singleton.py

def singleton(cls):
    instances = {}                                  # Line 2
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()                  # Line 5
        return instances[cls]
    return getinstance
 
class Counter:
    def __init__(self):
        self.count = 0
    def inc(self):
        self.count += 1

print type(Counter)    # <type 'classobj'>         # Line 15
Counter = singleton(Counter)
print type(Counter)    # <type 'function'>