Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > Python > reflection as in Java: how to create an instance from a classname

Reply
Thread Tools

reflection as in Java: how to create an instance from a classname

 
 
guss
Guest
Posts: n/a
 
      01-05-2009
I cannot find a satisfying answer to this question on the web so let's
try here.

My problem is the following, I would like to instantiate some object
from a configuration file that would contain class names like for
example classname=org.common.resource.MyResource.
Here my resource is the class to instanciate and it is in the module
resource that is in a package hierachy.

In fact I would like to do something very similar to the Java:

klass = Class.forname("org.common.resource.MyResource")

instance = klass.newInstance()

The second line is easy once I have a classobj but I have some
problems to find the right recipe for getting it.

I know how to create a class from scratch with new.classobj but how do
you get a class object and then create an object ?

I would like a recipe working for all cases (whatever the module is
not the local one ...)

Maybe I should follow another idiom I don't know ?

Thanks your help

Guillaume
 
Reply With Quote
 
 
 
 
Bruno Desthuilliers
Guest
Posts: n/a
 
      01-05-2009
guss a écrit :
> I cannot find a satisfying answer to this question on the web so let's
> try here.
>
> My problem is the following, I would like to instantiate some object
> from a configuration file that would contain class names like for
> example classname=org.common.resource.MyResource.
> Here my resource is the class to instanciate and it is in the module
> resource that is in a package hierachy.
>
> In fact I would like to do something very similar to the Java:
>
> klass = Class.forname("org.common.resource.MyResource")
>
> instance = klass.newInstance()
>
> The second line is easy once I have a classobj but I have some
> problems to find the right recipe for getting it.
>
> I know how to create a class from scratch with new.classobj but how do
> you get a class object and then create an object ?
>
> I would like a recipe working for all cases (whatever the module is
> not the local one ...)


use __import__ to get the module object, then getattr(module, classname)
to get the class object (sorry, no much time right now to give you a
full recipe, but that should be enough to get you started).

HTH
 
Reply With Quote
 
 
 
 
guss
Guest
Posts: n/a
 
      01-06-2009
hi Thanks for the tip but I had to play with the __import__ func a
bit.
Indeed to load not only the top module with __import__ one needs to
try to load an object from the module:

Here is my forname:

def forname(modname, classname):
module = __import__(modname,globals(),locals(),['NoName'],-1)
classobj = getattr(module, classname)
return classobj

Like that I can load MyError from the module org.myapp.exceptions

>>> c = forname('org.myapp.exceptions','MyError')
>>> instance = c('My Message')


If I do not put 'NoName' that is a fake object only module will be org
and not org.myapp.exceptions. This is strange ?

I think Python has all the elements for doing java like reflection and
introspection and even more but the API is not as mature and it is
quite difficult to find the information.
There is the need for a high level API.

Maybe it already exists, if anyone knows please tell me.
Thanks.

Guillaume

On Jan 5, 5:34*pm, Bruno Desthuilliers <bruno.
(E-Mail Removed)> wrote:
> guss a écrit :
>
>
>
> > I cannot find a satisfying answer to this question on the web so let's
> > try here.

>
> > My problem is the following, I would like to instantiate some object
> > from a configuration file that would contain class names like for
> > example classname=org.common.resource.MyResource.
> > Here my resource is the class to instanciate and it is in the module
> > resource that is in a package hierachy.

>
> > In fact I would like to do something very similar to the Java:

>
> > klass = Class.forname("org.common.resource.MyResource")

>
> > instance = klass.newInstance()

>
> > The second line is easy once I have a classobj but I have some
> > problems to find the right recipe for getting it.

>
> > I know how to create a class from scratch with new.classobj but how do
> > you get a class object and then create an object ?

>
> > I would like a recipe working for all cases (whatever the module is
> > not the local one ...)

>
> use __import__ to get the module object, then getattr(module, classname)
> to get the class object (sorry, no much time right now to give you a
> full recipe, but that should be enough to get you started).
>
> HTH


 
Reply With Quote
 
Carl Banks
Guest
Posts: n/a
 
      01-06-2009
On Jan 6, 2:24*am, guss <(E-Mail Removed)> wrote:
> hi Thanks for the tip but I had to play with the __import__ func a
> bit.
> Indeed to load not only the top module with __import__ one needs to
> try to load an object from the module:
>
> Here is my forname:
>
> def forname(modname, classname):
> * * module = __import__(modname,globals(),locals(),['NoName'],-1)
> * * classobj = getattr(module, classname)
> * * return classobj
>
> Like that I can load MyError from the module org.myapp.exceptions
>
> >>> c = forname('org.myapp.exceptions','MyError')
> >>> instance = c('My Message')

>
> If I do not put 'NoName' that is a fake object only module will be org
> and not org.myapp.exceptions. This is strange ?


