception if any.

Pythonic Way to go: LBYL or EAFP? Python programmers often wonder which coding style, LBYL or EAFP, is the best approach.

In practice, both styles are equally important and depend on the specific use cases. LBYL is more effective when you know all possible use cases and want to prevent exceptions from happening.

This coding style can help you define how the code should be executed and avoid errors related to input format, type, or range.

EAFP, on the other hand, is more suitable when you have no control over the inputs or cannot predict exceptions beforehand.

In such situations, the code will run first and handle any exceptions that result from unexpected inputs or actions. In conclusion, both coding styles serve their respective purposes, and Python programmers should use them in the right scenarios.

The LBYL and EAFP in Python

Avoiding Unnecessary Check Repetition

The frequent repetition of checking input values can make the code cluttered. The LBYL coding style can be improved by avoiding such repetition.

One way to overcome this is by incorporating an “assert” statement. The assertion statement checks if the input values meet the required conditions, and if not, the code stops executing immediately.

For instance, suppose you wanted to check that two integer values, a and b, are greater than zero and take their product. Instead of using LBYL coding style, you can use the “assert” statement, which allows you to perform a single check on both values.

As shown below, the “assert” statement makes the code cleaner and easier to read. “`

def multiply_positive_numbers(a, b):

assert a > 0 and b > 0, ‘Inputs must be greater than zero’

return a * b


Improving Readability and Clarity

LBYL coding style can also improve the clarity and readability of the code. For instance, the “if” statements used in LBYL can include comments to further explain how the code works.

Including comments can help other programmers understand your code and follow the logic.

Additionally, LBYL can make the use of optional arguments in the code easier.

The optional argument is a value that can be included or excluded from the function. LBYL coding style can verify that the optional argument is valid before executing the function with or without the optional argument.

Avoiding Race Conditions

Multi-threaded environments can be tricky, especially when multiple threads, each with their own resource needs, access a computational resource. This scenario may lead to a race condition, where two or more threads modify a shared resource that results in unexpected behavior.

LBYL coding style can detect such scenarios and prevent useless or erroneous computations. You can use locks to control the thread’s access to the shared resources and coordinate the operations.

A lock ensures that only one thread can access the resource at any given time, resulting in predictable behavior.


In summary, the LBYL and EAFP coding styles are both essential in Python programming, depending on the specific use cases. LBYL is more effective when preventing errors from happening, while EAFP is more useful in handling unexpected exceptions.

To improve LBYL, programmers can avoid repetition, use assert statements, and include comments. Additionally, in multi-threaded environments, locks can prevent LBYL from causing race conditions.

By understanding both coding styles, Python programmers can write effective and readable code that can handle unexpected conditions.

3) Common Gotchas with LBYL and EAFP

While the LBYL and EAFP coding styles have their strengths, they can also lead to potential errors if not used efficiently. One of the most significant challenges of working with either LBYL or EAFP coding is that programmers may accidentally use one style of exception handling where the other could have been more efficient.

For instance, If a programmer uses EAFP to address a predictable condition, such as validating user data, this may result in an inefficient process since the code will try to execute before realizing that the input is invalid, leading to a slowdown. The opposite is also true when a programmer applies the LBYL coding style to erroneous scenarios.

This results in the performance significantly reducing as the code keeps executing checks before making advancements. It’s essential to apply the correct style consistently throughout the code to avoid performance issues and other errors.

Another challenge with LBYL is that it may still result in hidden errors or race conditions. For example, if you are performing a computation with two variable values that are subject to change by other expressions, checking the values before computing may be insufficient since the value might have changed between the checking variable and computing.

In such an instance, some improvements may include locking the data to prevent race conditions or using the EAFP coding style. On the other hand, a common concern with EAFP coding style is performance.

The try…except statement can be inefficient in cases where you have a large number of potential exceptions. If you need to iterate over a large amount of data, the overuse of try/except blocks can lead to significant slowdowns.

To avoid such instances, a code with minimal try…except statements in EAFP coding is advisable.

