serializable object references

[Martin Drautzburg]

Is it possible to convert an object into a string that identifies the
object in a way, so it can later be looked up by this string.
Technically this should be possible, because things like

<__main__.Foo instance at 0xb7cfb6ac>

say everything about an object. But how can I look up the real object,
when I only have this string?

I know such a thing can be achieved with a dictionary that holds
reference-object pairs. Is there another way?

[7stud]

Re: serializable object references


Martin Drautzburg wrote:

Is it possible to convert an object into a string that identifies the
object in a way, so it can later be looked up by this string.
Technically this should be possible, because things like
>
<__main__.Foo instance at 0xb7cfb6ac>
>
say everything about an object. But how can I look up the real object,
when I only have this string?
>
I know such a thing can be achieved with a dictionary that holds
reference-object pairs. Is there another way?

How about:

class Dog(object):
def __init__(self, name):
self.name = name

d = Dog("Spot")
print globals()["d"].name

[7stud]

Re: serializable object references

On Apr 22, 5:07 am, Martin Drautzburg <Martin.Drautzb ...@web.de>
wrote:

>
<__main__.Foo instance at 0xb7cfb6ac>
>
But how can I look up the real object,
when I only have this string?
>

You can't because that identifies the instance with an address, and
pointers are not part of the python language.

[Gabriel Genellina]

Re: serializable object references

En Sun, 22 Apr 2007 08:07:27 -0300, Martin Drautzburg
<Martin.Drautzb urg@web.deescri bió:

Is it possible to convert an object into a string that identifies the
object in a way, so it can later be looked up by this string.
Technically this should be possible, because things like
>
<__main__.Foo instance at 0xb7cfb6ac>
>
say everything about an object. But how can I look up the real object,
when I only have this string?
>
I know such a thing can be achieved with a dictionary that holds
reference-object pairs. Is there another way?

Without further information, this would be the way.
What do you want to achieve? Maybe you are looking for the pickle module.

--
Gabriel Genellina

[Daniel Nogradi]

Re: serializable object references

Is it possible to convert an object into a string that identifies the

object in a way, so it can later be looked up by this string.
Technically this should be possible, because things like
>
<__main__.Foo instance at 0xb7cfb6ac>
>
say everything about an object. But how can I look up the real object,
when I only have this string?
>
I know such a thing can be achieved with a dictionary that holds
reference-object pairs. Is there another way?

There is the pickle module for serializing objects into strings:

http://docs.python.org/lib/module-pickle.html

If you need to store objects and later look them up by some key I
would recommend using a database. There are plenty:

http://docs.python.org/lib/persistence.html

HTH,
Daniel

[Martin Drautzburg]

Re: serializable object references

Gabriel Genellina wrote:

En Sun, 22 Apr 2007 08:07:27 -0300, Martin Drautzburg
<Martin.Drautzb urg@web.deescri bió:
>

>Is it possible to convert an object into a string that identifies the
>object in a way, so it can later be looked up by this string.
>Technically this should be possible, because things like
>>
><__main__.Fo o instance at 0xb7cfb6ac>
>>
>say everything about an object. But how can I look up the real
>object, when I only have this string?
>>
>I know such a thing can be achieved with a dictionary that holds
>reference-object pairs. Is there another way?

>
Without further information, this would be the way.
What do you want to achieve? Maybe you are looking for the pickle
module.
>

I was thinking that it would be nice if a web application could talk to
real objects. The client side does not need to know the internals of an
object, it acts as a "view" for server-side models. All it has to be
able to do is invoke methods on "its" model. So a view could just
store "its" object-reference in an instance variable and pass it to the
server, where my problem of looking it up comes in.

I am currently learning about web services but my feeling is
that "state" is not an integral part of this concept, rather an add-on,
but I may be mistaken here.

But for any reasonable web application you have to introduce state one
way or the other. My impression is, that this is achieved with "session
objects", which hold all the state of a session (again I may be
mistaken). But this would be a strange concept in an OO world. You
might as well put "ALL" state into a global "state" state object, where
all the class methods deposit their state and classes would hold
behavior only and no state. This is of course nothing an OO programmer
would want to do.

[Gabriel Genellina]

Re: serializable object references

En Sun, 22 Apr 2007 12:47:10 -0300, Martin Drautzburg
<Martin.Drautzb urg@web.deescri bió:

I was thinking that it would be nice if a web application could talk to
real objects. The client side does not need to know the internals of an
object, it acts as a "view" for server-side models. All it has to be
able to do is invoke methods on "its" model. So a view could just
store "its" object-reference in an instance variable and pass it to the
server, where my problem of looking it up comes in.

