__slots__ in Python: Save some space and prevent member variable additions

Today I’m going to be writing about a feature of Python I’d never read before, namely __slots__. In a nutshell, using __slots__ allows you to decrease the memory needed by your classes, as well as prevent unintended assignment to new member variables.

By default, each class has a dictionary which it uses to map from attribute names to the member variable itself. Dictionaries are extremely well designed in Python, yet by their very nature they are somewhat wasteful of space. Why is this? Hash tables strive to minimize collisions by ensuring that the load factor (number of elements/size of internal array) does not get too high. In general hash tables use O(n) space, but with a constant factor nearer to 2 than 1 (again, in order to minimize collisions). For classes with very small numbers of member variables, the overhead might be even greater.

class DictExample:
  def __init__(self):
    self.int_var = 5
    self.list_var = [0,1,2,3,4]
    self.nested_dict = {'a':{'b':2}}

# Note that this extends from 'object'; the __slots__ only has an effect
# on these types of 'new' classes
class SlotsExample(object):
  __slots__ = ('int_var','list_var','nested_dict')

  def __init__(self):
    self.int_var = 5
    self.list_var = [0,1,2,3,4]
    self.nested_dict = {'a':{'b':2}}

# jump to the repl
>>> a = DictExample()
# Here is the dictionary I was talking about.
>>> a.__dict__
{'int_var': 5, 'list_var': [0, 1, 2, 3, 4], 'nested_dict': {'a': {'b': 2}}}
>>> a.x = 5
# We were able to assign a new member variable
>>> a.__dict__
{'x': 5, 'int_var': 5, 'list_var': [0, 1, 2, 3, 4], 'nested_dict': {'a': {'b': 2}}}



>>> b = SlotsExample()
# There is no longer a __dict__ object
>>> b.__dict__
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AttributeError: 'SlotsExample' object has no attribute '__dict__'
>>> b.__slots__
('int_var', 'list_var', 'nested_dict')
>>> getattr(b, 'int_var')
5
>>> getattr(a, 'int_var')
5
>>> a.x = 5
# We cannot assign a new member variable; we have declared that there will only
# be member variables whose names appear in the __slots__ iterable
>>> b.x = 5
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AttributeError: 'SlotsExample' object has no attribute 'x'

Note that for the __slots__ declaration to have any effect, you must inherit from object (i.e. be a ‘new style class’). Furthermore, if you extend a class with __slots__ defined, you must also declare __slots__ in that child class, or else it will have a dict allocated, obviating the space savings. See this StackOverflow question for more.

This feature was useful to me when using Python to implement a packed binary message format. The specification spells out in exquisite detail how each and every byte over the wire must be sent. By using the __slots__ mechanism, I was able to ensure that the client could not accidentally modify the message classes and add new member variables, which would not be serialized anyways.