[Visualpython-users] Using Python copy function to copy vector by value (was "strange math")

[Parker S. <Par...@li...>]

Hi Lenore (& Robert & the list),

If you're used to passing parameters by value (as in languages like C), it can seem strange to pass them by reference (as with mutable objects in Python). You can dig into the Python docs to find out more about it. 

Python provides for copying mutable objects by value by its copy (and deepcopy) functions:

>>> from copy import *
>>> from visual import *
>>> vector1=vector(1,2,3)
>>> vector2=vector1                           {copy by reference: vector2 and vector1 are two names referring to the same object}
>>> vector2
vector(1, 2, 3)
>>> vector2.y=4
>>> vector2
vector(1, 4, 3)
>>> vector1           
vector(1, 4, 3)                                    
but:
>>> vector3=vector(copy(vector1))     {copy by value: vector3 is a new vector: each of its components is a copy of the corresponding component of vector1}
>>> vector3
vector(1, 4, 3)
>>> vector1.y=5
>>> vector1
vector(1, 5, 3)
>>> vector3
vector(1, 4, 3)

The "clumsy process" you mention accomplishes the same thing in a piecewise fashion.

Hope this helps! And I hope you enjoy using Python as much as thousands of the rest of us do. Best regards,
Parker

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Message: 2
Date: Sat, 12 Sep 2009 10:56:58 -0500
From: Lenore Horner <lh...@si...>
Subject: Re: [Visualpython-users] strange math

So a vector is a list which also has strange properties.  I still want  
to know why it is more useful to have two names for the same thing  
than it is to copy the values from one thing to another thing.

Granted, I have since found out I can work around this problem with  
the following clumsy process (the least clumsy of the three I have  
found).
>>> from visual import *
>>> a = vector(0,0,1)
>>> b = vector(a.x,a.y,a.z)
>>> print a,b
<0, 0, 1> <0, 0, 1>
>>> b.x=1
>>> print a,b
<0, 0, 1> <1, 0, 1>

Lenore

On Sep 12, 2009, at 10:12 , Robert Xiao wrote:

> Vector objects are mutable, which means they get passed by reference  
> instead of by value. It also means that statements like "a=b" do not  
> copy the value, but they copy the reference (making a and b point to  
> the same vector). So,
>
> a = vector(1,2,3)
> b = a
> b.x = 3
> print a
>
> yields
> <3, 2, 3>
>
> as expected, because a and b refer to the same vector. The same  
> thing happens with list objects in Python.
>
> Numbers, tuples and strings are immutable in Python, so they are  
> passed by value.
>
> Robert
>