This is more or less what several web frameworks do. You publish objects;
URLs are mapped to method objects; URL parameters become method
parameters. See http://wiki.python.org/moin/WebFrameworks

I am currently learning about web services but my feeling is
that "state" is not an integral part of this concept, rather an add-on,
but I may be mistaken here.
>
But for any reasonable web application you have to introduce state one
way or the other. My impression is, that this is achieved with "session
objects", which hold all the state of a session (again I may be
mistaken). But this would be a strange concept in an OO world. You
might as well put "ALL" state into a global "state" state object, where
all the class methods deposit their state and classes would hold
behavior only and no state. This is of course nothing an OO programmer
would want to do.

You can have a "persistent " state (stored in a backend database) and
"transitory " state (kept in session objects).

--
Gabriel Genellina

[Martin Drautzburg]

Re: serializable object references

Gabriel Genellina wrote:

En Sun, 22 Apr 2007 12:47:10 -0300, Martin Drautzburg
<Martin.Drautzb urg@web.deescri bió:
>

>I was thinking that it would be nice if a web application could talk
>to real objects. The client side does not need to know the internals
>of an object, it acts as a "view" for server-side models. All it has
>to be able to do is invoke methods on "its" model. So a view could
>just store "its" object-reference in an instance variable and pass it
>to the server, where my problem of looking it up comes in.

>
This is more or less what several web frameworks do. You publish
objects; URLs are mapped to method objects; URL parameters become
method parameters. See http://wiki.python.org/moin/WebFrameworks
>

Okay will look. I have checked out cherrypy, but it does not seem to
support direct object references, i.e. the server-side objects are
really stateless and all calls to an object method will see the same
state unless you do something about it youself.

I have also looked at the wonderful qooxdoo javascript framework and in
the examples they have, the data I receive on a published object method
on my cherrypy server is:
dict: {
'_ScriptTranspo rt_id': '11',
'_ScriptTranspo rt_data': '{
"service":"qoox doo.test",
"method":"sleep ",
"id":13,
"params":["10"],
"server_data":n ull
}',
'nocache': '1177256001914'
}

I am not sure what all of them mean, but my impression is that none of
them denote an object in the sense of an INSTANCE, at least "service"
and "method" definitely do not. The "id" is simply incremented with
every call, so it is again not an instance.

Now I could of course add an object reference do the "params" field and
have qooxdoo.text dispatch the call to an INSTANCE of an object and
invoke sleep() there. But first it is a shame, that I have to provide
this magic myself, and second it raises again my original question: how
to I pass an object reference and look up the object in qooxdoo.test.

I know I can do this with a dictionary, I just thought that the
__repr__() of an object could be used, as it seems the most obvious way
to do it.

[Gabriel Genellina]

Re: serializable object references

En Mon, 23 Apr 2007 03:35:42 -0300, Martin Drautzburg
<Martin.Drautzb urg@web.deescri bió:

Gabriel Genellina wrote:
>

>En Sun, 22 Apr 2007 12:47:10 -0300, Martin Drautzburg
><Martin.Drautz burg@web.deescr ibió:
>>

>>I was thinking that it would be nice if a web application could talk
>>to real objects. The client side does not need to know the internals
>>of an object, it acts as a "view" for server-side models. All it has
>>to be able to do is invoke methods on "its" model. So a view could
>>just store "its" object-reference in an instance variable and pass it
>>to the server, where my problem of looking it up comes in.

>>
>This is more or less what several web frameworks do. You publish
>objects; URLs are mapped to method objects; URL parameters become
>method parameters. See http://wiki.python.org/moin/WebFrameworks

>
Okay will look. I have checked out cherrypy, but it does not seem to
support direct object references, i.e. the server-side objects are
really stateless and all calls to an object method will see the same
state unless you do something about it youself.

The description I wrote above aplies exactly to ZOPE: URLs map exactly to
object methods and those objects can (and should) maintain state. (Zope
uses an OO database, ZODB, to hold those objects).
But Zope is a big framework and not easy to grasp, so I would not recomend
it for a small site or web application.
Using CherryPy you can keep state across requests, if you use some sort of
session machinery; at least you could build a mapping SessionID-State (the
state being as complex as you want).
I think TurboGears has something built in for managing sessions, but I'm
not sure.

I have also looked at the wonderful qooxdoo javascript framework and in
the examples they have, the data I receive on a published object method
on my cherrypy server is:

Never used qooxdoo...

--
Gabriel Genellina