Object Oriented example in Python

Question

I know its dumb question but i am trying to grasp the concepts of OOP in Python. Suppose i want to write the program for factorial in Procedural form, i would do something like this

def factorial(n):
    num = 1
    while n >= 1:
        num = num * n
        n = n - 1
    return num

f = factorial(3)
print f # 6

Now i want to rewrite the same factorial program in OO way. i am not getting how to write this same function using objects and classes.

class Factorial():
    def fact(n):
        num = 1
        while n >= 1:
            num = num * n
            n = n - 1
        return num

f = Factorial()
a = f.fact(3)
print a # TypeError: fact() takes exactly 1 argument (2 given)

I know this can be done more precisely in Functional way by using lambdas and other things, but i am learning the concepts of OOP. What i am doing wrong ?

thefourtheye · Accepted Answer · 2013-12-23T07:00:11.490

When you are calling an instance method, by default the current object is passed as the first parameter to the function. In your case,

def fact(n):

when you call it like this

a = f.fact(3)

it will be actually treated like this

a = fact(f, 3)

The reason why the current object is passed is, to let the instance method make changes to the object. (Remember Python doesn't have any other keyword like this, like in some other languages).

But your function's signature doesn't match this (it expects only one parameter). That's why it is throwing

# TypeError: fact() takes exactly 1 argument (2 given)

To fix this, you have to change the signature to something like this

def fact(self, n):

Now, the self parameter will receive the current object being passed.

score 2 · Answer 2 · edited May 23 '17 at 11:43

2

You forgot the self parameter:

class Factorial():
    def fact(self,n): #Forgot the self parameter huh?
        num = 1
        while n >= 1:
            num = num * n
            n = n - 1
        return num

I recommend you read these too:

edited May 23 '17 at 11:43

Community

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answered Dec 23 '13 at 06:42

K DawG

13,287
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Ugh, i got it. This **self** thing is confusing. – ajknzhol Dec 23 '13 at 06:44
Also might want to consider: class Factorial(Object): It works without doing that though. – Tony Dec 23 '13 at 06:49

score 1 · Answer 3 · answered Dec 24 '13 at 00:06

1

I would also recommend these article for reading:

An Introduction to Object-Oriented Concepts in Python, Part 1

Unfortunately I cannot put more that 2 links But I'm sure you'll find other 4 parts of these.

answered Dec 24 '13 at 00:06

STK

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Object Oriented example in Python

3 Answers3