Is it possible to increment/access a class variable from inside a constructor in Python?

Question

I want to keep track via a counter, every time a class is instantiated. I was trying to do this by incrementing a counter defined as a class variable from inside the class' consructor ala:

class Cl:
   cntr = 0
   def __init__(self):
      cntr += 1

However, when I instantiate the class, I get an "UnboundLocalError: local variable 'cntr' referenced before assignement" I'm assuming this is because the constructor is treating it as a local variable. How do I reference the class variable from inside a method or constructor in Python 2.7?

Answer 1

You just have to call the variable through the class:

class Cl:
    cntr = 0
    def __init__(self):
        Cl.cntr += 1  # <---Like this


print(Cl().cntr)  # prints 1
print(Cl().cntr)  # prints 2
print(Cl().cntr)  # prints 3

Answer 2

class Foo:
    n = 0
    def __init__(self):
        self._increment()

    @classmethod
    def _increment(cls):
        cls.n += 1

f1 = Foo()
f2 = Foo()

>>> f1.n
2

>>> f2.n
2