Python static class variable in nested class

Question

I have a nested class that uses static vars to have class wide parameters and accumulators.

If I do it as a standalone class it works.

If I do a nested class and inherit the standalone class, it works.

But I can't get a nested class to have static class variables, the interpreter gets confused. What am I doing wrong?

Code snippet:

class Cl_static_parameter_standalone:
 #static var common to all instances. Two uses: common settings, common accumulator
 c_n_counter : int = 0
 @staticmethod
 def increment() -> int:
 Cl_static_parameter_standalone.c_n_counter += 1
 return Cl_static_parameter_standalone.c_n_counter
 
class Cl_some_class:
 class Cl_static_parameter_inherited(Cl_static_parameter_standalone):
 pass
 class Cl_static_parameter_nested:
 c_n_counter : int = 0
 @staticmethod
 def increment() -> int:
 Cl_static_parameter_nested.c_n_counter += 1
 return Cl_static_parameter_nested.c_n_counter
 def __init__(self):
 return
 def do_something(self):
 
 print(f"Execute Standalone: {Cl_static_parameter_standalone.increment()}")
 print(f"Execute Inherited: {self.Cl_static_parameter_inherited.increment()}")
 print(f"Execute Nested: {self.Cl_static_parameter_nested.increment()}")
 return
my_instance = Cl_some_class()
my_instance.do_something()

Output:

Execute Standalone: 1
Execute Inherited: 2 
Traceback (most recent call last):
 File "stack_overflow_class_static_parameter.py", line 52, in <module>
 my_instance.do_something()
 ~~~~~~~~~~~~~~~~~~~~~~~~^^
 File "stack_overflow_class_static_parameter.py", line 48, in do_something
 print(f"Execute Nested:{self.Cl_static_parameter_nested.increment()}") 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^ 
 File "stack_overflow_class_static_parameter.py", line 38, in increment
 Cl_static_parameter_nested.c_n_counter += 1 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
NameError: name 'Cl_static_parameter_nested' is not defined. Did you mean: 'Cl_static_parameter_standalone'?

Answer 1

You can use the classmethod decorator, which is more appropriate for this situation anyways:

class C:
 class Nested:
 counter = 0
 @classmethod
 def increment(cls) -> int:
 cls.counter += 1
 return cls.counter
print(C().Nested.increment()) # prints 1

If you are wondering why increment can't find Cl_static_parameter_nested in your example: you would have to write Cl_some_class.Cl_static_parameter_nested to access it from the global namespace.

• 05032 Mendicant Bias: @classmethod works great and it's a lot cleaner, thanks!
• 05032 Mendicant Bias: the inherited version has some wierd quirks anyway. It seems to copy the static var at first call, then use a separate version of it, making it's behaviour weird. Execute Standalone: 1 Execute Inherited: 2 Execute Nested: 1 Execute Standalone: 2 Execute Inherited: 3 Execute Nested: 2