How to instantiate a class in python

Question

So, I'm trying to learn Python. It seems pretty easy, but evidently, I don't understand how classes are used. The following code gives me an error when I try to use the class.

class mystuff:
        def average(a,b,c): #get the average of three numbers
                result=a+b+c
                result=result/3
                return result

#now use the function average from the mystuff class
print mystuff.average(9,18,27)


File "class.py", line 7, in <module>
    print mystuff.average(9,18,27)
TypeError: unbound method average() must be called with mystuff instance as first argument (got int instance instead)

What's wrong?

Answer 1

I won't answer this question. Sorry to act like this, but it really won't help you.

Please read the tutorial on the Python website. There is a section on classes, but please read the whole thing. It should only take a few hours at the most.

I shall then be happy to answer any questions you have.

Answer 2

You can instantiate the class by declaring a variable and calling the class as if it were a function:

x = mystuff()
print x.average(9,18,27)

However, this won't work with the code you gave us. When you call a class method on a given object (x), it always passes a pointer to the object as the first parameter when it calls the function. So if you run your code right now, you'll see this error message:

TypeError: average() takes exactly 3 arguments (4 given)

To fix this, you'll need to modify the definition of the average method to take four parameters. The first parameter is an object reference, and the remaining 3 parameters would be for the 3 numbers.

Answer 3

In python member function of a class need explicit self argument. Same as implicit this pointer in C++. For more details please check out this page.

Answer 4

From your example, it seems to me you want to use a static method.

class mystuff:
  @staticmethod
  def average(a,b,c): #get the average of three numbers
    result=a+b+c
    result=result/3
    return result

print mystuff.average(9,18,27)

Please note that an heavy usage of static methods in python is usually a symptom of some bad smell - if you really need functions, then declare them directly on module level.

Answer 5

You never created an instance.

You've defined average as an instance method, thus, in order to use average you need to create an instance first.

Answer 6

You need to spend a little more time on some fundamentals of object-oriented programming.

This sounds harsh, but it's important.

• Your class definition is incorrect -- although the syntax happens to be acceptable. The definition is simply wrong.

• Your use of the class to create an object is entirely missing.

• Your use of a class to do a calculation is inappropriate. This kind of thing can be done, but it requires the advanced concept of a @staticmehod.

Since your example code is wrong in so many ways, you can't get a tidy "fix this" answer. There are too many things to fix.

You'll need to look at better examples of class definitions. It's not clear what source material you're using to learn from, but whatever book you're reading is either wrong or incomplete.

Please discard whatever book or source you're using and find a better book. Seriously. They've mislead you on how a class definition looks and how it's used.

You might want to look at http://homepage.mac.com/s_lott/books/nonprog/htmlchunks/pt11.html for a better introduction to classes, objects and Python.

Answer 7

Every function inside a class, and every class variable must take the self argument as pointed.

class mystuff:
    def average(a,b,c): #get the average of three numbers
            result=a+b+c
            result=result/3
            return result
    def sum(self,a,b):
            return a+b


print mystuff.average(9,18,27) # should raise error
print mystuff.sum(18,27) # should be ok

If class variables are involved:

 class mystuff:
    def setVariables(self,a,b):
            self.x = a
            self.y = b
            return a+b
    def mult(self):
            return x * y
    def sum(self):
            return self.x + self.y

 print mystuff.SetVariables(9,18) # Setting mystuff.x and mystuff.y
 print mystuff.mult() # should raise error
 print mystuff.sum()  # should be ok