Object Initialization (Constructors) When we create an instance of a class, we often want to initialize its data members (fields).
We could create an instance of time set to12:30:30 with the following code in main:
# main
t = Time()
t.hour=12
t.minute=30
t.second=30
This is quite laborious. A constructor is a special method for initializing fields on creation.
In Python, a constructor is a method with the special name __init__ (two underscores around "init")
Using a constructor, the code in the sample main can be reduced to one line.
Here's a modified class Time and modified main program that creates a Time instance:
class Time:
def __init__(self, hours=0, minutes=0, seconds=0):
self.hours = hours
self.minutes = minutes
self.seconds = seconds
# main
t = Time(12, 30, 30)
Note that the parameters have default values given to them. This allows a programmer to also create a Time class without specifying the hour, minute, and second on the creation:
t2 = Time()
Object Equality Consider the following main program:
t1 = Time(4,5,22)
t2= Time(4,5,22)
if (t1==t2)
print "t1 is same as t2"
Will anything be printed?
With objects, "==" is defined to compare the addresses of the two objects.
Python allows a programmer to redefine == for a class by using the special name __eq__.
class Time:
def __init__(self, hours=0, minutes=0, seconds=0):
self.hours = hours
self.minutes = minutes
self.seconds = seconds
def __eq__(self, other):
if ((self.hours==other.hours) and
(self.minutes==other.minutes) and
(self.seconds=other.seconds)):
return True
else
return False;
Given this __eq__, we'll get a different result for:
if __name__ == '__main__':
t1 = Time(4,5,22)
t2= Time(4,5,22)
if (t1==t2)
print "t1 is same as t2"
When Python sees == on two objects, it checks if the class of the objects has an __eq__. If not, it calls the default comparison (compare addresses).
String Representation When we call print on an object, Python does not print the contents of the object, it prints its address.
A programmer can write a method with the name __str__ to provide a string representation for an object (which can be printed)
class Time:
# ...
def __str__(self):
return str (self.hours) +":"+ str(self.minutes) + ":" +str(self.seconds)
With __str__ defined, when one prints a time object, it will display the data.
In-Class Problem Modify your Coordinate code so that it has a proper constructor, equality method, and string representation. Write a main method that tests all of these functions. |
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