[Tutor] user-given variable names for objects

[Che M]

> Date: Thu, 13 Dec 2007 01:58:10 -0600
> From: rabidpoobear at gmail.com
> To: pine508 at hotmail.com
> CC: tutor at python.org
> Subject: Re: [Tutor] user-given variable names for objects
> 
> Che M wrote:
> > I'm sure this is a classic beginner's topic, and I've read a bit about 
> > it online already, but I'd like to ask about it here as well.  I want 
> > to assign names to objects based on what a user inputs so that I can 
> > later keep track of them.
> Yes, this comes up quite a bit.
> > In particular, I want to time multiple events by getting each of their 
> > start times and later comparing them to their respective stop times--I 
> > don't want to user timers.  In order to do this, I need to give each 
> > start time a name associated with the name of the event.  If there is 
> > an "event A" it's start time could be eventA_start, whereas event B 
> > could be called eventB_start, i.e.:
> >
> > eventA_start = datetime.datetime.now()
> >
> > The problem is, I don't know the "A" part...the user could choose 
> > eventPotato or eventDinosaur.  I won't know in advance.  I want the 
> > name chosen/inputted by the user to be attached to the name for the 
> > time that event started.
> >
> > I have read that the way to do this properly in Python is with use of 
> > dictionaries, but I haven't found a reference online that shows how to 
> > do it is in a complete way.  Any help is appreciated.
> There are a multitude of different ways this can be accomplished with 
> dictionaries.  It really depends on what structure you want your data to 
> follow.
> By way of example, suppose we want to store the start time and length of 
> timers from the user.
> We could have an input loop as such:
> import time
> eventData = {}
> while True:
>     name, duration = raw_input("Name your event (done to exit): "), 
> int(raw_input("How long should it last? "))
>     if name.strip().lower() == "done":
>         break
> 
> now we want to add the user's data to our dictionary so we can keep 
> track of events.
> 
>     eventData[name] = time.time() + duration
> 
> note that I used the time.time() function to get the current time and 
> just added duration to it,
> so every event now stores the ending time of the event (when we want to 
> trigger it, say)
> 
> given this time, we can generate a list of all events that have happened 
> in the past,
> using list comprehensions or by other methods:
> [a for a in eventData.keys() if eventData[a] < time.time()]
> 
> then we can simply print out all events that have already occurred at 
> any point in time:
> while True:
>     finished = [a for a in eventData.keys() if eventData[a] < time.time()]
>     print "The following events have occurred: "
>     for name in finished:
>         print name
>     time.sleep(1)
>     if len(finished) == len(eventData.keys()):
>         break
> 
> Of course this is by no means an ideal solution, it merely serves as an 
> example.
> 
> Does this make sense, and do you understand why it's better to use 
> dictionaries than to use variables
> to achieve this?
> [note - no code was tested, not guaranteed to work :)]
> -Luke

Thanks, yes, I am getting clearer by the minute in my understanding of dictionaries and why they are better than variables for these cases.  Although I was not familiar with what you can do with a list such as you did here:
> [a for a in eventData.keys() if eventData[a] < time.time()]
I think it basically makes sense.  I guess .keys() is a built-in method for dictionaries to return a list of all their values, then?  By the way, what was the purpose of the line with
time.sleep(1)
But otherwise, it seems a good start.  Thank you, and I'll try more of it tomorrow.


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