Why Do Lambda Functions in Python Loops Sometimes Fail to Bind Properly?

Lambda Functions within Loops

In Python, lambda functions provide a concise way to define anonymous functions. However, when used within loops, they can lead to unexpected behavior if proper binding is not ensured.

Consider the following code snippet:

directorys = {'login': <object>, 'home': <object>}
for d in directorys:
    self.command["cd " + d] = (lambda : self.root.change_directory(d))

The intent is to create a dictionary mapping command strings to functions that change the directory. However, the result is unexpected: the two lambda functions created are identical and execute the same command, changing the directory to "login."

Why Does This Happen?

Lambda functions within a loop share the same function scope. This means that the variable captured by the lambda function is the same for all iterations of the loop. In this case, "d" represents the value of the last item in the "directorys" dictionary, which is "login."

Solving the Issue

To ensure proper binding, we need to ensure that each lambda function has access to its own unique value of "d." One approach is to pass "d" as a parameter with a default value:

lambda d=d: self.root.change_directory(d)

By setting the default value of "d" to the current value of "d" in each iteration of the loop, we create a unique closure for each lambda function. Alternatively, we can use additional closure techniques to achieve the same result:

(lambda d=d: lambda: self.root.change_directory(d))()

This outer lambda function creates a closure for "d" and the inner lambda function uses "d" from the closure.

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