How to Avoid Variable Closure When Creating Lambdas Inside Loops?

Creating Lambdas Inside a Loop without Variable Closure

When creating lambdas inside a loop that iterate over a list of objects, it is important to prevent variable closure. Variable closure occurs when a function retains access to a variable outside of its immediate scope. This can lead to unexpected behavior, such as all lambdas referencing the same final value of a variable.

The Problem:

The following code demonstrates the problem:

lambdas_list = []
for obj in obj_list:
    lambdas_list.append(lambda: obj.some_var)

for f in lambdas_list:
    print(f())

When invoking the lambdas in this code, all lambdas will produce the same result: the value of obj.some_var from the last object in obj_list. This is because the lambdas all refer to the same obj variable, which changes with each iteration of the loop.

The Solution:

To prevent variable closure, we can use the syntax lambda x=x: where x is the variable we want to capture. This captures a copy of obj and makes it local to each lambda:

lambdas_list = []
for obj in obj_list:
    lambdas_list.append(lambda obj=obj: obj.some_var)

for f in lambdas_list:
    print(f())

Now, each lambda will reference the correct obj and produce the expected value for obj.some_var.

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