[Python] Newbie question about nested lists and constructor functions...

Could you please post example(s) so we know exactly what is giving you
trouble? I at least don't quite know what you are asking.

The constructor function (Python calls it "__init__") is created thus:

class Spam:
def __init__(self):
...

and of course __init__ might have additional arguments. If you are creating
a nested list with known initializers, you might say:

myvar = [ [ "list", 2, 3, 4 ], [ 'A', 'B', 'C' ] ]

so if you want to create a nested list in an __init__ function:

class Spam:
def __init__(self):
self.eggbasket = [ [ "list", 2, 3, 4 ], [ 'A', 'B', 'C' ] ]

to assign a nested list to a class attribute. If you are passing in
information
which you wish to assign that way, you would embed it in the given syntax.
For
instance:

class Spam:
def __init__(self, eggs):
self.eggbasket = [ eggs, [ "spam", "spam", "spam" ] ]

my_spam = Spam([ "egg", "egg", "egg"])

which would nest the given list of eggs within the eggbasket member of
my_spam.
You might alternately want to make a copy of the initializer instead of
storing
a reference:

class Spam:
def __init__(self, eggs):
self.eggbasket = [ eggs[:], [ "spam", "spam", "spam" ] ]

[:] (slice of all) is the Pythonic "shallow" list copier.

Of course this is all potentially nonsense as, again, I don't exactly know
what
you are wanting to do.

----- Original Message -----
From: "Bartok" <JollyRoger at hotmail.com>
Subject: Newbie question about nested lists and constructor functions...
help?

Sorry about that Chris.

I am trying to create instantiated objects with the following properties:

list_var = [(key, value), left, right]

where the "key, value" pair are a tuple, and the variables "left" and
"right" are empty lists [ ] which will be manipulated to hold values later
in the program after they have already been initialized to empty lists. The
purpose of this object is to hold the necessary data for nodes in a binary
search tree. I know how to implement this type of data structure in C++ but
I seem to be getting hung up on the syntax differences in the Python class
and member variable declarations.

An example of one of my attempts would be:

class Node:
def __init__(self, key = ''):
left = []
right = []
data = (key, 'value')
newnode = [data, left, right]
def info(self):
return self.data

The "info" function is only intended to return the values of the variables
"key and value". The other values are used as pointers to child nodes in
the tree. As I mentioned earlier, I have been through the online tutorial a
few times and have not found any information that clears this issue up for
me. Obviously, the code above is incomplete as it should also contain the
appropriate "if" and "elif" statements to properly branch according to the
information passed to the constructor function. Unfortunately, I do not
know what I am doing wrong. Any suggestions?

Norm.

P.S. Sorry about the contact information on the last message. Someone was
fooling around with my newsgroup account settings and I didn't notice it
until the last message was already posted.




Chris Gonnerman wrote in message ...
in

----- Original Message -----
From: "Norm Lyons" <norm at cybersurf.net>
Sent: Sunday, March 04, 2001 10:18 PM
Subject: Re: Newbie question about nested lists and constructor functions...

OKAY. Now we are getting somewhere. This seems to be a common mistake for
users of other languages. Your code likely should be like this:

class Node:
def __init__(self, key = ''):
left = []
right = []
data = (key, 'value')
self.data = [data, left, right]
def info(self):
return self.data[0]

SPECIFICALLY, you need to assign your list to a data attribute of self. I
would write
your code more like this:

class Node:
def __init__(self, key = ''):
self.left = []
self.right = []
self.data = (key, 'value')
def info(self): # not really needed with this version IMHO
return self.data

or perhaps:

class Node:
def __init__(self, key = '', value = ''):
self.left = None # these get object references (later)
self.right = None # so I don't think they need to be lists.
self.key = key
self.value = value # is this a default?
def info(self): # not really needed with this version IMHO
return (self.key, self.value)

but that is a personal thing.
I think I have this all right, but I haven't test run it yet. Good luck :-)

"Bartok" <JollyRoger at hotmail.com> wrote in message
news:W1Do6.4683$ke1.121750 at jekyl.ab.tac.net...
Constructing a list, nested or not, within __init__() is no different from
doing so anywhere else.
help?

It would be much easier to do so if you post both a specific example of
what you want to do and what you have tried.