Re: [Tutor] using shelve
Sorry it took me so long to get back - from your posts and my experimentation I
can see that when you access one item in the shelve dictionary, it only gets
the one item, not all of them. I am going to use shelve, and only refactor or
change if performance becomes an issue - which I don't see happening for a long
time.
On Thursday, June 21, 2007, at 10:27PM, John Fouhy [EMAIL PROTECTED] wrote:
On 22/06/07, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote:
I created a shelf called 'myshelf' and put two objects in it, a string and a
list. When I open the shelf I get:
d=shelve.open('/Users/development/Desktop/myshelf')
d.keys()
['dir1', 'dir2']
d
{'dir2': '/Users/development/Desktop/RSSReaderApp/RSS.db', 'dir1':
['.DS_Store', '.localized', 'access the latest version', 'Python Quick
Reference.webloc', 'rssdb', 'workspace']}
It seems that when you use shelve.open(), it actually brings the entire
shelf dictionary into memory, accessible through d.
Well ... all that tells me is that when you ask python for a string
representation of a shelf, it reads the entire thing.
What if you had 100 objects in myshelf, or 1000 or 100,000? Wouldn't it get
bogged down?
If you try to print out the whole thing, probably. Let's try some test:
import shelve
d = shelve.open('test.shelf')
for i in range(1000):
... d[str(i)] = 'x'*i
...
d.close()
Morpork:~/tmp repton$ ls -l test.shelf
-rw-r--r-- 1 repton repton 1M Jun 22 14:23 test.shelf
First, we'll measure how long it takes python to open the shelf using
shelve.open:
Morpork:~/tmp repton$ python -m timeit -s 'import shelve' 'd =
shelve.open(test.shelf)'
1000 loops, best of 3: 1.95 msec per loop
Now we'll measure how long it takes python to open a shelf and turn it
into a string:
Morpork:~/tmp repton$ python -m timeit -s 'import shelve' 'd =
shelve.open(test.shelf)' 'str(d)'
10 loops, best of 3: 51.5 msec per loop
So, that's about a factor of 25 difference.
HTH!
--
John.
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[Tutor] using shelve
I created a shelf called 'myshelf' and put two objects in it, a string and a
list. When I open the shelf I get:
d=shelve.open('/Users/development/Desktop/myshelf')
d.keys()
['dir1', 'dir2']
d
{'dir2': '/Users/development/Desktop/RSSReaderApp/RSS.db', 'dir1':
['.DS_Store', '.localized', 'access the latest version', 'Python Quick
Reference.webloc', 'rssdb', 'workspace']}
It seems that when you use shelve.open(), it actually brings the entire shelf
dictionary into memory, accessible through d.
What if you had 100 objects in myshelf, or 1000 or 100,000? Wouldn't it get
bogged down?
If so, what is it good for and how would you store a large number of objects?
(Storing strings and lists in an sql database is probably the way to go for
this simple example, but what if you had more complex objects?)
Thanks
Chris V.
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[Tutor] Subclassing vs. stand alone functions
Hi there, I am new to Python and trying to get my head around the OO stuff. I guess my question is - when do you go with subclassing vs. making a standalone function? Let's say you want to load a dictionary. Do I create a function that accepts some argument (say a file name) and returns a dictionary, or do I subclass dict and override the __init__ and __setitem__ functions to make 'self-loading' dictionary? It seems the end result is the same. Here is a follow-up if you will indulge me... I created a class called WebPage which is a stand-alone class (I get that). It loads a web page template, and has a function to update the replacement vars with your data (updHtmlVar), and another to spit out the html to a file (wrtHtml). Do you subclass WebPage for each particular page you want (because you can customize it with load functions for each piece of data) or do you just use it as is, and create separate functions outside the class that load the data and you just use updHtmlVar to load it into your WebPage object? Again, the end result is the same. (I can send code samples if it will help). Thanks, Chris ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Subclassing vs. stand alone functions
I am new to Python and trying to get my head around the OO stuff. I guess my question is - when do you go with subclassing vs. making a standalone function? OK, I'll take a slightly different approach than the other answers so far. First: procedural and OO styles of programming are diffrent ways of thinking about a problem. Any programming problem can be solved using either approach and both approaches are equally good, neither is intrinsically better than the other. Second: Some problems are more amenable to an OO aproach than a procedural and vice versa. And the majority can be done either way with very little to choose between them. I will now assume that you understand the procedural way and already know how to apply good procedural design, including modularity featuring loose coupling and tight cohesion. In addition data structure design and its relationship to your procedural design should be a concept familiar to you. ( In a perfect world you'll also be familiar with the princuiples of functional programming and the lambda calculus, but that's possibly asking too much.) Not sure about the lambda calculus, but I have been doing procedural programming for about 10 years. (I try my best for modularity and all that good stuff :) That leaves the question of why and wjen should we use OOP? OOP suits programs that feature a high level of correspondence between the real world and the software modfel we are building. For example simulation software (including many games) usually involves the representation and control of a number of objects. It is a natural link to model these objects as classes and create corresponding objects in our solution. Similarly GUIs are made up of windows, widgets, etc. Again these have a fairtly clear translation into objects. When we get into problems primarily of algorithms, or of transforms to fixed data then an OOP style is not always such an obvious fit. Similarly when modelling complex state machines the applicability of OOP can be less obvious and a traditional table driven procedural style may seem better suited. In those cases the decision to use OOP is likely to be driven by the desire to create a reusable component. Something that can be utilised across multiple projects. or it may be driven by the desire to abstract away a complex process or data structure. Hiding it behind a simplere API. This can be done using traditional approaches but usually only at the cost od writing an awful lot of code or by exposing the data structure at least for initialisation purposes. Thanks, this is just what I needed! A way to think about which to use. Now to your examples: Let's say you want to load a dictionary. Why would anyone ever want to load a dictionary? I just want to create dictionary with some data in it. The data comes from a file, let's say. I would then go on to do something with the dictionary - like use it as input to another function. (Sorry, I am thinking procedurally, or are dictionaries typically populated for you by the functions you call... maybe it's just a bad example. What is the higher level goal you are trying to achieve? Is the dictionary part of the solution or the problem? If it is part of the problem a dictionary object may be appropriate. If its part of the solution, and you are already using a non OOP approach why would you want an object? Unless its for the reasons above - reuse or abstraction that is hard using procedures. But you should very rarely be making decisions at this level unless you have alrwady decided on amn overall approach and you are considering an exception to the overall style. ie Should I create a function in an OOP design or should I create a class in a procedural design. (Mixing styles is OK but will normally involve some compromises) Do I create a function that accepts some argument or do I subclass dict and override It seems the end result is the same. Quite so and the andswer will depend on what you are trying to achieve. There is no definitive right answer. I created a class called WebPage which is a stand-alone class (I get that). Sorry, I don't get it! :-). Do you mean you only have a class and never create any instances? No. Or do you mean you don;t subclass anything in defining it? Yes. Or do you mean you only create a single instance? You could have multiple instances. It loads a web page template, and has a function to update the replacement vars with your data (updHtmlVar), and another to spit out the html to a file (wrtHtml). The data that this page holds is an html template. Does it also hold the data displayed by the html? in which case its not updating with 'your' data but with *its own* data. But it may allow you to pass some data to it. Or is it that it renders the html only when given some data? Which it doesn't store? It stores the data and the template. When it is instantiated, you just have the template and the variables. You
