Constructors and classes in python

Question

I am novice to python and I am trying to learn the concept of classes. There are some some questions I want to ask:

Is it mandatory to write __init__(self) constructor?
How do you run a script that has classes?
In this example, it is throwing an error for self?

Code:

 #An example of a class
    class Shape:
        def __init__(self,x,y):
            self.x = x
            self.y = y
        description = "This shape has not been described yet"
        author = "Nobody has claimed to make this shape yet"
        def area(self):
            return self.x * self.y
        def perimeter(self):
            return 2 * self.x + 2 * self.y
        def describe(self,text):
            self.description = text
        def authorName(self,text):
            self.author = text
        def scaleSize(self,scale):
            self.x = self.x * scale
        self.y = self.y * scale

Error:

Traceback (most recent call last):
  File "C:/Python27/cls.py", line 2, in <module>
    class Shape:
  File "C:/Python27/cls.py", line 18, in Shape
    self.y = self.y * scale
NameError: name 'self' is not defined

Check the indent of the last line. Something looks "off", no? (In Python, self is just a parameter, and thus only applies to the given function scope which is determined by the indent.) — user2864740
– user2864740, Commented Feb 27, 2014 at 5:39

Christian Tapia · Accepted Answer · 2014-02-27 05:46:01Z

2

1) No, it's not mandatory. Constructor are usually used for initialization. If you implement a constructor or not, depends on what you are trying to do.

3) It seems like it's an indentation issue:

class Shape:
    def __init__(self,x,y):
        self.x = x
        self.y = y
        self.description = "This shape has not been described yet"
        self.author = "Nobody has claimed to make this shape yet"
    def area(self):
        return self.x * self.y
    def perimeter(self):
        return 2 * self.x + 2 * self.y
    def describe(self,text):
        self.description = text
    def authorName(self,text):
        self.author = text
    def scaleSize(self,scale):
        self.x = self.x * scale
        self.y = self.y * scale

edited Feb 27, 2014 at 5:46

answered Feb 27, 2014 at 5:38

Christian Tapia

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3 Comments

user3201916 Over a year ago

2. How to run a script that have classes and many functions in one go

Christian Tapia Over a year ago

The answer to that question can be found in any book (or tutorial) about OOP in Python.

Yogesh D Over a year ago

@user3201916 If the correct indentation solved your problem which should be the case here you should accept the answer.

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