creating and naming objects

[Brian]

I have a question that some may consider silly, but it has me a bit
stuck and I would appreciate some help in understanding what is going
on.

For example, lets say that I have a class that creates a student
object.

Class Student:
def setName(self, name)
self.name = name
def setId(self, id)
self.id = id

Then I instantiate that object in a method:

def createStudent():
foo = Student()
/add stuff

Now, suppose that I want to create another Student.  Do I need to name
that Student something other than foo?  What happens to the original
object?  If I do not supplant the original data of Student (maybe no id
for this student) does it retain the data of the previous Student
object that was not altered?  I guess I am asking how do I
differentiate between objects when I do not know how many I need to
create and do not want to hard code names like Student1, Student2 etc.

I hope that I am clear about what I am asking.

Thanks,
Brian

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Re: creating and naming objects

[Tim Chase]

 def createStudent():
 foo = Student()
 /add stuff
 
 Now, suppose that I want to create another Student.  Do I need
  to name that Student something other than foo?  What happens 
 to the original object?

If you want to keep the old student around, you have to keep a
reference to it somewhere.  This can be saving the reference 
before you tromp on foo, or make foo a list and keep a list of 
students (which is what it seems like you want to do).

  foo = Student() # foo is a Student
  bar = foo # bar now refers to the same student
  foo is bar
True
  bar.setName(George)
  foo.name  #altering the thing refered to by bar changes 
foo because they both refer to the same object
'George'
  foo = Student() # make foo refer to a new student
  foo is bar
False
  foo.name
''
  bar.name
'George'
  students = []
  students.append(Student())
  students.append(Student())
  students[0].setName(Alice)
  students[1].setName(Bob)
  students[0].name
'Alice'
  students[1].name
'Bob'
  students[0] is students[1]
False

Hopefully the above gives you some ideas as to

-how references work
-how to store multiple students (use a list)

 I hope that I am clear about what I am asking.

I hope I am clear in explaining what I understand that you are 
asking. :)

-tkc




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Re: creating and naming objects

[Diez B. Roggisch]

Brian wrote:

 I have a question that some may consider silly, but it has me a bit
 stuck and I would appreciate some help in understanding what is going
 on.
 
 For example, lets say that I have a class that creates a student
 object.
 
 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id
 
 Then I instantiate that object in a method:
 
 def createStudent():
 foo = Student()
 /add stuff
 
 Now, suppose that I want to create another Student.  Do I need to name
 that Student something other than foo?  What happens to the original
 object?  If I do not supplant the original data of Student (maybe no id
 for this student) does it retain the data of the previous Student
 object that was not altered?  I guess I am asking how do I
 differentiate between objects when I do not know how many I need to
 create and do not want to hard code names like Student1, Student2 etc.
 
 I hope that I am clear about what I am asking.

You seem to confuse the terms class and instance. An object is the instance
of a class. You can have as many Students as you like. You just have to
keep a reference around for retrieval. E.g.

students = [Student(student %i % i) for i in xrange(100)]

will create a list of 100 (boringly named) students.

The 

foo = Student(foo)

will create a Student-object and the name foo refers to it. You are free to
rebind foo to another Student or something completely different.

foo = Student(foo)
foo = 10

When you do so, and no other references to the Student-objects are held, it
will actually disappear - due to garbage collection. 

Diez
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Re: creating and naming objects

[Dennis Benzinger]

Brian wrote:
 [...]
 For example, lets say that I have a class that creates a student
 object.
 
 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id
 
 Then I instantiate that object in a method:
 
 def createStudent():
 foo = Student()
 /add stuff
 
 Now, suppose that I want to create another Student.  Do I need to name
 that Student something other than foo?  What happens to the original
 object?  If I do not supplant the original data of Student (maybe no id
 for this student) does it retain the data of the previous Student
 object that was not altered?  I guess I am asking how do I
 differentiate between objects when I do not know how many I need to
 create and do not want to hard code names like Student1, Student2 etc.
 [...]


Just return your Student object from createStudent() and put it in a 
list. For example:

all_students = []

for i in range(10):
 one_student = createStudent()

 # Do what you want with one_student here

 all_students.append(one_student)

print all_students


BTW: Why don't you use a constructor to create Student objects?

Bye,
Dennis
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Re: creating and naming objects

[Steve Holden]

Brian wrote:
 I have a question that some may consider silly, but it has me a bit
 stuck and I would appreciate some help in understanding what is going
 on.
 
