Finding the instance reference of an object

[Astley Le Jasper]

Sorry for the numpty question ...

How do you find the reference name of an object?

So if i have this

bob = modulename.obje ctname()

how do i find that the name is 'bob'

[Carsten Haese]

Re: Finding the instance reference of an object

Astley Le Jasper wrote:

Sorry for the numpty question ...
>
How do you find the reference name of an object?
>
So if i have this
>
bob = modulename.obje ctname()
>
how do i find that the name is 'bob'

Why do you need to find that? You know that its name is 'bob'.

--
Carsten Haese

http://informixdb.sourceforge.net

[Michele Simionato]

Re: Finding the instance reference of an object

On Oct 16, 4:01 pm, Astley Le Jasper <Astley.lejas.. .@gmail.com>
wrote:

Sorry for the numpty question ...
>
How do you find the reference name of an object?
>
So if i have this
>
bob = modulename.obje ctname()
>
how do i find that the name is 'bob'

This is a FAQ:

http://www.python.org/doc/faq/programming/#how-can-my-code-discover-the-name-of-an-object

[Astley Le Jasper]

Re: Finding the instance reference of an object

On 16 Oct, 16:52, Carsten Haese <carsten.ha...@ gmail.comwrote:

Astley Le Jasper wrote:

Sorry for the numpty question ...

>

How do you find the reference name of an object?

>

So if i have this

>

bob = modulename.obje ctname()

>

how do i find that the name is 'bob'

>
Why do you need to find that? You know that its name is 'bob'.
>
--
Carsten Haesehttp://informixdb.sour ceforge.net

I'm creating mulitple instances, putting them in a list, iterating
through the list to send them to some functions where process them
with some instance specific parameters. Something along the lines of:

bob = someobject()
harry = someobject()
fred = someobject()

parameterdict = {'bob':(0,1,2), 'harry':(3,4,5) ,'fred':(6,7,8) }
people_list = (bob, harry, fred)

for person in people_list:
add_parameters( person)

def add_parameters( person)
mytuple = parameterdict[??????instance. name????]
person.x = mytuple[0]
person.y = mytuple[1]
person.z = mytuple[2]

.... alternatively there is probably a very much easier way of doing
it.

[Steven D'Aprano]

Re: Finding the instance reference of an object

On Thu, 16 Oct 2008 08:04:23 -0700, Astley Le Jasper wrote:

I'm creating mulitple instances, putting them in a list, iterating
through the list to send them to some functions where process them with
some instance specific parameters. Something along the lines of:
>
bob = someobject()
harry = someobject()
fred = someobject()
>
parameterdict = {'bob': (0,1,2), 'harry': (3,4,5), 'fred': (6,7,8)}
people_list = (bob, harry, fred)
>
for person in people_list:
add_parameters( person)
>
def add_parameters( person)
mytuple = parameterdict[??????instance. name????]
person.x = mytuple[0]
person.y = mytuple[1]
person.z = mytuple[2]
>
... alternatively there is probably a very much easier way of doing it.

Assuming that someobject() objects are hashable, you can do this:

# use the objects themselves as keys, not their names
parameterdict = {bob: (0,1,2), harry: (3,4,5), fred: (6,7,8)}
people_list = [bob, harry, fred]

but of course that doesn't work if someobject() items aren't hashable
(say, lists or dicts).

But in my opinion, this is probably the best way (untested):

def add_parameters( person, x, y, z):
# note we don't use any global variables
person.x = x
person.y = y
person.z = z

parameters = [(0,1,2), (3,4,5), (6,7,8)]
people = [bob, harry, fred]
for person, args in zip(people, parameters):
add_parameters( person, *args)



--
Steven

[Diez B. Roggisch]

Re: Finding the instance reference of an object

Astley Le Jasper schrieb:

On 16 Oct, 16:52, Carsten Haese <carsten.ha...@ gmail.comwrote:

>Astley Le Jasper wrote:

