Scope of Lambda Functions and Their Parameters

Lambda functions, introduced in Python as concise anonymous functions, offer convenience in various scenarios. However, understanding their parameter scoping can be crucial to avoid unexpected behavior. Let's explore a common issue that arises when using lambda functions within loops.

The Problem

Consider the following code, which aims to create a list of callback functions for GUI events:

def callback(msg):
    print msg

# Incorrect approach using an iterator
funcList = []
for m in ('do', 're', 'mi'):
    funcList.append(lambda: callback(m))

# Correct approach creating one at a time
funcList = []
funcList.append(lambda: callback('do'))
funcList.append(lambda: callback('re'))
funcList.append(lambda: callback('mi'))

# Execute the callback functions
for f in funcList:
    f()

When executed, the code unexpectedly prints:

mi
mi
mi
do
re
mi

Instead of the expected output:

do
re
mi
do
re
mi

The Explanation

Understanding the scope of lambda functions and their parameters is crucial here. Unlike regular functions, lambda functions reference the surrounding environment in which they are created. This reference includes variables used within the lambda's body.

When using an iterator in the incorrect approach, for each element m in the loop, it creates a new lambda function that references the same variable m. However, after the loop completes, the variable m references the last element in the loop, i.e., 'mi'. Therefore, when the callback functions are executed, they all print 'mi' using the updated reference.

The Solution

To resolve this issue, we can capture the value of the parameter m at the time the lambda function is created by using it as the default argument of an optional parameter:

for m in ('do', 're', 'mi'):
    funcList.append(lambda m=m: callback(m))

Now, within each lambda function, m is captured as a local variable, and its value is preserved when the loop completes. Consequently, when the callback functions are executed, they print the correct values, resulting in the expected output.