[Tutor] Instantiating large numbers of objects? [lists and dictionaries]

Danny Yoo dyoo@hkn.eecs.berkeley.edu
Sun, 3 Feb 2002 14:03:20 -0800 (PST)

On Sun, 3 Feb 2002, Britt Green wrote:
> If you had three accounts, you'd create three instances in your code:
> bob = new Account()
> sue = new Accout()
> jim = new Account()
> and then you'd call the class methods like so:
> bob.makeWithdrawl(500)
> sue.checkBalance()
> jim.makeDeposit(200)
> So far this is the only way I know of to instantiate classes.
> Obviously when a bank uses this software, they wouldn't open up the
> code to add new accounts. How would they create new instances of the
> Account class? I seem to recall being told once that in C or Java one
> would create an array or Vector of objects. I was thinking I would do
> the same thing with my Rooms, except Python doesn't have arrays or
> Vectors.

Although Python doesn't have arrays or Vectors, it does have a similar
container class: Python has the 'list' type.  Here's an example:

>>> class Person:
...     def __init__(self, name):
...         self.name = name
...     def sing(self):
...         print "tra la la, my name is", self.name
>>> names = ['bart', 'lisa', 'marge', 'homer']
>>> people = []
>>> for n in names:
...     people.append(Person(n))
>>> people
[<__main__.Person instance at 80a2b28>, <__main__.Person instance at
80d3498>, <
__main__.Person instance at 80d34d0>, <__main__.Person instance at

In this example, we assign 'people' initially to the empty list, '[]'.  
If we're given some list of names, we can easily create instances for each
name, and add them to our list.  By using a list, we avoid hardcoding the
number of people our program can deal with.

The append() method on Python lists is equivalent to the "push_back()"
method in C++ vectors.  Once we have this list of people, we can pick any
one of them by index:

>>> people[0].sing()
tra la la, my name is bart

But people often don't like to be labeled with numbers.  It might be nicer
to be able to say something like:


That is, it might be nice to use the person's name to pull them from our
list container.  In that case, we can use a more appropriate data
structure, the dictionary:

>>> people_dict = {}
>>> for n in names:
...     people_dict[n] = Person(n)
>>> people_dict['lisa'].sing()
tra la la, my name is lisa

A Python dictionary is equivalent to Java's concept of a 'Map'.  What's
nice about Python's dictionary is that it uses very similar syntax to that
of arrays --- this emphasizes the idea that Lists and Dictionaries
implement the same idea of a container.

Also, both of these data types are considered "primitive" in Python, which
makes them very convenient to work with.  Take a look at:


for an example of List usage.  I think you'll find that Python lists are
pretty darn casual compared to a C++ array.  *grin*

For information on Python dictionaries, see:


There's a lot of good information on these data types in almost every
Python tutorial:


and please feel free to ask questions about them; lists and dictionaries
are fun to talk about.  *grin*

> I apologize if I'm not explaning myself clearly. This is still pretty
> new to me and I'm not 100% sure how to phrase my question.

Don't worry: your question makes perfect sense.  I hope this answer
somewhat reciprocates that.  Good luck to you!