Class Variables or Instance Variables: Which is Best for a Singleton Class in Python?

Class Variables vs. Instance Variables for Single-Instance Classes

When creating Python classes with only one required instance, you may face the dilemma of where to store the class attributes: in class variables or instance variables. Both options have their pros and cons.

Instance Variables

Instance variables are specific to each instance of a class. They are defined within the __init__ method, as in the second code example provided:

class MyController(Controller):

  def __init__(self):
    self.path = "something/"
    self.children = [AController, BController]

  def action(self, request):
    pass

Pros:

• Faster access as there is no need to inherit attributes from the class.
• No confusion over which attributes belong to which instance.

Cons:

• Duplicate attributes for each instance, which can be wasteful if the attributes are large or complex.

Class Variables

Class variables, on the other hand, are shared among all instances of a class. They are defined outside of the __init__ method, as in the first code example provided:

class MyController(Controller):

  path = "something/"
  children = [AController, BController]

  def action(self, request):
    pass

Pros:

• Single source of truth for attributes, reducing potential errors and inconsistencies.
• No memory overhead associated with duplicate attributes.

Cons:

• Slightly slower access due to the additional level of inheritance.
• Potential confusion if instances need to have different values for the same attribute.

Recommendation

If you know for certain that you will only ever have one instance of a class, it is generally recommended to use instance variables. This provides slightly faster access and eliminates any potential confusion or problems with duplicate attributes.

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