CODEWITHSUNDEEP
python
vinita
vinita

Reputation: 597

Python methods in a class

So I saw this code somewhere.

1] So why are we returning self at the end of each method?what purpose does it serve?

2] How to interpret the method chaining (last line)- doug.set_legs(4).count_legs().sleep().sleep().sleep() 

class pet:
    number_of_legs = 0
    def sleep(self):
        print "zzz "
        return self
    def set_Legs(self, legs):
        self.number_of_legs = legs
        return self
    def count_legs(self):
        print "I have %s legs" % self.number_of_legs
        return self

class dog(pet):
    def bark(self):
    print "Woof"

doug = dog()
doug.set_Legs(4).count_legs().sleep().sleep().sleep() # Any explanation for this chaining & how its operating particularly in this case ??
>>>I have 4 legs
   ZZZ
   ZZZ
   ZZZ
   Out[249]: <__main__.dog instance at 0x7fd5d81917e8> # Why & from where this part getting printed ??

doug.set_legs(4).count_legs().sleep().set_legs(10).count_legs().sleep().sleep()
>>>I have 4 legs
   ZZZ
   I have 10 legs
   ZZZ
   ZZZ
   Out[242]: <__main__.dog instance at 0x7fd5d81917e8> # Why & from where this part getting printed ??

Upvotes: 0

Views: 126

Answers (4)

Stefan Seemayer
Stefan Seemayer

Reputation: 2067

This is an example of the Method Chaining pattern. While it's unusual in Python, it can generally be useful if you want to perform multiple actions on the same object. Because of this, it's most often seen in jQuery programming, e.g. for modifying elements on a webpage.

Since the sleep, set_legs and count_legs methods don't return anything of importance, you could just as well return the object the method was called on instead of not returning anything.

By this, if you want to do something else on the same object right after, you can compress it into one line of code.

To explain your line of code:

doug                  # the Dog object
    .set_Legs(4)      # call set_legs on doug, returning doug
    .count_legs()     # call count_legs on doug, returning doug
    .sleep()          # call sleep on doug, returning doug
    .sleep()          # call sleep on doug, returning doug
    .sleep()          # call sleep on doug, returning doug

The last printed line is your python interpreter showing the return value of the function, i.e. doug. You could call even more set_legs, count_legs, etc. on that.

The above code is equivalent to doing:

 doug.set_Legs(4)
 doug.count_legs()
 doug.sleep()
 doug.sleep()
 doug.sleep()

Upvotes: 2

Wjars
Wjars

Reputation: 99

1)

def sleep(self):
    print "zzz "
    return self

Your methods can return anything. The only obligation, for a method in a class, is to have 'self' as the first parameter. That's all. You have no reason to return the object. Some of your functions do not need any return statement! 

def sleep(self):
    print "zzz "

2) Interpret it in the way it's written: from left to right. Those previous returns were there for you to be able to chain this way (you chain on the return value).

Upvotes: 0

user2357112
user2357112

Reputation: 282026

Returning self is useful for method chaining, so you can do

doug.set_Legs(4).count_legs().sleep().sleep().sleep()

instead of

doug.set_Legs(4)
doug.count_legs()
doug.sleep()
doug.sleep()
doug.sleep()

This doesn't look like a very appropriate place to apply it. It can be useful when you want to apply a sequence of transformations to an object, like in jQuery.

As for the method chaining,

doug.set_Legs(4).count_legs().sleep().sleep().sleep()

is equivalent to

temp = doug.set_Legs(4)
temp = temp.count_legs()
temp = temp.sleep()
temp = temp.sleep()
temp = temp.sleep()

which, since all the methods here return self, is equivalent to

doug.set_Legs(4)
doug.count_legs()
doug.sleep()
doug.sleep()
doug.sleep()

Upvotes: 1

mmmmmm
mmmmmm

Reputation: 32720

Both questions are related. The methods return self which is the object you are working on and this allows you to chain methods as the result of the first method is the object you call the second method on.

e.g.

doug = dog()
doug.set_Legs(4).count_legs().sleep()

is equivalent to

doug = dog()
doug.set_Legs(4)
doug.count_legs()
doug.sleep()

Upvotes: 2

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