Re: dynamically instantiate a model at run time

[bruno desthuilliers]

On 24 déc, 15:01, Jeff FW  wrote:
> I think you've got a small typo in the code there, that might be
> confusing to the OP--shouldn't the get_model() call have quotes around
> "tag"?  Like so:
> model_class = get_model("test", "tag")

Nope. In the OP's code, 'tag' is a variable which contains the name of
the model class.


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Re: dynamically instantiate a model at run time

[Jeff FW]

I think you've got a small typo in the code there, that might be
confusing to the OP--shouldn't the get_model() call have quotes around
"tag"?  Like so:
model_class = get_model("test", "tag")

-Jeff

On Dec 23, 5:03 pm, bruno desthuilliers
 wrote:
> On 23 déc, 20:23, "dick...@gmail.com"  wrote:
>
> > i'm trying to do some instantiation of models, based on run time
> > parameters. i'm new to django/python so not sure the term, but the
> > relative java term is reflection.
>
> The term in Python is... well, it isn't. With a dynamic language, it's
> just daily programming, you know ?-)
>
> (ok, nitpicking, sorry. Please don't hold it against me - or at least
> read the practical solution at the bottom before).
>
>
>
> > for example, if i have a model, in my django models.py:
>
> > class Foo(models.Model):
> >     name = models.CharField(max_length=20)
>
> > now in some views.py code, i'm given, say some xml that defines a new
> > Foo object, so i want to dynamically create a new Foo object and save
> > it.  bar
>
> > i've tried a few things below, but namely, what i'm trying to do is
> > parse the xml, see that i need to create "Foo" and set it's name to
> > bar:
>
> > tag = "Foo"
> > key = "name"
> > value = "bar"
>
> >  _temp = __import__('project.test.models', globals(), locals(),
> > ['Foo'], -1)
> >  createobj = getattr(_temp, tag)
>
> This returns a class object, not an instance.
>
> > setattr(createobj, key, value)
>
> This sets the "name" attribute of *class* Foo to "bar".
>
> > createobj.save()
>
> > i'm getting unbound method on save,
>
> Not surprinsing since you're calling it on a class object... An
> unbound method (IOW: an instance method looked up on the class) takes
> an instance as first param - you know, the (in)famous  'self'
> argument ?
>
> 
> What is defined (using the 'def' statement) in a class statement is
> _really_ a function. It only becomes a "method" (in fact just a thin
> wrapper around the class, instance and function - name it, a partial
> evaluation of the function) when looked up on an instance - and an
> "unbound method" (same thing modulo the instance) when looked up on
> the class.
>
> IOW and to make a long story short:
>
> instance.method(arg)
> # is equivalent to
> cls.method(instance, arg)
> # which - if and only if "method" is defined in "cls"  (_not_
> inherited)
> # is equivalent to:
> cls.__dict__['method'](instance, arg)
>
> If you want to learn more about it, google for python+descriptor
> +protocol. It's the same thing that powers computed attributes (aka
> properties), and it's something worth learning about.
> 
>
> > and tried various ways to save.
> > but i'm thinking it is in the class instantiation where the problem
> > is.
>
> Indeed. You didn't create a Foo instance.
>
> Ok, now for the solution(s):
>
> # 1/ Django's models are all loaded at startup and cached.
> #    To get a model by appname/model, just use:
>
> from db.models.loading import get_model
> model_class = get_model("test", tag)
>
> # NB : at this point, "model_class" is what you named "createobj"
>
> # 2/ once you have the class, you need an instance:
>
> instance = model_class()
>
> # 3/ then you can set the attribute and save the instance:
>
> setattr(instance, key, value)
> instance.save()
>
> # ALT 1
> # you can save on typing, using keyword args
> # to instanciate your model:
>
> from db.models.loading import get_model
> model_class = get_model("test", tag)
> instance = model_class(**{key:value})
> instance.save()
>
> # ALT2
> #  and save even more typing using the
> #  model's manager "create" method:
>
> from db.models.loading import get_model
> model_class = get_model("test", tag)
> # create and save the instance in one step
> instance = model_class.objects.create(**{key:value})
>
> HTH.
>
> Oh, and welcome onboard BTW - we all hope you'll enjoy the trip !-)
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Re: dynamically instantiate a model at run time

[bruno desthuilliers]

On 23 déc, 20:23, "dick...@gmail.com"  wrote:
> i'm trying to do some instantiation of models, based on run time
> parameters. i'm new to django/python so not sure the term, but the
> relative java term is reflection.

