Class assigning index to object using static class variable

Question

I have a class which represents an object to be kept in a set. I would like the class itself to remember how many it has created so that when you call SetObject() and __init__() a new object is created, which receives a unique index. Maybe something like this

class SetObject(object):
  # static class variable 
  object_counter = 0

  def __init__(self, params):
    self.params=params
    self.index = self.get_index()

  def get_index(self):
    object_counter += 1
    return object_counter-1

a = SetObject(paramsa)
b = SetObject(paramsb)
print a.index
print b.index

would produce

0
1

or something like this. Currently it seems that this approach gives a "variable referenced before assignment" error.

Remember that objects need to be hashable if they go in a set, and they may not be mutable. — Katriel
– Katriel, Commented Dec 17, 2012 at 8:58
Not actually true. The state used to compute the object's eq and hash must be immutable. If you don't override a class'eq and hash than the object's id is used to deduce the hash. And since the object's id never changes (even if it's attributes change) => you can use it in a set without any problems. — Ioan Alexandru Cucu
– Ioan Alexandru Cucu, Commented Dec 17, 2012 at 10:54

Ioan Alexandru Cucu · Accepted Answer · 2012-12-17 08:57:00Z

3

You need to write:

  def get_index(self):
     SetObject.object_counter += 1
     return SetObject.object_counter-1

otherwise it would only work if object_counter was a global variable.

answered Dec 17, 2012 at 8:57

Ioan Alexandru Cucu

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Comments

Martijn Pieters · Accepted Answer · 2012-12-17 10:26:46Z

2

You need to use a reference to the class to refer to it's variables; you could perhaps use a class method (with the @classmethod decorator), but there is really no need to.

Better use itertools.count() to get a fool-proof 'static' counter; no need to reassign back to the class attribute then:

import itertools

class SetObject(object):
   object_counter = itertools.count().next

   def __init__(self, params):
       self.params=params
       self.index = self.object_counter()

(code above assumes Python 2; on Python 3 iterables do not have a .next method and you'd need to use functools.partial(next, itertools.count()) instead).

Because the counter is an iterator, we don't need to assign to SetObject.object_counter at all. Subclasses can provide their own counter as needed, or re-use the parent class counter.

edited Dec 17, 2012 at 10:26

answered Dec 17, 2012 at 9:11

Martijn Pieters

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3 Comments

Duncan Over a year ago

Why not just use self.index = self.object_counter()? The class attribute is found automatically when there's no overriding instance attribute.

Duncan Over a year ago

I'm not complaining about assigning to self.index. All I'm saying is that calling self.__class__.object_counter() is (in the absence of an instance attribute) exactly the same as the simpler self.object_counter()

Martijn Pieters Over a year ago

@Duncan: Duh. Of course. Updated, thanks for waking me up this morning (need more caffeine, I guess).

Katriel · Accepted Answer · 2012-12-17 08:57:59Z

1

The line

object_counter += 1

translates to

object_counter = object_counter + 1

When you assign to a variable inside a scope (e.g. inside a function), Python assumes you wanted to create a new local variable. So it marks object_counter as being local, which means that when you try to get its value (to add one) you get a "not defined" error.

To fix it, tell Python where to look up object_counter. In general you can use the global or nonlocal keywords for this, but in your case you just want to look it up on the class:

self.__class__.object_counter += 1

answered Dec 17, 2012 at 8:57

Katriel

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3 Comments

Ioan Alexandru Cucu Over a year ago

I think SetObject.object_counter is cleaner than self.__class__.object_counter

Katriel Over a year ago

@IoanAlexandruCucu I don't: it hardcodes and duplicates the name of the class. That breaks inheritance (a class LoggedSetObject(SetObject), say) and refactoring (if you want to rename SetObject).

l4mpi Over a year ago

+1 for explaining what actually happens. But nonlocal only works in python 3, OP seems to be using python 2.

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