Here is an example implementation of a singleton class in Python:

```python
class Singleton:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            print("Creating new instance")
            cls._instance = super().__new__(cls)
        else:
            print("Using existing instance")
        return cls._instance
```

In this implementation, the `_instance` variable keeps track of whether an instance of the class has already been created. The `__new__` method is called when an instance of the class is requested. If an instance has already been created, it returns that instance. Otherwise, it creates a new instance and returns that.