4) EAFP vs LBYL with Examples

Dealing with Too Many Errors or Exceptional Situations

When dealing with user data or other input validation, handling all errors might become a challenging and time-consuming task. In such cases, EAFP might be the way to go.

If there are many possible exceptions, every single one will be handled within a single try…except block.

Here is an example of utilizing EAFP when verifying user input:


def validate_user_input(user_data):


assert user_data[‘name’]

assert user_data[‘age’] > 0

assert user_data[’email’]

# additional validation can be checked here

except (KeyError, ValueError, TypeError):

print(“Invalid user information”)


The EAFP style ensures that all possible exceptions are handled within the same try…except block, which makes for more efficient operation.

Checking for Object Types and Attributes

When working with objects, it’s vital to verify the existence of a specific attribute or object type. LBYL coding style comes in handy in such cases.

For instance, suppose you wanted to access an attribute of an object and you’re unsure if the attribute exists. You can use an “if…else” statement with the hasattr() function to check for the object’s attribute before proceeding with the operation.


class Example:

def __init__(self, value):

self.value = value

example = Example(“hello world”)

if hasattr(example,’value’):



print(“Attribute not found”)


This LBYL code ensures that the attribute exists before attempting to access it, which helps users avoid attribute errors during runtime.

Working with Files and Directories

When working with files and directories, LBYL coding style maintains code clarity by checking for files and directories’ existence when required. For instance, using the os module and LBYL, a python programmer can check if a file or a directory exists before proceeding.


import os

file_path = “/path/to/the/file.txt”

if os.path.isfile(file_path):

# File exists, so proceed with an operation


# Handle the case where the file doesn’t exist


This LBYL coding style verifies the existence of a file before proceeding with the operation, increasing its efficiency.


In Python programming, both LBYL and EAFP coding styles are essential for writing efficient and effective code. While both styles have their strengths, it’s important to apply them under the right circumstances to avoid potential errors, race conditions, or efficiency problems.

By using the correct coding style, Python programmers can write clean, readable code that delivers results without causing system slowdowns or errors. 5)


Choosing between LBYL and EAFP coding styles comes down to the individual problem and developer preference. There is no one hard-and-fast rule for which coding style is the best option.

When trying to prevent errors, the LBYL coding style shines since it can define a clear path of execution and avoid ambiguity. LBYL is particularly useful when dealing with data validation tasks as it allows validations to take place before proceeding with an operation.

However, LBYL coding style might still result in hidden errors or race conditions, which means, In cases where you have a highly concurrent and multi-threaded environment, consider EAFP coding style as it allows you to handle unforeseen situations that you may have missed in the initial coding phase. In contrast, EAFP coding style shines in situations where unexpected inputs can occur, and Conditional statements might be impossible or impractical.

It is an excellent choice when handling file or network operations, where the input is often unpredictable and must be handled carefully.

However, EAFP might result in inefficient code due to relying on “try…except” blocks.

Here, decide based on the circumstances and ensure the desired functionality is not compromised. Python developers often find themselves working with both coding styles.

It is essential to recognize that both coding styles are equally valuable. They allow developers to write flexible, efficient, and robust code.

By knowing when LBYL or EAFP is best suited for specific scenarios, developers can streamline their workflow and write better code. In conclusion, as a Python developer, you should weigh the pros and cons of each coding style and choose a specific style based on the problem’s requirements, development goals, and workflow.

Ultimately, it’s essential to remember that there is no perfect approach to coding and that choosing the right style requires good decision-making and a good understanding of the problem to be solved. In conclusion, the LBYL and EAFP coding styles are both equally essential for preventing errors and handling exceptional situations in Python programming.

Both styles have their strengths, and the choice of which approach to use depends on the specific problem and developer preference. When trying to prevent errors, LBYL coding style is ideal, while EAFP is more suitable when handling unpredicted scenarios.

Python developers should weigh the pros and cons of each style before choosing the right approach, and applying the style consistently throughout the code will ensure optimal efficiency. Choosing the right coding style is essential to write clean, readable, and effective code that delivers results without causing system slowdowns or errors.