[Tutor] constructor

Dave Angel davea at ieee.org
Mon Apr 5 00:45:29 CEST 2010



Shurui Liu (Aaron Liu) wrote:
> I am studying about how to create a constructor in a Python program, I
> don't really understand why the program print out "A new critter has
> been born!" and "Hi.  I'm an instance of class Critter." twice. I
> guess is because "crit1 = Critter()     crit2 = Critter()"  But I
> don't understand how did computer understand the difference between
> crit1 and crit2? cause both of them are equal to Critter(). Thank you!
>
> # Constructor Critter
> # Demonstrates constructors
>
> class Critter(object):
>     """A virtual pet"""
>     def __init__(self):
>         print "A new critter has been born!"
>
>     def talk(self):
>         print "\nHi.  I'm an instance of class Critter."
>
> # main
> crit1 = Critter()
> crit2 = Critter()
>
> crit1.talk()
> crit2.talk()
>
> raw_input("\n\nPress the enter key to exit.")
>
>
>   
Critter is a class, not a function.  So the syntax
   crit1 = Critter()

is not calling a "Critter" function but constructing an instance of the 
Critter class.  You can tell that by doing something like
  print crit1
  print crit2

Notice that although both objects have the same type (or class), they 
have different ID values.

Since you supply an __init__() method in the class, that's called during 
construction of each object.  So you see that it executes twice.

Classes start to get interesting once you have instance attributes, so 
that each instance has its own "personality."  You can add attributes 
after the fact, or you can define them in __init__().  Simplest example 
could be:

crit1.name = "Spot"
crit2.name = "Fido"

Then you can do something like
  print crit1.name
  print crit2.name

and you'll see they really are different.

DaveA





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