Re: [Visualpython-users] strange math

[Tony R. <To...@br...>]

Additionally, many objects that I'm used to working with contain open
streams, database links, and other singular, resource type
references.  How does one copy these by value?

I am most experienced with php, and since 5.x, all objects are passed
by reference; one must use the "copy" keyword which would invoke the
__copy__() special method.

I used the pass stuff around by reference all the time in php, so
python makes perfect sense to me.  Also, pretty much all systems have
a "copy on write" policy anyway, so your not really passing by value
until you modify the new reference.  Most new references created are
read only, and of they are not, I like the python way of making it
obvious in the code by explicity creating a new object, as in the

b = vector(a)

examples.

On Sep 12, 2009, at 4:06 PM, "Jacob Schmidt" <schmidjw@rose-
hulman.edu> wrote:

> There are many reasons for this type of behavior.
>
> And there are two sides to this coin.
>
> A) Why would you want the names to be references of an object?
>
> Imagine you want to pass a custom object into a function. Python
> doesn't
> know how to pass it in by value -> it is a distinct instance.
>
> For example -> you make a Rational class that you can use for rational
> number arithmetic. What are you supposed to do when you add two of
> them
> together? Change the first one? Change the second one? Make a new one?
> Alright. That's simple enough. Make a new one. But what if you go
> deeper, make more complex functions, more complex objects? You can't
> make a new one every time you want to change something. That's time
> consuming AND memory consuming.
>
>
> B) Why can't you just copy and pass everything by value?
>
> How do you copy an object? Maybe for integers, it is a simple matter
> of
> copying the bits into a new object and you're done. How do you copy a
> list? Do you copy the references of all the items in the list to the
> new
> list, or do you try to copy recursively all the items in the list? How
> do you copy a custom object? What if that object is based on dynamic
> data? Each object should have a unique identifier, right? But that
> sometimes defeats the point of the copy. You want the two objects to
> be
> identical.
>
> Python was written in such a way that it balances these arguments.
> Tuples and lists are separate entities because they behave differently
> and are used in the situations for which they are best suited.
>
> Assignment just creates a new reference because it is the cheapest
> quickest way to do it, and in most situations it works.
>
>
>
> On Sat, 2009-09-12 at 15:07 -0400, Bruce Sherwood wrote:
>> None of us readers of this mailing list participated in the
>> creation of Python,
>> so I doubt that anyone here can tell you why Python is built that
>> way, nor argue
>> the merits of the case. Nor was the intent of the experienced.html
>> article to
>> explain this behavior, only to alert you to it.
>>
>> As for utility of this behavior: It's a bit of a stretch for me,
>> but I can
>> imagine a situation where you want to refer to some very long name
>> by using a
>> short name, as in
>>
>>    s = the_sphere_whose_outer_surface_represents_Earth = sphere()
>>    s.pos = vector(10,20,30)
>>
>> I tried a little googling to find some history and found this
>> laconic comment:
>>
>> "It is not weird in Python. Implicit copies are never made, partly
>> because it's very hard to implement it correctly for every object."
>>
>> Also, http://merd.sourceforge.net/inoutness.html seems to summarize
>> these issues
>> for diverse programming languages.
>>
>> Bruce Sherwood
>>
>> Lenore Horner wrote:
>>>
>>>> http://vpython.org/contents/experienced.html
>>>
>>> I'm sorry to say that it was entirely unenlightening.  The only
>>> relevant
>>> statement seems to be this "Making a second, independent copy of an
>>> object is a fairly unusual thing to do in Python, but there exist
>>> special methods for doing this."  But there is zilch explanation
>>> of why
>>> this is so.  I repeat:  why ever would I want two identical
>>> objects that
>>> _must remain so forever_ ?  I completely fail to grasp the utility
>>> of
>>> this concept.  I cannot think of a single thing that I can
>>> accomplish
>>> with two identical objects that I cannot accomplish with only one
>>> of them.
>>>
>>> Sorry to be so boneheaded about this, but I just don't get it.
>>>
>>> Lenore
>>
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Tony Risinger
Application Development Specialist
Tel: 507-535-7563 | Fax: 507-292-5747 | Toll Free: 866-241-0639

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