为类变量python赋值

Question

I have the following class which has two class variables the "value" and the "gradient".I assign two values to the class variables and then print them out. But then I assign 20 to a class variable("limit") that doesn't exist. but the compiler is happy and it seems that stores the variable. How is this possible? My assumption was that if the variable name was incorrect I should see an error message. shouldn't I ?

class wire:
    def __init__(self,value,gradient):
        self.value     = value
        self.gradient  = gradient

XX = wire(10,0.05)
print XX.value , XX.gradient

XX.limit = 20
print XX.value , XX.gradient, XX.limit

Thanks for your help

Answer 1

These are instance variables, not class variables.

But I don't know why you expect an error. There is no difference between defining those attributes in __init__ or anywhere else. Either way, you're adding new attributes to the instance.