>>Sorry for the numpty question ...
>>How do you find the reference name of an object?
>>So if i have this
>>bob = modulename.obje ctname()
>>how do i find that the name is 'bob'

>Why do you need to find that? You know that its name is 'bob'.
>>
>--
>Carsten Haesehttp://informixdb.sour ceforge.net

>
I'm creating mulitple instances, putting them in a list, iterating
through the list to send them to some functions where process them
with some instance specific parameters. Something along the lines of:
>
bob = someobject()
harry = someobject()
fred = someobject()
>
parameterdict = {'bob':(0,1,2), 'harry':(3,4,5) ,'fred':(6,7,8) }
people_list = (bob, harry, fred)
>
for person in people_list:
add_parameters( person)
>
def add_parameters( person)
mytuple = parameterdict[??????instance. name????]
person.x = mytuple[0]
person.y = mytuple[1]
person.z = mytuple[2]
>
... alternatively there is probably a very much easier way of doing
it.

Why not simply do

bob = someobject(0, 1, 2)

?

Diez

[Larry Bates]

Re: Finding the instance reference of an object

Astley Le Jasper wrote:

Sorry for the numpty question ...
>
How do you find the reference name of an object?
>
So if i have this
>
bob = modulename.obje ctname()
>
how do i find that the name is 'bob'

Short answer is that you can't. This because Python's names (bob) are bound to
objects (modulename.obj ectname()). They are NOT variables as they are in
"other" programming languages. It is perfectly legal in Python to bind multiple
names to a single object:

a=b=c=modulenam e.objectname()

a, b, and c all point to the same object. An object can have an unlimited
number of names bound to it. This is one of the most difficult concepts for
many beginning Python programmers to understand (I know I had a difficult time
at first). It is just not how we taught ourselves to think about "variables"
and you can write quite a lot of Python treating the names you bind to objects
like they were "variables" .

To accomplish what you want, put your instances in a dictionary.

instances = {}
instances['bob'] = modulename.obje ctname()
instances['joe'] = modulename.obje ctname()
..
..
..

Then you can reference them as:

instances[name]

and/or you can pass the name in as an argument to the __init__ method of
objectname so that it can "hold" the name of the dictionary key that references
it (good for debugging/logging).

-Larry

[Joe Strout]

Re: Finding the instance reference of an object

On Oct 16, 2008, at 10:59 AM, Larry Bates wrote:

>how do i find that the name is 'bob'

>
Short answer is that you can't. This because Python's names (bob)
are bound to objects (modulename.obj ectname()). They are NOT
variables as they are in "other" programming languages.

Which other programming languages? I've never seen an OOP language
that didn't work the same way as Python.

However, 'bob' here really is a variable. It's a variable whose value
(at the moment) is a reference to some object.

It is perfectly legal in Python to bind multiple names to a single
object:
>
a=b=c=modulenam e.objectname()

Right -- three variables (a, b, and c) that all have the same value,
i.e. all refer to the same object. There's nothing more mysterious
here than

i=j=k=42

where i, j, and k all have the same value. (The OP's question would
be like asking "what is the name of the variable referring to 42? And
while you might, in theory, be able to produce a list of all such
variables by trolling through the Python's internals, it's a bit of a
silly thing to do.)

a, b, and c all point to the same object. An object can have an
unlimited number of names bound to it. This is one of the most
difficult concepts for many beginning Python programmers to
understand (I know I had a difficult time at first). It is just not
how we taught ourselves to think about "variables" and you can write
quite a lot of Python treating the names you bind to objects like
they were "variables" .

Well, they are variables. I'm not quite grasping the difficulty
here... unless perhaps you were (at first) thinking of the variables
as holding the object values, rather than the object references. That
is indeed something important to grasp, since it explains why if you do

a = b # where b was some object with an attribute 'foo'...
a.foo = 42

....you now find that b.foo is 42 too. Nothing mysterious once you
realize that the value of a and b is a reference to some object that
has a "foo" attribute.

Not sure if all this was helpful to anyone, but I hope so!

