self in Python classes

Question

I know first argument in Python methods will be an instance of this class. So we need use "self" as first argument in methods. But should we also specify attribures (variables) in method starting with "self."?

My method work even if i don't specify self in his attributes:

class Test:
  def y(self, x):
    c = x + 3
    print(c)

t = Test()
t.y(2)
5

and

class Test:
  def y(self, x):
    self.c = x + 3
    print(self.c)

t = Test()
t.y(2)
5

For what i would need specify an attribute in methods like "self.a" instead of just "a"?

In which cases first example will not work but second will? Want to see situation which shows really differences between two of them, because now they behave the same from my point of view.

Answer 1

The reason you do self.attribute_name in a class method is to perform computation on that instances attribute as opposed to using a random variable.For Example

class Car:
  def __init__(self,size):
      self.size = size

  def can_accomodate(self,number_of_people):
      return self.size> number_of_people

  def change_size(self,new_size):
      self.size=new_size

   #works but bad practice      

   def can_accomodate_v2(self,size,number_of_people):
       return size> number_of_people

 c = Car(5)
 print(c.can_accomodate(2))
 print(c.can_accomodate_v2(4,2))

In the above example you can see that the can_accomodate use's self.size while can_accomodate_v2 passes the size variable which is bad practice.Both will work but the v2 is a bad practice and should not be used.You can pass argument into a class method not related to the instance/class for example "number_of_people" in can_accomodate funtion. Hope this helps.