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Date: Sat, 11 Feb 2012 00:01:52 +0100
Subject: Postpone evaluation of argument
From: Righard van Roy <pluijzer@gmail.com>
To: python-list@python.org
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:20202

Hello,

I want to add an item to a list, except if the evaluation of that item
results in an exception.
I could do that like this:

def r(x):
    if x > 3:
        raise(ValueError)

try:
    list.append(r(1))
except:
    pass
try:
    list.append(r(5))
except:
    pass

This looks rather clumbsy though, and it does not work with i.e. list
comprehensions.

I was thinking of writing a decorator like this:

def tryAppendDecorator(fn):
    def new(*args):
        try:
            fn(*args)
        except:
            pass
    return new

@tryAppendDecorator
def tryAppend(list, item):
    list.append(item)

tryAppend(list, r(1))
tryAppend(list, r(5))

This does not work however because the 'item' argument gets evaluated
before the decorator does it's magic.

Is there a way to postpone the evaluation of 'item' till it gets used
inside the decorator. Like it is possible to quote a form in Lisp.

Thank you,
Righard