 For example, lets say that I have a class that creates a student
 object.
 
 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id
 
 Then I instantiate that object in a method:
 
 def createStudent():
 foo = Student()

Of course here you'd need an argument to  correspond to the student 
name, but we'll overlook that.

 /add stuff
 
This actually looks like a Student factory function, though I have 
reservations about it (see below).

 Now, suppose that I want to create another Student.  Do I need to name
 that Student something other than foo?  What happens to the original
 object?  If I do not supplant the original data of Student (maybe no id
 for this student) does it retain the data of the previous Student
 object that was not altered?  I guess I am asking how do I
 differentiate between objects when I do not know how many I need to
 create and do not want to hard code names like Student1, Student2 etc.
 
 I hope that I am clear about what I am asking.
 
 Thanks,
 Brian
 
Presumably your definition of createStudent() ends with something like

 return foo

otherwise the created Student object has no references once the function 
complete, and will therefore become non-referenced garbage immediately.

Assuming that createStudent() does indeed return the created Student 
instance then you would use it like this:

s1 = createStudent()
 ...
s2 = createStudent()

and so on. Of course there's nothing to stop you creating lists of 
students or dicts of students either. Objects don't really have names 
in Python, it's better to think of names being references to objects. 
Each object can be referenced by zero, one or more names, but references 
can also be stored in the items of container objects.

A point you don't appear to have thought of, though, is that all the 
functionality implemented in your createStudent() function could just as 
easily become part of the Student.__init__() method. Then there's no 
need to create a factory function at all - the __init__() method just 
initialises each newly-created instance before you get your hands on it. 
Then your code would be more like


s1 = Student()
 ...
s2 = Student()

and so on. Plus, of course, the __init__() method can take arguments if 
they are needed to initialize the object successfully. So I'm not really 
sure quite what createStudent() is for.

Hope this rambling helps at least a bit.

regards
  Steve
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Holden Web LLC/Ltd  http://www.holdenweb.com
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Re: creating and naming objects

[Brian]

Thank you all for your response.  I think that I am getting it.  Based
on those responses, would I be correct in thinking that this would be
the way to initialize my Student object and return the values?

class Student:
def __init__(self, name, id):
self.name = name
self.id = id

def getName(self):
return self.name

def getId(self):
return self.id

Additionally, correct me if I am wrong but I can recycle:

foo = Student()

And doing so will just create a new instance of Student (with whatever
attributes it is given) and the original foo is gone?

Thanks for your help and patience.  After reading my original post and
then this one, I could see that it may look suspiciously like a home
work assignment.  Trust me it's not.  The student example just seemed
like a good fit to discuss this.

Thanks again,
Brian

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Re: creating and naming objects

[Gerard Flanagan]

Brian wrote:
 Thank you all for your response.  I think that I am getting it.  Based
 on those responses, would I be correct in thinking that this would be
 the way to initialize my Student object and return the values?

 class Student:
 def __init__(self, name, id):
 self.name = name
 self.id = id

 def getName(self):
 return self.name

 def getId(self):
 return self.id

 Additionally, correct me if I am wrong but I can recycle:

 foo = Student()

 And doing so will just create a new instance of Student (with whatever
 attributes it is given) and the original foo is gone?

 Thanks for your help and patience.  After reading my original post and
 then this one, I could see that it may look suspiciously like a home
 work assignment.  Trust me it's not.  The student example just seemed
 like a good fit to discuss this.

 Thanks again,
 Brian

Here's a page from the Python tutorial wiki which may be helpful to
you:

  http://pytut.infogami.com/node11-baseline.html

It's an introduction to classes which, appositely, uses a Student class
as an example.  Still a work in progress.  Feel free to leave a comment
if you find it helpful or not - your input would be very much valued.
(Note the StudentTracker class).

Gerard

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Re: creating and naming objects

[bruno at modulix]

Brian wrote:
 Thank you all for your response.  I think that I am getting it.  Based
 on those responses, would I be correct in thinking that this would be
 the way to initialize my Student object and return the values?
 
 class Student:

Do yourself a favour: use new-style classes

class Student(object):

 def __init__(self, name, id):
 self.name = name
 self.id = id

ok until here.

 def getName(self):
 return self.name
 
 def getId(self):
 return self.id

These to getters are totally useless in Python. Python doesn't have
access restrictors [1], so you can directly access the attributes.
Python also has support for computed attributes (looks like an ordinary
attribute but is accessed thru a getter/setter), so there's no problem
wrt/ encapsulation. Just get rid of these two methods and you'll be fine.

