CODEWITHSUNDEEP
pythonpython-3.xclassmethods
stu
stu

Reputation: 8805

Why do I get a TypeError if I try to call a method directly from the class, supplying all arguments?

When I try this code:

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))

I get an error that says:

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 does this mean? Why can't I call average this way?

Upvotes: 71

Views: 145307

Answers (8)

cmoman
cmoman

Reputation: 341

To minimally modify your example, you could amend the code to:

class myclass(object):
        def __init__(self): # this method creates the class object.
                pass

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


mystuff=myclass()  # by default the __init__ method is then called.      
print mystuff.average(a,b,c)

Or to expand it more fully, allowing you to add other methods.

class myclass(object):
        def __init__(self,a,b,c):
                self.a=a
                self.b=b
                self.c=c
        def average(self): #get the average of three numbers
                result=self.a+self.b+self.c
                result=result/3
                return result

a=9
b=18
c=27
mystuff=myclass(a, b, c)        
print mystuff.average()

Upvotes: 13

Rob123
Rob123

Reputation: 1095

Try this:

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)

or this:

class mystuff:
    def average(self,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)

Upvotes: 1

lmount
lmount

Reputation: 1302

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  # This line will raise an error
    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

Upvotes: 4

Suraj
Suraj

Reputation: 4767

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.

Upvotes: 1

S.Lott
S.Lott

Reputation: 391852

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.

Upvotes: 1

recursive
recursive

Reputation: 86084

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.

Upvotes: -2

rob
rob

Reputation: 37644

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.

Upvotes: 34

Ryan
Ryan

Reputation: 1012

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.

Upvotes: 88

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