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Date: Mon, 29 Oct 2012 15:20:02 +0000
Subject: Immutability and Python
From: andrea crotti <andrea.crotti.0@gmail.com>
To: python-list <python-list@python.org>
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Xref: csiph.com comp.lang.python:32383

I have a philosofical doubt about immutability, that arised while doing
the SCALA functional programming course.

Now suppose I have a simple NumWrapper class, that very stupidly does:

class NumWrapper(object):
    def __init__(self, number):
        self.number = number

and we want to change its state incrementing the number, normally I
would do this

    def increment(self):
        self.number += 1


But the immutability purists would instead suggest to do this:

    def increment(self):
        return NumWrapper(self.number + 1)


Now on one hand I would love to use only immutable data in my code, but
on the other hand I wonder if it makes so much sense in Python.

My impression is that things get more clumsy in the immutable form, for
example in the mutable form I would do simply this:

number = NumWrapper(1)
number.increment()

while with immutability I have to do this instead:
new_number = number.increment()

But more importantly normally classes are way more complicated than my
stupid example, so recreating a new object with the modified state might
be quite complex.

Any comments about this? What do you prefer and why?