The term in Python is... well, it isn't. With a dynamic language, it's
just daily programming, you know ?-)

(ok, nitpicking, sorry. Please don't hold it against me - or at least
read the practical solution at the bottom before).

> for example, if i have a model, in my django models.py:
>
> class Foo(models.Model):
> name = models.CharField(max_length=20)
>
> now in some views.py code, i'm given, say some xml that defines a new
> Foo object, so i want to dynamically create a new Foo object and save
> it.  bar
>
> i've tried a few things below, but namely, what i'm trying to do is
> parse the xml, see that i need to create "Foo" and set it's name to
> bar:
>
> tag = "Foo"
> key = "name"
> value = "bar"
>
>  _temp = __import__('project.test.models', globals(), locals(),
> ['Foo'], -1)
>  createobj = getattr(_temp, tag)

This returns a class object, not an instance.

> setattr(createobj, key, value)

This sets the "name" attribute of *class* Foo to "bar".

> createobj.save()
>
> i'm getting unbound method on save,

Not surprinsing since you're calling it on a class object... An
unbound method (IOW: an instance method looked up on the class) takes
an instance as first param - you know, the (in)famous  'self'
argument ?


What is defined (using the 'def' statement) in a class statement is
_really_ a function. It only becomes a "method" (in fact just a thin
wrapper around the class, instance and function - name it, a partial
evaluation of the function) when looked up on an instance - and an
"unbound method" (same thing modulo the instance) when looked up on
the class.

IOW and to make a long story short:

instance.method(arg)
# is equivalent to
cls.method(instance, arg)
# which - if and only if "method" is defined in "cls"  (_not_
inherited)
# is equivalent to:
cls.__dict__['method'](instance, arg)

If you want to learn more about it, google for python+descriptor
+protocol. It's the same thing that powers computed attributes (aka
properties), and it's something worth learning about.


> and tried various ways to save.
> but i'm thinking it is in the class instantiation where the problem
> is.

Indeed. You didn't create a Foo instance.


Ok, now for the solution(s):

# 1/ Django's models are all loaded at startup and cached.
#To get a model by appname/model, just use:

from db.models.loading import get_model
model_class = get_model("test", tag)

# NB : at this point, "model_class" is what you named "createobj"

# 2/ once you have the class, you need an instance:

instance = model_class()

# 3/ then you can set the attribute and save the instance:

setattr(instance, key, value)
instance.save()

# ALT 1
# you can save on typing, using keyword args
# to instanciate your model:

from db.models.loading import get_model
model_class = get_model("test", tag)
instance = model_class(**{key:value})
instance.save()

# ALT2
#  and save even more typing using the
#  model's manager "create" method:

from db.models.loading import get_model
model_class = get_model("test", tag)
# create and save the instance in one step
instance = model_class.objects.create(**{key:value})


HTH.

Oh, and welcome onboard BTW - we all hope you'll enjoy the trip !-)
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dynamically instantiate a model at run time

[dick...@gmail.com]

i'm trying to do some instantiation of models, based on run time
parameters. i'm new to django/python so not sure the term, but the
relative java term is reflection.

for example, if i have a model, in my django models.py:

class Foo(models.Model):
name = models.CharField(max_length=20)

now in some views.py code, i'm given, say some xml that defines a new
Foo object, so i want to dynamically create a new Foo object and save
it.  bar

i've tried a few things below, but namely, what i'm trying to do is
parse the xml, see that i need to create "Foo" and set it's name to
bar:

tag = "Foo"
key = "name"
value = "bar"

 _temp = __import__('project.test.models', globals(), locals(),
['Foo'], -1)
 createobj = getattr(_temp, tag)
setattr(createobj, key, value)
createobj.save()

i'm getting unbound method on save, and tried various ways to save.
but i'm thinking it is in the class instantiation where the problem
is.

thanks!

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