pysingleton

0.2.1 verified Mon Apr 27 auth: no python maintenance

A minimal Python library providing a @singleton decorator to turn classes into singletons. Version 0.2.1, released Oct 2014. It is in maintenance mode with rare updates.

pip install pysingleton

Common errors

error ImportError: No module named pysingleton ↓

cause Library not installed, or wrong Python environment.

fix

Run 'pip install pysingleton' and ensure the correct Python interpreter is active.

error TypeError: 'str' object is not callable when using @singleton ↓

cause Forgot to call the decorator; used @singleton instead of @singleton().

fix

Use @singleton (no parentheses) as it directly accepts the class.

error AttributeError: 'MyClass' object has no attribute '_instance' ↓

cause Manually accessing _instance before instantiation.

fix

Call MyClass() to create the singleton instance first; then use MyClass()._instance.

Warnings

gotcha Locking in __init__: The singleton decorator does not protect against concurrent instantiation. In multithreaded environments, __init__ may be called multiple times. ↓

fix Add a threading lock inside the class's __new__ method or use a thread-safe singleton pattern.

gotcha Subclassing singletons: Subclassing a singleton class may lead to unexpected behavior. The subclass is not automatically a singleton. ↓

fix Avoid subclassing singletons; apply @singleton to each class individually.

deprecated Python 2 support: The library was built for Python 2 and may have compatibility issues with Python 3 (e.g., metaclass syntax). ↓

fix Use Python 3 compatible libraries like 'singledispatch' or custom metaclass for production code.

Imports

singleton
```
from pysingleton import singleton
```
The decorator is at module level.

Quickstart

Apply @singleton to a class to ensure only one instance exists.

from pysingleton import singleton

@singleton
class MyClass:
    pass

a = MyClass()
b = MyClass()
assert a is b
print("Singleton works")