Newbie help- Can multiple instances with multiple names automatically created.

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Tue Jan 5 04:52:02 CET 2010


On Mon, 04 Jan 2010 19:12:53 -0800, Nav wrote:

> Okay, let me ask another question:
> 
> When we create instances of objects by doing 
> x = className()
> are we using globalnamespace?

That depends on whether you are doing x = className() inside a function 
(or class), or in the top level of the program.


If I do this:

x = 1234

def function():
    y = 4567

then x is defined in the global namespace and y is defined in the 
namespace which is local to function().


> if yes then:
>   if using globalnamespace is bad then why does every book or tutorial
> about python classes give the above  style of assignment as an example?

No, you're confused -- the problem isn't with using the global namespace. 
The problem is that you don't know what names you want to use ahead of 
time.

You use assignment like:

x = something()

when you know the name x when you are writing the code. That way you can 
write x in the code, and all is good:

x = something()
print x.method()
mydict = {x: -1}
assert mydict.keys() == [x]

Now, imagine that you didn't know what names you have to use. Say, for 
example, that you need a variable number of Somethings:

a = Something()
b = Something()
c = Something()
d = Something()  
# I never know when to stop...
z = Something()
# ... keep going? too far? who knows??? HELP!
process(a)
process(b)
process(c)
# when do I stop???
process(x)
process(y)  # ...


That's the wrong way to deal with it. So instead you use a list:


mylist = []  # define ONE NAME in the global namespace
for i in range(some_number):
    mylist.append(Something())
# later...
for x in mylist:  # again, we use ONE name, `x`
    process(x)



A list implicitly maps numbers (the position) to values. If you want to 
map strings (names) to values, use a dict. Here is an example. Notice we 
don't know how many players there are, or what their names are:


print "Welcome to the game."
print "Please enter the name of each player,"
print "or the word 'STOP' when there are no more players."
players = {}
i = 1
while True:
    # loop forever
    name = raw_input("Name of player %d? " % i)
    name = name.strip()  # get rid of extra whitespace
    if name.upper() == 'STOP':
        # we're done
        break
    players[name] = NewPlayer()

# much later...
for name, player in players.items():
    print "Welcome, player %s" % name
    play_game(player)
    

The only downside is that dicts are unordered, so the order of the 
players is not the same as the order they were entered in.


-- 
Steven



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