How to Properly Capture Variables in Lambda Functions in Loops?

Capturing Variables in Lambda Functions

When using lambda functions within a loop, it's important to understand their variable scope. Unlike regular functions which copy local variables, lambda functions reference them.

Consider the given code:

<code class="python">for m in ('do', 're', 'mi'):
    funcList.append(lambda: callback(m))</code>

Here, the lambda function captures the value of m from the enclosing scope. However, after the loop finishes, m retains the last value ('mi'). When each lambda function is called, it references this shared m variable, resulting in the output "mi" multiple times.

To overcome this issue and ensure each lambda captures a distinct value of m, use a technique called "default arguments":

<code class="python">for m in ('do', 're', 'mi'):
    funcList.append(lambda m=m: callback(m))</code>

By making m a default parameter with the same name, each lambda captures its own instance of the variable, ensuring the expected output:

"do"
"re"
"mi"

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