Understanding python functions - Instant Python tutorial

Chris Carlen crcarleRemoveThis at BOGUSsandia.gov
Fri Jul 13 18:41:39 CEST 2007


Ben Finney wrote:
> Chris Carlen <crcarleRemoveThis at BOGUSsandia.gov> writes:
 >
> def change(some_list):
>     some_list[1] = 4
>
> x = [1,2,3]
> change(x)
> print x # Prints out [1,4,3] 
 > ---
 > def nochange(x):
 >     x = 0
 >
 > y = 1
 > nochange(y)
 > print y # Prints out 1
 >
>>I don't understand Hetland's terminology though, when he is speaking
>>of "binding" and "reference."  Actually, Hetland's entire first
>>paragraph is unclear.
>>
>>Can anyone reword this in a way that is understandable?
> 
> I've had some success with the following way of thinking about it.
> 
> Some languages have "variables", which act like boxes that have names
> etched on the side. Once created, the box can contain an object, and
> it can be inspected while in the box; to change the variable, you
> throw out the object and put a different object in the same box.

Yes, so y = x takes a copy of the stuff in the x box and puts it in the 
y box.  Which is what really happens in the hardware.

> That's not how Python works. Every value is an object; the assignment
> operator binds a name to an object. This is more like writing the name
> on a sticky-note, and sticking it onto the object.
> 
>   * The object itself doesn't change or "move".
> 
>   * The object can be referred to by that name, but isn't "inside" the
>     name in any way.
> 
>   * Assigning multiple names to the same object just means you can
>     refer to that same object by all those names.
> 
>   * When a name goes away, the object still exists -- but it can't be
>     referred to if there are no longer any names left on it.
> 
>   * Assigning a different object to an existing name just means that
>     the same sticky-note has moved from the original object to the new
>     one. Referring to the same name now references a different object,
>     while the existing object keeps all the other names it had.

Excellent description.  This understandable to me since I can envision 
doing this with pointers.  But I have no idea how Python actually 
implements this.  It also appears that I am being guided away from 
thinking about it in terms of internal implementation.

> When you pass an object as a parameter to a function, the object
> receives a new sticky-label: the parameter name under which it was
> received into the function scope. Assignment is an act of binding a
> name to an object; no new object is created, and it still has all the
> other names it had before.

Ok, so I can understand the code above now.

In the first case I pass the reference to the list to change().  In the 
function, some_list is another name referring to the actual object 
[1,2,3].  Then the function changes the object referred to by the second 
element of the list to be a 4 instead of a 2.  (Oh, the concept applies 
here too!)  Out of the function, the name x refers to the list which has 
been changed.

In the second case, y refers to a '1' object and when the function is 
called the object 1 now gets a new reference (name) x inside the 
function.  But then a new object '0' is assigned to the x name.  But the 
y name still refers to a '1'.

I get it.  But I don't like it.  Yet.  Not sure how this will grow on me.

> When the function ends, all the names that were created inside that
> function's scope disappear; but the objects still exist under any
> names they had previously, and if you use those names you'll be
> looking at the same object as was manipulated inside the function.
> 
> When the object has lost all its names -- for example, they've
> disappeared because the scope they were in has closed, or they've been
> re-bound to other objects -- they can no longer be referenced. At some
> point after that, the automatic garbage collection will clean that
> object out of memory.
> 
> This sticky-note analogy, and the behaviour described, is what is
> meant by "references". A name refers to an object; changing the object
> means that by referring to that same object under its different names,
> you will see the same, modified, object.
> 
> In Python, all names are references to objects. The assignment
> operator '=' doesn't create or change a "variable"; instead, it binds
> a name as reference to an object. All functions receive their
> parameters as the existing object with a new name -- a reference to
> that object, just like any other name.
> 
> Hope that helps.


A great deal of help, thanks.  Excellent explanation.  Wow.  This is 
strange.  A part of me wants to run and hide under the nearest 8-bit 
microcontroller.  But I will continue learning Python.



-- 
Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis at BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and
"BOGUS" from email address to reply.



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