Ask Forgiveness, Not Permission: When Is This Programming Style Appropriate?

"Ask Forgiveness Not Permission" in Programming

The phrase "ask forgiveness not permission" is often used to describe a programming style that favors simplicity and robustness over correctness. This style is based on the idea that it is better to catch exceptions and handle them gracefully than to try to anticipate every possible failure and write code to prevent it.

Asking for Permission vs. Asking for Forgiveness

In the "ask for permission" style, code is written in a way that checks for conditions that may cause an exception before performing any action that could result in an exception. For example:

if os.path.exists(filename):
    with open(filename) as f:
        data = f.read()
else:
    raise FileNotFoundError(f"File {filename} not found.")

In the "ask for forgiveness" style, code is written to simply try to perform the action and catch any exceptions that may occur:

try:
    with open(filename) as f:
        data = f.read()
except FileNotFoundError:
    print(f"File {filename} not found.")

Why "Ask Forgiveness" Is Preferred

There are two main reasons why the "ask forgiveness" style is generally preferred over the "ask for permission" style:

1. Concurrency Issues:

In a concurrent environment, such as a multithreaded program or a program that interacts with external resources, the state of the system can change between the time when a permission check is made and the time when the action is performed. This can lead to incorrect results or exceptions being thrown unnecessarily.

2. Overly Restrictive Permission Checks:

Permission checks can be overly restrictive, preventing code from performing actions that are actually possible. For example, checking for file existence before opening it can fail if the file is created after the check but before the open operation.

When to Use "Ask Forgiveness"

The "ask forgiveness" style is appropriate when:

• The operation is likely to fail in predictable and recoverable ways.
• The cost of checking for permission is high.
• The permission check can change between the time of the check and the time of the operation.

Example

The example provided in the question demonstrates an appropriate use of the "ask for forgiveness" style. Checking for the presence of the bar attribute before accessing it can fail if the attribute is set later. Instead, the code should simply try to access the attribute and catch any AttributeError exception that may occur.

It is important to note that the "ask forgiveness" style does not mean that coders should be sloppy or write code that is prone to exceptions. Rather, it suggests that it is better to handle exceptions gracefully than to try to anticipate every possible failure.

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