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Date: Tue, 31 May 2011 18:38:40 +1100
Subject: Re: scope of function parameters (take two)
From: Daniel Kluev <dan.kluev@gmail.com>
To: python-list@python.org
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6715

On Tue, May 31, 2011 at 6:17 PM, Henry Olders <henry.olders@mcgill.ca> wrote:
> Clearly, making a copy within the function eliminates the possibility of the
> side effects caused by passing in mutable objects. Would having the
> compiler/interpreter do this automatically make python so much different?

As I've pointed, you can make decorator to do that. Adding @copy_args
to each function you intend to be pure is not that hard.

import decorator
import copy

@decorator.decorator
def copy_args(f, *args, **kw):
    nargs = []
    for arg in args:
        nargs.append(copy.deepcopy(arg))
    nkw = {}
    for k,v in kw.iteritems():
        nkw[k] = copy.deepcopy(v)
    return f(*nargs, **nkw)

@copy_args
def test(a):
    a.append(1)
    return a

>>> l = [0]
>>> test(l)
[0, 1]
>>> l
[0]


>>> inspect.getargspec(test)
ArgSpec(args=['a'], varargs=None, keywords=None, defaults=None)

So this decorator achieves needed result and preserves function signatures.


-- 
With best regards,
Daniel Kluev