Best,
- Joe

[Astley Le Jasper]

Re: Finding the instance reference of an object

On 16 Oct, 18:53, "Diez B. Roggisch" <de...@nospam.w eb.dewrote:

Astley Le Jasper schrieb:
>
>
>

On 16 Oct, 16:52, Carsten Haese <carsten.ha...@ gmail.comwrote:

Astley Le Jasper wrote:
>Sorry for the numpty question ...
>How do you find the reference name of an object?
>So if i have this
>bob = modulename.obje ctname()
>how do i find that the name is 'bob'
Why do you need to find that? You know that its name is 'bob'.

>

--
Carsten Haesehttp://informixdb.sour ceforge.net

>

I'm creating mulitple instances, putting them in a list, iterating
through the list to send them to some functions where process them
with some instance specific parameters. Something along the lines of:

>

bob = someobject()
harry = someobject()
fred = someobject()

>

parameterdict = {'bob':(0,1,2), 'harry':(3,4,5) ,'fred':(6,7,8) }
people_list = (bob, harry, fred)

>

for person in people_list:
  add_parameters( person)

>

def add_parameters( person)
  mytuple = parameterdict[??????instance. name????]
  person.x = mytuple[0]
  person.y = mytuple[1]
  person.z = mytuple[2]

>

... alternatively there is probably a very much easier way of doing
it.

>
Why not simply do
>
bob = someobject(0, 1, 2)
>
?
>
Diez

Because that was pseudo code to demonstrate what I was trying to
achieve. The parameters are more complicated than that.

[Astley Le Jasper]

Re: Finding the instance reference of an object

Thanks for all the responses. That helps.

Ta

ALJ

[Aaron \Castironpi\ Brady]

Re: Finding the instance reference of an object

On Oct 16, 12:25 pm, Astley Le Jasper <Astley.lejas.. .@gmail.com>
wrote:

Thanks for all the responses. That helps.
>
Ta
>
ALJ

If you're sure it's unique, why not just scan through the pairs in
locals()?

for k, v in locals():
if v is the_object_im_l ooking_for:
name_im_looking _for= k

This method can sometimes return more than one name, due to the
variable ordering of objects in dictionaries.

[Steven D'Aprano]

Re: Finding the instance reference of an object

On Thu, 16 Oct 2008 11:24:28 -0600, Joe Strout wrote:

On Oct 16, 2008, at 10:59 AM, Larry Bates wrote:
>

>>how do i find that the name is 'bob'

>>
>Short answer is that you can't. This because Python's names (bob) are
>bound to objects (modulename.obj ectname()). They are NOT variables as
>they are in "other" programming languages.

>
Which other programming languages? I've never seen an OOP language that
didn't work the same way as Python.
>
However, 'bob' here really is a variable. It's a variable whose value
(at the moment) is a reference to some object.

Traditionally, a "variable" is a named memory location.

The main objection I have to using "variable" to describe Python name/
value bindings is that it has connotations that will confuse programmers
who are familiar with C-like languages. For example:

def inc(x):
x += 1

n = 1
inc(n)
assert n == 2

Why doesn't that work? This is completely mysterious to anyone expecting
C-like variables.

At this point people will often start confusing the issue by claiming
that "all Python variables are pointers", which is an *implementation
detail* in CPython but not in other implementations , like PyPy or Jython.

Or people will imagine that Python makes a copy of the variable when you
call a function. That's not true, and in fact Python explicitly promises
never to copy a value unless you explicitly tell it to, but it seems to
explain the above, at least until the programmer starts *assuming* call-
by-value behaviour and discovers this:

def inc(alist):
alist += [1] # or alist.append(1) if you prefer
return alist

a = [1, 2, 3]
b = inc(a)
assert a == [1, 2, 3]

Are functions call by value or call by reference???

(Answer: neither. They are call by name.)


I myself often talk about variables as shorthand. But it's a bad habit,
because it is misleading to anyone who thinks they know how variables
behave, so when I catch myself doing it I fix it and talk about name
bindings. Noobs might not know what that means, but that's a feature, not
a bug, because it gets them paying attention instead of making faulty
assumptions.

