Your current system has Neuralnetwork inheriting from data. What you are currently trying to do is best done in the __init__ method. You also really shouldn't do a print in the body of a class like this.
Python doesn't have the concept of private methods and function and as such everything can be accessed with dot notation. With that being said adding a _ to the start of a function/method/field will tell the users of your class/library that it should be treated as though it was private.
class Neuralnetwork(object):
def __init__(self, data):
self.data = data
print(self.data)
if __name__ == "__main__":
nuralnetwork1 = Neuralnetwork(3)
print(nuralnetwork1.data) # Prints the number 3 to the console
nuralnetwork2 = Neuralnetwork(2)
print(nuralnetwork2.data) # Prints the number 2 to the console
print(nuralnetwork1.data + nuralnetwork2.data) # Prints the number 5 to the console
class Neuralnetwork:orclass Neuralnetwork(object), further, the print statement should be inside the__init__(fix the identation).