[1] it's on purpose - we prefer to use conventions like using
_names_starting_with_a_leading_underscore to denote implementation stuff
that should not be accessed by client code.

 Additionally, correct me if I am wrong but I can recycle:
 
 foo = Student()

This won't work - will raise a TypeError. You defined Student.__init__()
to take 2 mandatory parameters name and id,so you must pass them at call
time.

FWIW, you would have noticed this just by trying it into your Python
shell. The interactive Python shell is a very valuable tool. It makes
exploring APIs and testing ideas a breeze, with immediate feedback. It
is one of the things that makes Python so easy and usable (not that it's
a Python invention - but it's a GoodThing(tm) Python have it).

 And doing so will just create a new instance of Student (with whatever
 attributes it is given) and the original foo is gone?

Yes. Or more exactly, the identifier 'foo' will refer to the newly
created Student object, and if there are no other references to the
object previously pointed by 'foo', it will be disposed of...

 Thanks for your help and patience.  After reading my original post and
 then this one, I could see that it may look suspiciously like a home
 work assignment.  Trust me it's not.

It could have been homework, this is not a problem - as long as you
effectively tried to do your homework by yourself, and only ask for help
when stuck. FWIW, in this job, knowing when and how to ask for help is
as important as knowing how to first try to solve the problem oneself !-)

HTH
-- 
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p in '[EMAIL PROTECTED]'.split('@')])
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Re: creating and naming objects

[Maarten van Veen]

In article [EMAIL PROTECTED],
 Brian [EMAIL PROTECTED] wrote:

 I have a question that some may consider silly, but it has me a bit
 stuck and I would appreciate some help in understanding what is going
 on.
 
 For example, lets say that I have a class that creates a student
 object.
 
 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id
 
 Then I instantiate that object in a method:
 
 def createStudent():
 foo = Student()
 /add stuff
 
 Now, suppose that I want to create another Student.  Do I need to name
 that Student something other than foo?  What happens to the original
 object?  If I do not supplant the original data of Student (maybe no id
 for this student) does it retain the data of the previous Student
 object that was not altered?  I guess I am asking how do I
 differentiate between objects when I do not know how many I need to
 create and do not want to hard code names like Student1, Student2 etc.
 
 I hope that I am clear about what I am asking.
 
 Thanks,
 Brian

Hi Brian,

If you would do it like this:
Class Student:
def setName(self, name)
self.name = name
def setId(self, id)
self.id = id


def createStudent():
foo = Student()
foo.setName(Brian)
foo = Student()
print foo.getName()

You would get an error here, because foo now equals the second Student 
object which has no name. And you can't get to the first one. 
Perhaps you could do something like this.

Class Student:
def setName(self, name)
self.name = name
def setId(self, id)
self.id = id

listWithStudents = []

def createStudent():
listWithStudent.append(Student())

Now every student you create is put in the list with students so you can 
access them but don't have to give them names.
You could do something to all of them like this:
for student in listWithStudent:
   student.doSomething()

Or change just one student:
listWithStudent[23].doSomething()  # this is the 24th student though!!!

Hope this is what you're looking for.
Maarten
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Re: creating and naming objects

[Brian]

Maarten van Veen wrote:
snip

 Hi Brian,

 If you would do it like this:
 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id


 def createStudent():
 foo = Student()
 foo.setName(Brian)
 foo = Student()
 print foo.getName()

 You would get an error here, because foo now equals the second Student
 object which has no name. And you can't get to the first one.
 Perhaps you could do something like this.

 Class Student:
 def setName(self, name)
 self.name = name
 def setId(self, id)
 self.id = id

 listWithStudents = []

 def createStudent():
 listWithStudent.append(Student())

 Now every student you create is put in the list with students so you can
 access them but don't have to give them names.
 You could do something to all of them like this:
 for student in listWithStudent:
student.doSomething()

 Or change just one student:
 listWithStudent[23].doSomething()  # this is the 24th student though!!!

 Hope this is what you're looking for.
 Maarten

This is of great help.  Thank you,
Brian

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