On Wed, 30 Mar 2005, Kevin wrote:
> I was wondering, can you make a program the uses alot of classes do the
> exact same thing with out useing classes?
Hi Kevin,
Yes. We can even do a lot of object oriented programming without classes,
although it might be slightly painful.
You asked an earlier question about constructors, so I'll try to flesh
that out too. Let's use a small toy example.
If we have a Person class:
######
class Person:
def __init__(self, name):
self.name = name
self.friends = []
def addFriend(self, otherPerson):
self.friends.append(otherPerson)
def sayHelloToAll(self):
for friend in self.friends:
print "hi", friend.name, "it is I,", self.name
######
then we can just play with it:
######
>>> alan = Person("alan")
>>> tim = Person("tim")
>>> alan.addFriend(tim)
>>> alan.sayHelloToAll()
hi tim it is I, alan
######
But we can write fairly close code that just uses functions!
######
def makePerson():
return {'name': None,
'friends' : None}
def initPerson(self, name):
self['name'] = name
self['friends'] = []
def addFriend(self, otherPerson):
self['friends'].append(otherPerson)
def sayHelloToAll(self):
for friend in self['friends']:
print "hi", friend['name'], "it is I,", self['name']
######
Let's see how this might work:
######
>>> (kevin, danny, sean) = (makePerson(), makePerson(), makePerson())
>>> initPerson(kevin, "Kevin")
>>> initPerson(danny, "Danny")
>>> initPerson(sean, "Sean")
>>> addFriend(kevin, danny)
>>> addFriend(kevin, sean)
>>> sayHelloToAll(kevin)
hi Danny it is I, Kevin
hi Sean it is I, Kevin
######
What's different here from the code with the explicit 'class' stuff is
only mere appearance. Syntactially, it has the appearance of:
verb(subject, object)
If we were to use classes, things would look more like:
subject.verb(object)
which some people prefer to read.
But it's a bit clunky to do this all by hand. As a concrete example,
Python's class support automatically does stuff like makePerson() for us:
we just have to write an appropriate initializer to shape the object the
way we want. When we do something like this with Classes:
######
personUsingClasses = Person("Kevin")
######
we are actually doing two things: we first "construct" an object, and then
we "initialize" it to fill the object in.
In the function code above, we can see these two steps:
######
personUsingFunctions = makePerson()
initPerson(personUsingFunctions, "Kevin")
######
It's a common idiom to do construct-and-init, since an uninitialized
object is pretty useless, so Python's class support always does
construction for us. This allows us to concentrate on the part that's
really important to us --- the initializer.
If we use Python's class support, stuff like this tends to be briefer and
easier to read. So a little bit of Python's class support is syntax to
make it nicer to write code that focuses on "objects".
Does this make sense so far?
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