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    <p>Let’s take a simple vector class:</p> <pre><code>class Vector: def __init__(self, x, y): self.x = x self.y = y </code></pre> <p>We want to have a method which calculates the length. What would it look like if we wanted to define it inside the class?</p> <pre><code> def length(self): return math.sqrt(self.x ** 2 + self.y ** 2) </code></pre> <p>What should it look like when we were to define it as a global method/function?</p> <pre><code>def length_global(vector): return math.sqrt(vector.x ** 2 + vector.y ** 2) </code></pre> <p>So the whole structure stays the same. How can me make use of this? If we assume for a moment that we hadn’t written a <code>length</code> method for our <code>Vector</code> class, we could do this:</p> <pre><code>Vector.length_new = length_global v = Vector(3, 4) print(v.length_new()) # 5.0 </code></pre> <p>This works because the first parameter of <code>length_global</code>, can be re-used as the <code>self</code> parameter in <code>length_new</code>. This would not be possible without an explicit <code>self</code>.</p> <hr> <p>Another way of understanding the need for the explicit <code>self</code> is to see where Python adds some syntactical sugar. When you keep in mind, that basically, a call like</p> <pre><code>v_instance.length() </code></pre> <p>is internally transformed to</p> <pre><code>Vector.length(v_instance) </code></pre> <p>it is easy to see where the <code>self</code> fits in. You don't actually write instance methods in Python; what you write is class methods which must take an instance as a first parameter. And therefore, you’ll have to place the instance parameter somewhere explicitly.</p>
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    1. POWhat is the purpose of self?
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    1. USKevin Johnsrude
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    1. COVector.length_new = length_global... I actually started to use syntax like this in my class declarations. Whenever I only want to inherit some of the methods from another class, I just explicitly copy the reference to the methods.
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      1. USJeeyoung Kim
    2. COwould it be fair to say that python's "instance method" is simply a syntactic sugar of static global methods (as in Java or C++) with an instance object passed in to package multiple attributes? --- well this is kind of half-true since in polymorphism, the more important purpose of "this" (as in java) or "self" is to give u the correct implementation of methods. Python does have this. so calling myobj.someMethod() is equal to TheClassOfMyObj.someMethod(myobj) in python. note that the "TheClassOfMyObj" is automatically figured out by python from "self", otherwise u'd have to find that out.
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      1. USteddy teddy
    3. COInfact, not only are instance methods just class methods, but methods are just functions which are members of a class, as the `Vector.length_new = length_global` shows.
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