9cfdbfd255
git-svn-id: svn://svn.cy55.de/Zope3/src/cybertools/trunk@631 fd906abe-77d9-0310-91a1-e0d9ade77398
72 lines
2.1 KiB
Text
72 lines
2.1 KiB
Text
Setting up and using LazyVars objects
|
|
=====================================
|
|
|
|
Lazy variables are only calculated when really needed, and they are
|
|
calculated only once during the lifetime of the LazyVars objects they
|
|
live in.
|
|
|
|
This is especially useful when rendering web pages as during this
|
|
rendering often the same data are used again and again, whereas you don't
|
|
know at the beginning of the rendering process which data you will really
|
|
need.
|
|
|
|
We first need a function that will be used to provide a value
|
|
for our lazy variable. This function will expect one parameter that will
|
|
be set to the LazyVars object when the function is called.
|
|
|
|
Our demonstration function will increment a counter on a context object
|
|
(provided via the vars parameter) and return this counter so that we can easily
|
|
follow the calls to the function:
|
|
|
|
>>> def getNumber(vars):
|
|
... context = vars.context
|
|
... context.counter += 1
|
|
... return context.counter
|
|
|
|
We now register the function with our LazyVars class under the name we later
|
|
want to use for accessing the variable:
|
|
|
|
>>> from cybertools.lazyvars.lazyvars import LazyVars
|
|
>>> LazyVars.registerVariable('number', getNumber)
|
|
|
|
We also need a context object - that one which carries the above mentioned
|
|
counter:
|
|
|
|
>>> class Number(object):
|
|
... counter = 0
|
|
>>> context = Number()
|
|
|
|
This object is now used as the context parameter when creating a LazyVars
|
|
object:
|
|
|
|
>>> lv = LazyVars(context)
|
|
|
|
So let's look if the LazyVars object can give us a value for the variable
|
|
we have registered:
|
|
|
|
>>> lv.number
|
|
1
|
|
|
|
The getNumber() function has been called that apparently has
|
|
incremented the counter.
|
|
|
|
What happens if we access the variable again?
|
|
|
|
>>> lv.number
|
|
1
|
|
|
|
Same result, no incrementation, as it is now stored in the LazyVars object and
|
|
retrieved without recalculation. Really lazy...
|
|
|
|
We can even use the same function for more than one variable. When we first
|
|
access the new variable the function is called again:
|
|
|
|
>>> LazyVars.registerVariable('number2', getNumber)
|
|
>>> lv.number2
|
|
2
|
|
|
|
Our first variable is not affected by this:
|
|
|
|
>>> lv.number
|
|
1
|
|
|