Of course, you're entitled to define "variable" any way you like, and
then insist that Python variables don't behave like variables in other
languages. Personally, I don't think that's helpful to anyone.

[snip]

Well, they are variables. I'm not quite grasping the difficulty here...
unless perhaps you were (at first) thinking of the variables as holding
the object values, rather than the object references.

But that surely is what almost everyone will think, almost all the time.
Consider:

x = 5
y = x + 3

I'm pretty sure that nearly everyone will read it as "assign 5 to x, then
add 3 to x and assign the result to y" instead of:

"assign a reference to the object 5 to x, then dereference x to get the
object 5, add it to the object 3 giving the object 8, and assign a
reference to that result to y".

Of course that's what's really happening under the hood, and you can't
*properly* understand how Python behaves without understanding that. But
I'm pretty sure few people think that way naturally, especially noobs.
References are essentially like pointers, and learning pointers is
notoriously difficult for people. Python does a magnificent job of making
references easy, but it does so by almost always hiding the fact that it
uses references under the hood. That's why talk about variables is so
seductive and dangerous: Python's behaviour is *usually* identical to the
behaviour most newbies expect from a language with "variables" .



--
Steven

[Steven D'Aprano]

Re: Finding the instance reference of an object

On Thu, 16 Oct 2008 11:51:43 -0700, Aaron \"Castironpi \" Brady wrote:

If you're sure it's unique, why not just scan through the pairs in
locals()?
>
for k, v in locals():
if v is the_object_im_l ooking_for:
name_im_looking _for= k
>
This method can sometimes return more than one name, due to the variable
ordering of objects in dictionaries.

Because:

(1) in general, objects can have no name at all, or multiple names, so
this won't work in general (although it may work in a specific case);

(2) it's awfully inefficient if you are doing it a lot; and

(3) even if it works and you can afford to pay the cost, it is almost
certainly the Wrong Way to solve the problem at hand.




--
Steven

[Aaron \Castironpi\ Brady]

Re: Finding the instance reference of an object

On Oct 16, 8:30 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com .auwrote:

On Thu, 16 Oct 2008 11:51:43 -0700, Aaron \"Castironpi \" Brady wrote:

If you're sure it's unique, why not just scan through the pairs in
locals()?

>

for k, v in locals():
  if v is the_object_im_l ooking_for:
    name_im_looking _for= k

>

This method can sometimes return more than one name, due to the variable
ordering of objects in dictionaries.

>
Because:
>
(1) in general, objects can have no name at all, or multiple names, so
this won't work in general (although it may work in a specific case);
>
(2) it's awfully inefficient if you are doing it a lot; and
>
(3) even if it works and you can afford to pay the cost, it is almost
certainly the Wrong Way to solve the problem at hand.
>
--
Steven

In fact, what's more called-for, is a mapping in reverse:

name_im_looking _for= lookup[ the_object_im_l ooking_for ]

You'll just have to modify it in parallel with your local variables,
which is a (yet another) bad sign.

[Terry Reedy]

Re: Finding the instance reference of an object

Astley Le Jasper wrote:

I'm creating mulitple instances, putting them in a list, iterating
through the list to send them to some functions where process them
with some instance specific parameters. Something along the lines of:
>
bob = someobject()
harry = someobject()
fred = someobject()
>
parameterdict = {'bob':(0,1,2), 'harry':(3,4,5) ,'fred':(6,7,8) }
people_list = (bob, harry, fred)
>
for person in people_list:
add_parameters( person)
>
def add_parameters( person)
mytuple = parameterdict[??????instance. name????]
person.x = mytuple[0]
person.y = mytuple[1]
person.z = mytuple[2]

If you want an object to have a 'personal' name, give it a name
attribute, just like python does to modules, classes, and functions.

class someob():
def __init__(self,n ame):
self.name = name

people = {someob('bob'), someob('harry') , someob('fred')
....
def add_param(perso n):
mytuple = parameterdict[person.name]
...