Singleton

One shared instance for the whole app

Singleton guarantees a class has exactly one instance and gives everyone a single point of access to it. The classic use is a shared resource: one app Config, one logger, one Stripe client that every part of PawWalk talks to.

Here is the honest part you rarely hear: in Python you almost never need the textbook singleton. A plain module is already a singleton — imported once, cached in sys.modules, shared everywhere. A module-level config = Config() gives you a shared instance with zero ceremony.