Is it possible for a python lambda function to update an object's attribute?

Question

Before I start, let me say I know what I am trying to do is unorthodox and perhaps people will have an aversion to the whole concept, however I am curious as to whether it is possible.

So here is the question.

Given an instance k of a class Kthat has an integer attribute z initialised to zero; can we call a lambda function from within k that modifies k.z?

An example would be simply incrementing k.z from 0 to 1.

I've played around a little to see if I could get it to work, and despite success I have included some of that code below.

class K():
  def __init__(self):
    self.z = 0
    self.functions = [
        lambda : "Returns string",
        lambda : self.z, # returns value of z
        # lambda : self.z+=1, # Fails to execute
    ]

k = K()
print(k.functions[0]())
print(k.functions[1]())
print(k.z)

John Forbes · Accepted Answer · 2020-04-16 05:27:07Z

2

lambda expressions can only have simple expressions inside of them. In this case, you could do:

lambda : setattr(self, 'z', self.z + 1)

But this is obviously very ugly, and gives you no benefit over:

def _update_z(): self.z += 1
self.functions = [
    lambda : "Returns string",
    lambda : self.z, # returns value of z,
    _update_z
]

edited Apr 16, 2020 at 5:27

John Forbes

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answered Apr 16, 2020 at 4:59

juanpa.arrivillaga

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5 Comments

John Forbes Over a year ago

In a case where there are many functions like _update_z, and they are only being called in one place in the same way, why is it better to separate each of these into their own functions? E.g. say there were 100 of these simple one line functions.

juanpa.arrivillaga Over a year ago

@JohnForbes what do you mean? They always are their own function. a lambda creates a function just like any other. In your particular case, you could go either way shown above. I'm just saying, if you can't get something to work with a lambda, just write a regular function definition statement, lambda is only sugar, it doesnt give you anything special

John Forbes Over a year ago

So I was trying to avoid excessive if branching, so i had implemented a switch like structure with a dictionary where given some signal from an event a particular function would fire. After refactoring most of these functions became very short so I was considering having them in place in the dictionary. Thanks for your thoughts, it is good to know what is possible.

juanpa.arrivillaga Over a year ago

@JohnForbes what? You can still put the function in a dictionary, I'm putting it in a list above...

John Forbes Over a year ago

Yeah, I saw that. I was thanking you for the demonstration.

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