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Kirill Smelkov
Zope
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44a49b07
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44a49b07
authored
May 09, 2006
by
Philipp von Weitershausen
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update docs, put reminder in there where to put LazyExpr in the future.
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lib/python/Products/PageTemplates/DeferExpr.py
lib/python/Products/PageTemplates/DeferExpr.py
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lib/python/Products/PageTemplates/DeferExpr.py
View file @
44a49b07
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@@ -10,32 +10,16 @@
# FOR A PARTICULAR PURPOSE
#
##############################################################################
"""
Defer and
Lazy expression handler
"""Lazy expression handler
defer expressions can be usesd for a design pattern called deferred evaluation.
Example:
<div tal:define="xis defer:string:x is $x">
<p tal:repeat="x python:range(3)"
tal:content="xis"></p>
</div>
Output:
<div>
<p>x is 0</p>
<p>x is 1</p>
<p>x is 2</p>
</div>
A lazy expressions is implemented in a similar way but has a different result. While
a defer expression is evaluated every time it is used according to its context a lazy
expression is evaluted only the first time it is used. Lazy expression are known
under the name lazy initialization of variables, too.
A common use case for a lazy expression is a lazy binding of a costly expression.
While one could call an expression only when it's required it makes sense to define
it only one time when it could be used multiple times.
A lazy expressions is implemented similarly to the defer expression
but has a different result. While a defer expression is evaluated
every time it is used according to its context a lazy expression is
evaluted only the first time it is used. Lazy expression are known
under the name lazy initialization of variables, too. A common use
case for a lazy expression is a lazy binding of a costly expression.
While one could call an expression only when it's required it makes
sense to define it only one time when it could be used multiple times.
Example
...
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@@ -46,11 +30,11 @@ Example
</div>
"""
from
zope.tales.expressions
import
DeferWrapper
,
DeferExpr
# lazy expression
_marker
=
object
()
# TODO These should really be integrated into the Zope 3 ZPT
# implementation (zope.tales)
class
LazyWrapper
(
DeferWrapper
):
"""Wrapper for lazy: expression
"""
...
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