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Groups > comp.lang.python > #20202

Postpone evaluation of argument

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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

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Postpone evaluation of argument Righard van Roy <pluijzer@gmail.com> - 2012-02-11 00:01 +0100
  Re: Postpone evaluation of argument Paul Rubin <no.email@nospam.invalid> - 2012-02-10 15:57 -0800
    Re: Postpone evaluation of argument 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-11 03:16 -0800
  Re: Postpone evaluation of argument Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2012-02-11 10:54 +0200

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