Hi, Hugo !
Excellent explanation. Thank you.
All the best,
hilton
On Mon, Dec 6, 2010 at 5:05 PM, Hugo Arts <hugo.yo...@gmail.com> wrote:
> On Mon, Dec 6, 2010 at 6:09 PM, Joel Schwartz <j...@joelschwartz.com>
> wrote:
> > Chris,
> >
> > Can you say more about number (7) in your list? What does "pass by value"
> > mean and what are the alternatives?
> >
> > Thanks,
> > Joel
> >
>
> Generally, pass-by-* refers to how the arguments to functions are treated.
> * In call-by-value, the value of the arguments are copied into the
> function. There is no way to modify variables outside of the function
> since you don't have access to them, only to copies. C uses this,
> among many others:
>
> int a = 5
> void func(int b) { b = 6; }
> func(a);
> a == 5; /* evaluates to true, variable outside function scope remains
> unchanged */
>
> The value b, inside the function, contains a copy of a. So when it is
> modified, the original a remains unchanged.
>
> * in call-by-reference, the function is given implicit *references* to
> its arguments. When modifying the variables inside of the function,
> the variable outside is also changed. you can simulate it in many
> languages by passing an expicit reference rather than an implicit one
> (such as C's pointers):
>
> int a = 5
> void func(int * b) { *b = 6; }
> func(&a);
> a == 6; /* evaluates to true. the function was able to modify a
> variable outside of its scope */
>
> * python uses something that wikipedia calls "call-by-sharing." It is
> not call-by-value, nor is it call-by-reference. It means, in short,
> that while the function has access to the callers, *values*, it does
> NOT have access to the callers *variables*. To demonstrate:
>
> a = []
> def f(b):
> b.append(1)
> b = [2]
> f(a)
> print a # prints "[1]"
>
> As in pass-by-reference, the function f could modify it's callers
> values by appending 1 to the list. However, unlike *real*
> pass-by-reference, when trying to *re-assign* the variable into
> something entirely different, there was no effect (a did not become
> equal to [2]).
>
>
> Many people call python pass-by-reference, even though this is
> technically incorrect. The difference comes from the semantics of
> variables and values. In languages such as C, a variable is an area of
> memory that contains something. An assignment then, copies the value
> on the right into the variable (memory) on the left.
>
> python doesn't have variables, but names. a name is essentially itself
> a reference to some *object* that lives somewhere in memory. An
> assignment is something completely different in this context, it
> merely sets the reference (variable) on the left to *point to* the
> object on the right. So, when evaluating function arguments, names
> inside the function are set to point to the *objects* supplied as
> arguments, (not to names!). Since we don't have access to the caller's
> names, python is not a true pass-by-reference language.
>
> for more on why python is neither call-by-value nor call-by-reference:
> http://effbot.org/zone/call-by-object.htm
>
> for more on python's variable semantics and how it differs from
> languages like C:
>
> http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables<http://python.net/%7Egoodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables>
>
> Hugo
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