Why Do Python Lambda Functions Capture the Final Value of a Loop Variable, and How Can This Be Fixed?

Understanding Closure Capture in Lambda Functions

Lambda functions, a concise and convenient tool in programming, allow for anonymous functions stored in closures. In Python, closures capture the enclosing scope to gain access to external variables, but this behavior can be surprising at times.

Consider a scenario where a set of lambda functions are initialized within a loop to add a specific number to their input. Intuitively, one might expect these functions to retain the value of the loop variable at the time of their creation. However, the actual result is different - all lambda functions capture the final value of the loop variable.

This happens because closures in Python capture the enclosing scope, not the references to specific variables or objects. In other words, the lambda functions effectively embed a reference to the loop variable i, which changes as the loop progresses. Consequently, when any of these functions are called, they utilize the final value of i, leading to unexpected outputs.

Enforcing Value Capture in Lambda Functions

To circumvent this issue and capture the intended value of the loop variable, it is necessary to force the capture using a parameter with a default value. By assigning the variable to a parameter within the lambda function and supplying a default value of its intended value, it is possible to preserve the desired capture behavior.

Example:

# Initialize an array of lambda functions
adders = [None, None, None, None]

# Create lambda functions with forced capture of i
for i in [0, 1, 2, 3]:
    adders[i] = lambda a, i=i: i + a

# Call a lambda function with input
print(adders[1](3))  # Output: 4

Conclusion

By understanding the capture mechanism in lambda functions and utilizing this technique, it is possible to control the values captured by closures, ensuring the desired behavior in code execution.

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