How would you implement the Singleton pattern in Python, and what are the caveats of doing so?
Singleton in Python: module-level state, new, or decorator, with caveats
The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. In Python, the simplest and most Pythonic way to achieve this is a module-level instance, because modules are singletons by nature: they are imported once and cached in sys.modules. If you need a class-based singleton, you can override new to return a cached instance, use a metaclass to control instantiation, or use a decorator. Each approach has caveats. The pattern introduces global mutable state, which makes testing harder and hides dependencies. It can be thread-unsafe unless you add a lock. It complicates inheritance and can be broken by multiple interpreters or processes. In most cases, dependency injection is a better choice for testability and flexibility.
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Module-level instance: define a module with a single instance and import it. This is the most Pythonic and avoids metaclass complexity.
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Override new: check if an instance exists and return it. Must handle thread safety with a lock.
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Metaclass: override call to control instantiation. More complex but works with inheritance.
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Decorator: wrap the class to return a single instance. Simple but can break isinstance.
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Caveats: global mutable state, hard to test, hides dependencies, not thread-safe unless locked, and breaks with multiple interpreters or processes.
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Trade-off: singletons are convenient but often a design smell. Prefer dependency injection for testability.
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Common mistake: using a singleton for a database connection without pooling, which serializes access.
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Version note: Python's module system already provides singletons. No language-level singleton exists.
Scenario Questions
0-2 years experience
- 1What is a singleton and why might you want one?
- 2How do you create a module-level singleton in Python?
2-5 years experience
- 1You need a configuration object that is shared across your application. How do you implement it without a singleton?
- 2A singleton is causing test failures because state leaks between tests. How do you fix it?
5-8 years experience
- 1You have a singleton logger that is used everywhere. How do you make it testable?
- 2You need thread-safe lazy initialization for a singleton. How do you implement it?
8+ years experience
- 1Design a system that avoids singletons entirely while still providing shared configuration and resources.
- 2Compare the singleton pattern with dependency injection and module-level state on testability, concurrency, and maintainability.
Follow-up Questions
- How would you test a class that depends on a singleton?
- What is the difference between a singleton and a Borg (monostate) pattern?