Yes, it's strange. It's that way for historical and logistical
reasons. Here's how I'd write the function; no need to specify
modname and classname separately.


def forname(name):
parts = name.split(".")
obj = __import__(".".join(parts[:-1]))
for part in parts[1:]:
obj = getattr(obj,part)
return obj


> I think Python has all the elements for doing java like reflection and
> introspection and even more but the API is not as mature and it is
> quite difficult to find the information.
> There is the need for a high level API.
>
> Maybe it already exists, if anyone knows please tell me.
> Thanks.


I'm going to suggest that the reason high-level reflection APIs are
used so often in Java is to compensate for Java's lack of run-time
flexibility. If you don't know what class to use or method to call at
compile-time, the easiest thing to do is to store the name in a string
and use the Reflection API to get at it at run-time.

Python, OTOH, is very dynamic, so there is not much demand for spiffy
introspection APIs. Functions and classes are ordinary objects, so if
you don't know what function to call or class to use at compile-time,
you can just pass the objects around. No strings required.

So what I'm saying is: the Python developers didn't bother to make an
high-level, easy-to-use __import__ because there really isn't much
demand for it.


Carl Banks
 
Reply With Quote
 
guss
Guest
Posts: n/a
 
      01-06-2009
Hi Carl

thanks for your improved forname method.

Regarding the high level reflection API, it is true that we don't need
an API as complex as in Java considering the dynamic aspect of Python
but you have a forname function I needed one so
it could be nice to have it (and other services related to reflection
and introspection) standardized in an api supported by default by
Python.

Who doesn't need to load or create a class (a plugin object ...) from
a configuration file these days.

Cheers Guillaume

> On Jan 6, 2:24*am, guss <(E-Mail Removed)> wrote:
>
>
>
> > hi Thanks for the tip but I had to play with the __import__ func a
> > bit.
> > Indeed to load not only the top module with __import__ one needs to
> > try to load an object from the module:

>
> > Here is my forname:

>
> > def forname(modname, classname):
> > * * module = __import__(modname,globals(),locals(),['NoName'],-1)
> > * * classobj = getattr(module, classname)
> > * * return classobj

>
> > Like that I can load MyError from the module org.myapp.exceptions

>
> > >>> c = forname('org.myapp.exceptions','MyError')
> > >>> instance = c('My Message')

>
> > If I do not put 'NoName' that is a fake object only module will be org
> > and not org.myapp.exceptions. This is strange ?

>
> Yes, it's strange. *It's that way for historical and logistical
> reasons. *Here's how I'd write the function; no need to specify
> modname and classname separately.
>
> def forname(name):
> * * parts = name.split(".")
> * * obj = __import__(".".join(parts[:-1]))
> * * for part in parts[1:]:
> * * * * obj = getattr(obj,part)
> * * return obj
>
> > I think Python has all the elements for doing java like reflection and
> > introspection and even more but the API is not as mature and it is
> > quite difficult to find the information.
> > There is the need for a high level API.

>
> > Maybe it already exists, if anyone knows please tell me.
> > Thanks.

>
> I'm going to suggest that the reason high-level reflection APIs are
> used so often in Java is to compensate for Java's lack of run-time
> flexibility. *If you don't know what class to use or method to call at
> compile-time, the easiest thing to do is to store the name in a string
> and use the Reflection API to get at it at run-time.
>
> Python, OTOH, is very dynamic, so there is not much demand for spiffy
> introspection APIs. *Functions and classes are ordinary objects, so if
> you don't know what function to call or class to use at compile-time,
> you can just pass the objects around. *No strings required.
>
> So what I'm saying is: the Python developers didn't bother to make an
> high-level, easy-to-use __import__ because there really isn't much
> demand for it.
>
> Carl Banks


 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Re: How include a large array? Edward A. Falk C Programming 1 04-04-2013 08:07 PM
Getting a reference to enclosing instance without using literal classname Robert Dodier Java 5 09-15-2008 05:10 AM
CSS: "tagname.classname" or ".classname" Ante Perkovic Javascript 2 12-25-2003 03:02 AM
Is `new classname' the same as `new classname()' ? Hongzheng Wang C++ 32 12-05-2003 05:29 PM
classname::classname a type ? Oliver S. C++ 1 09-11-2003 07:47 PM



Advertisments