Calling a class method raises a TypeError in Python


Question

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))

Error:

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?

1
71
12/27/2018 10:52:49 PM

Accepted Answer

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.

87
12/28/2008 11:48:05 PM

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.


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