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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?
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You cannot call them explicitly. For instance, when you create a new object, Python automatically calls the __new__ method, which in turn calls the __init__ method. The __str__ method is called when you print() an object.
The __call__ method enables Python programmers to write classes where the instances behave like functions and can be called like a function. When the instance is called as a function; if this method is defined, x(arg1, arg2, ...) is a shorthand for x. __call__(arg1, arg2, ...) .
In Python, the syntax for instantiating a new class instance is the same as the syntax for calling a function. There's no new needed: we just call the class. When we call a function, we get its return value. When we call a class, we get an “instance” of that class.
The __add__() method in Python specifies what happens when you call + on two objects. When you call obj1 + obj2, you are essentially calling obj1. __add__(obj2). This works, because int implements the __add__() method behind the scenes.
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.
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