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

G

guss

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
 
B

Bruno Desthuilliers

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
 
G

guss

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

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
 
C

Carl Banks

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

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
 
G

guss

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
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
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
 

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