Infinite recursion using lambda in python

Question

I have a class. This class contains a function. I want to change this function in the same way every once in a while. If I use lambda I get infinite recursion. I understand why I get this, I want to find an elegant solution.

def func(s):
    return  1 # some not interesting function

class cls: # a class

    def __init__(self , f):
    self.f = f

c = cls(func)
c.f = lambda x: c.f(x) + 1 #  i want c.f to return c.f(x) + 1
print(c.f(1)) # causes infinite recursion

I don't want to do

c.f = lambda x: func(x) + 1

because I want to change c.f in the same way more than once.

mgilson · Accepted Answer · 2014-08-01 15:10:51Z

8

The infinite recursion is happening because inside the lambda function, c.f is resolved at call-time, so it's already the "new" lambda function instead of the original function passed to cls.__init__.

You could do something like this:

c.f = lambda x, f=c.f: f(x) + 1

Since the argument default is evaluated at the time the lambda function is created, it will be the original function on the class.

answered Aug 1, 2014 at 15:10

mgilson

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2 Comments

Yair Daon Over a year ago

This works, thanks. I can't see what this is doing though. Care to give explanations\ references?

mgilson Over a year ago

@YairDaon -- I did explain ... f=c.f gets evaluated when the lambda function is created, not when it is called, so the f that gets passed to the function is c.f (the original).

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