Python - class attributes, methods and parameters

Question

I am starting to study class in python, and am trying to get my head around the concepts of attributes, methods and parameters in OOP.

I am working with 3 examples:

---

example 1

   class Clock(object):
        def __init__(self, time):
        self.time = time
        def print_time(self):
        time = '6:30'
        print self.time

with:

clock = Clock('5:30')
clock.print_time()

It prints 5:30

---

example 2

class Clock(object):
    def __init__(self, time):
    self.time = time
    def print_time(self, time):
    print time

with:

clock = Clock('5:30')
clock.print_time('10:30')

It prints 10:30.

---

example 3

  class Clock(object):
        def __init__(self, time):
        self.time = time
        def print_time(self):
        print self.time

finally, with:

boston_clock = Clock('5:30')
paris_clock = boston_clock
paris_clock.time = '10:30'
boston_clock.print_time()

It prints 10:30

could please someone explain me how attributes, methods and parameters are being bound to objects in these examples?

Answer 1

Attributes are the variables within a class or instance. In something like this the variable hello is an attribute of the class Hi.

class Hi:
  hello = "Hello World!"

Methods are functions within the class, so for something like this, function greet is a method of the class Hi.

class Hi:
  def greet(self):
    pass

Parameters are input(s) that go into a method. So the string, "Hello World!" is a parameter of the method say in the class Hi.

class Hi:
  def say(self, saying):
    print(saying)

Hi().say("Hello World!")

There's a nice question on the Software Engineering StackExchange site about OOPL. Explaining OOP Concepts to a non technical person.

Answer 2

This link here can explain more about class attributes/methods/parameters.

However, I do understand how complicated these concepts are, so I will answer your question (although in the future, try to ask a more specific question!).

In example one:

init (the initializer) and print_time are both class attributes. When you initialize the clock variable and pass in the parameter '5:30', it accesses the init function and hits the self.time = time line of code. Since time is accessed using a dot notation, time is an INSTANCE attribute (specific to the individual object).

When you call self.print_time(), the time there is a local variable specific to the function call, therefore the instance attribute is not changed. That is why when you print self.time it is still 5:30.

In example two:

In this case, the init and print_time functions are both class attributes (similar to the example above). The initialization of the clock object is the same as above. However, when it calls print_time, time is a the parameter '10:30', and therefore when we just print time (notice we did not use any dot notation), it prints only the local variable of '10:30'.

In example three:

init and print_time are both class attributes, same as the above two examples. When you initialize the boston_clock object, it is similar to both example one and two. Then you assign the name paris_clock to the object boston_block (notice that paris_clock and boston_clock are just names pointing to the same object, like how I could have two names). Therefore when we execute the line of code paris_clock.time = '10:30', the INSTANCE attribute of this single object is changed to '10:30'. However, since boston_clock is pointing to the same object as paris_clock, boston_clock's time attribute is also '10:30'.