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Python Recursive Lambda Function Example

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A recursive lambda function in Python is something you are probably never going to have to do. You should probably look into other solutions first, but if you're curious like me you may want to try it. I ran into a situation where I was looping through objects at runtime and creating lambda functions as a wrapper function with my object as the default value. In my case I was linking a bunch of function calls to menus in PyQt. I wanted to connect a menu option to a function that ran all of the other menu options.

I decided to try to write a lambda function that worked similarly to a recursive function and worked back through the list of lambda functions that were created. It probably would have been better to just create a list of the lambda functions to call, but I wanted to see if I could make it work and keep all the code for the menus in the same place. I ended up with something similar to the code below. This gave me a function that would loop through and call all the previous lambda functions created in my loop.

def foo(x):
return x**2
r = lambda x = None: x
for i in range(10):
    f = lambda x = i:foo(x)
    r = lambda x = r, y = f:(y(), x())
print r()

In this case I've made a function foo that just squares a value and a for loop that will create ten lambda functions f() that each call this function for a different value of x. r() is my recursive function. The first r (r = lambda x = None: x) just returns None. I need this because all later iterations of r() call the previous r(). This could be done another way by putting an if statement in the r() lambda; I think this is more readable as an example though. All the other r() that are created in the group call the current f() and then call the previous r(). That's how the recursion happens. So in the end calling r() calls all the f()s that were created.

Last Updated on Thursday, 21 July 2011 23:06  


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