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POProblem with Functions which has default parameter(s)
 

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  1. POProblem with Functions which has default parameter(s)
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    <p>I have some question about functions which have default parameters. <br /></p> <pre><code>import sys from random import randint my_list = [1,2,3] def Veli(a, b = my_list): my_list.append(randint(1,1500)) print a + b[-1] print "Number of Refs:", sys.getrefcount(my_list) print "Def args:", Veli.func_defaults Veli(1) # This is 4 Veli(1) # We almost always obtain different result because of randint Veli(1) # Again different results. print Veli.func_defaults print "Number of Refs:", sys.getrefcount(my_list) Veli(1) my_list = [-5] # old my_list has different address print "Number of Refs:", sys.getrefcount(my_list) # Less than previous print "Def args:", Veli.func_defaults Veli(1) # Now gives same results. print "Def args:", Veli.func_defaults Veli(1) # Same result again... </code></pre> <p><br /> Outputs: (Some numbers depend on which values randint has returned, of course.) <br /> <blockquote></p> <p>Number of Refs: 3<br /> Def args: ([1, 2, 3],)<br /> 322<br /> 1119<br /> 740<br /> ([1, 2, 3, 321, 1118, 739],)<br /> Number of Refs: 3<br /> 303<br /> Number of Refs: 2<br /> Def args: ([1, 2, 3, 321, 1118, 739, 302],)<br /> 303<br /> Def args: ([1, 2, 3, 321, 1118, 739, 302],)<br /> 303<br /> </blockquote></p> <p></p> <p><br /> The following code gives you a number less than the previous one, because b and my_list is not referenced to same address anymore.</p> <pre><code>print "#ref:", sys.getrefcount(my_list) # Less than previous </code></pre> <p>Now, we have a way (the only way?) to reach b, default argument of the function:</p> <pre><code>Veli.func_defaults[0] </code></pre> <p><br /> Sorry about my long explanation. Here is my questions:</p> <ol> <li><p>Is this a problem? Dictionary of my module crush my_list variable then now my_list variable has default address than previous. Then function which uses global variable named my_list in its body is changing (growing) while my default argument is not. Why cannot <code>b</code> see global variable named <code>my_list</code> when <code>a + b[-1]</code> expression is executed? I know, <code>b</code> has different address with my_list (because <a href="http://docs.python.org/reference/datamodel.html#objects-values-and-types" rel="nofollow">mutual objects (like list) are guaranteed to refer to different, unique, newly created list</a>) now, but why were Python implemented so that <code>b</code> cannot see the global variables when b is a function arguments? Could you explain comprehensively?</p></li> <li><p>Is there any way to get same results with Veli.func_defaults[0]? This code executed then let say, I want to change my default arguments of my function named Veli. I cannot do this with my_list, because my_list and b have different addresses. One way is changing the elements of the list, Veli.func_defaults[0]. Is there any different way(s)?</p></li> <li><p>(It is not so related to code above) How can get addresses of variable? For example, how can get address of <code>b</code>? I use built-in function such as <code>__hash__</code> , but there should be more appropriate way.</p></li> </ol> <p><br /> Notes:</p> <p>a) This codes may be useless for any reason, but I want to learn opinions.<br/> b) Python 2.6.6 (r266:84292, Sep 15 2010, 15:52:39)<br/> [GCC 4.4.5] on linux2 </p>
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    1. COhttp://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument
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    2. CO@ncoghlan: Upvoted as it's important lecture, but since OP exploits this behaviour, not completely relevant here.
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    3. COOops, I actually thought I deleted that after rereading the OP's question...
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