Should I Use Instance Attributes or Class Attributes for Singleton Instances in Python?

Prefer Instance Attributes for Singleton Instances

In scenarios where only a single instance of a class is necessary, the choice between using instance attributes and class attributes warrants consideration. While both approaches effectively store attributes, they have subtle differences in terms of efficiency and scope.

Class Attributes

Class attributes are defined outside instance methods and are shared among all instances of the class. Using class attributes for a singleton instance can lead to redundancy, as the same data will be present in every instance. Additionally, modifying a class attribute will affect all instances.

Instance Attributes

Instance attributes, on the contrary, are defined within the constructor (__init__) and are unique to each instance. For a singleton, this approach ensures that the attributes are initialized only once and stored within the sole instance. This results in a small performance advantage over class attributes due to the reduced lookup depth.

Recommendation

In situations where a class will only have a single instance, it is recommended to use instance attributes. This provides efficiency in accessing the data and eliminates the risk of unintended modifications affecting multiple instances.

Example

Consider the following Python code that defines a class with attributes that will only be used by a single instance:

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

  def action(self, request):
    pass

In this example, using instance attributes ensures that there will be only one set of attributes for the sole instance of the MyController class, providing optimal efficiency and data isolation.

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