why am i getting NameError when accessing a function

Question

I couldn't figure out why I'm getting a NameError when trying to access a function inside the class.

This is the code I am having a problem with. Am I missing something?

class ArmstrongNumber:

    def cubesum(num):
        return sum([int(i)**3 for i in list(str(num))])

    def PrintArmstrong(num):
        if cubesum(num) == num:
            return "Armstrong Number"
        return "Not an Armstrong Number"

    def Armstrong(num):
        if cubesum(num) == num:
            return True
        return False

[i for i in range(1000) if ArmstrongNumber.Armstrong(i)] # this return NameError

Error-message:

NameError                                 Traceback (most recent call last)
<ipython-input-32-f3d39f24a48c> in <module>
----> 1 ArmstrongNumber.Armstrong(153)

<ipython-input-31-fd21586166ed> in Armstrong(num)
     10 
     11     def Armstrong(num):
---> 12         if cubesum(num) == num:
     13             return True
     14         return False

NameError: name 'cubesum' is not defined

You should decorate `cubesum` as `@staticmethod` and then use `ArmstrongNumber.cubesum(...)`. That being said, your class doesn't make much sense. — Selcuk, Feb 06 '19 at 06:46
@Jeril Again, what is `self`? It is not a magical variable in Python unlike other languages. You have to specifically include it in the method signature. These are all static or class methods — Selcuk, Feb 06 '19 at 06:48
he needs to create an object of the class before calling its methods, then `self` will work — Vaibhav Vishal, Feb 06 '19 at 06:48
@VaibhavVishal I recommend you to read [this](https://stackoverflow.com/questions/2709821/what-is-the-purpose-of-self) question. — Selcuk, Feb 06 '19 at 06:50
@Selcuk I would really appreciate if you can correct where I could improve? — Rakesh, Feb 06 '19 at 06:51
Please also extract a [mcve] from your code before asking any question here at all, most of your code is useless for demonstration purposes. — Ulrich Eckhardt, Feb 06 '19 at 06:52
@Rakesh There is no point using a class here as you are not keeping any state in it. Your class is just a collection of static functions, which can also be written as unbound (i.e. outside any class) methods. — Selcuk, Feb 06 '19 at 06:52
@Selcuk Got it. It seems I have to learn more about classes in python. — Rakesh, Feb 06 '19 at 06:58
@Selcuk. these are not static or classmethod s. these are std methods unexpectedly referring to the instance with *num* instead of *self* — JL Peyret, Feb 06 '19 at 07:07
@JLPeyret Not quite. `num` is not an instance, it is an `int`. — Selcuk, Feb 06 '19 at 07:08

Sociopath · Accepted Answer · 2019-02-06T07:05:12.627

2

Use classname before method:

class ArmstrongNumber:

    def cubesum(num):
        return sum([int(i)**3 for i in list(str(num))])

    def PrintArmstrong(num):
        if ArmstrongNumber.cubesum(num) == num:
            return "Armstrong Number"
        return "Not an Armstrong Number"

    def Armstrong(num):
        if ArmstrongNumber.cubesum(num) == num:
            return True
        return False

print([i for i in range(1000) if ArmstrongNumber.Armstrong(i)])

Unlsess you pass self to the functions those functions are not instance methods. Even if you define that within class you still need to access them using classname.

edited Feb 06 '19 at 07:05

answered Feb 06 '19 at 06:46

Sociopath

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1

@AkshayNevrekar Correction: They **are** class methods (or static methods, depending on the decorator used). What you probably wanted to mean is that they are **not** instance methods. – Selcuk Feb 06 '19 at 06:53
1

If you do it like this then there is no point in using class for this case, best would be just use static methods – Nihal Feb 06 '19 at 06:55

Nihal · Answer 2 · 2019-02-06T07:08:03.217

this should be your actual solution if you really want to use class

class ArmstrongNumber(object):

    def cubesum(self, num):
        return sum([int(i)**3 for i in list(str(num))])

    def PrintArmstrong(self, num):
        if self.cubesum(num) == num:
            return "Armstrong Number"
        return "Not an Armstrong Number"

    def Armstrong(self, num):
        if self.cubesum(num) == num:
            return True
        return False


a = ArmstrongNumber()

print([i for i in range(1000) if a.Armstrong(i)])

output

[0, 1, 153, 370, 371, 407]

2nd method:

if you dont want to use class then use static methods like this

def cubesum(num):
    return sum([int(i)**3 for i in list(str(num))])


def PrintArmstrong(num):
    if cubesum(num) == num:
        return "Armstrong Number"
    return "Not an Armstrong Number"


def Armstrong(num):
    if cubesum(num) == num:
        return True
    return False


# a = ArmstrongNumber()

print([i for i in range(1000) if Armstrong(i)])

This is still pointless. Maybe move `num` into an `__init__` method then have methods like `.is_armstrong_number()`? — Selcuk, Feb 06 '19 at 07:07

why am i getting NameError when accessing a function

2 Answers2