custom field for callable python object like class or function

[Michael Palumbo]

Hi,

Do you know a custom field for a callable python object like a Class or a function ?

It could store it as a string in the DB: "module1.module2.class"  or   "module1.module2.function"

With that string, it could load it as a real python object when you get it from the DB.

For example, you could do something like that:

from module1.module2 import testme

mymodel.process = testme

mymodel.save() => stores the string "module1.module2.testme"

# Later...

ins = mymodel.objects.get(..)

ins.process('OK') # ins.process can be called because it has been resolved to the testme function

Thanks.

[Kurtis Mullins]

It looks like you want to, basically, store a reference to a Python object in your Model. It shouldn't be too extremely difficult. I'd create a Model for this purpose though and just use extra methods to provide any functionality you need. Just as a quick prototypical example:

class ReferenceModel(Model):

    _reference = models.CharField(max_length=150)

    def get_reference(self):

        """

        Load the string from self._reference and use some sort of a process

        to dynamically load the object.

        e.g. http://stackoverflow.com/questions/547829/how-to-dynamically-load-a-python-class

        """

    def set_reference(self, input):

        self._reference = input

    foo = property(get_reference, set_reference)

Anyways, just a guess at one of many ways to possibly accomplish this task.

        

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[Kurtis Mullins]

Whoops,

foo = property(get_reference, set_reference)

should be

reference = property(get_reference, set_reference)

Sorry, was reading another page to make sure I have the property code right:

http://stackoverflow.com/questions/1454727/do-properties-work-on-django-model-fields

[Michael Palumbo]

ok thanks, so you'd rather do it with a Model than a custom field.

I actually started to make a custom field but I had a few issues while displaying it in the admin so that's why I asked the question then.

[Melvyn Sopacua]

On 23-8-2012 0:37, Michael Palumbo wrote:
> ok thanks, so you'd rather do it with a Model than a custom field.
> I actually started to make a custom field but I had a few issues while
> displaying it in the admin so that's why I asked the question then.

You can pickle an object and store the result in a blob-type field or
encode it and put it in a TextArea field:
<http://docs.python.org/library/pickle.html#pickle>
It's basically what the session module is doing. Perhaps look there for
inspiration:
<https://docs.djangoproject.com/en/1.4/topics/http/sessions/#technical-details>

--
Melvyn Sopacua

[Michael Palumbo]

I did not know the pickle module. This is interesting. Thanks.

However, after playing with it, I am not sure it fits totally my need because I'd like humans to be able to write a string like "modulea.moduleb.MyClass" in the admin or so.

So I kept trying making a custom field (instead of a Model) by creating a custom field, formfield and widget.

The list view works well, even though it displays <class 'modulea.moduleb.MyClass'> instead of modulea.moduleb.MyClass... 

The add page works as well, I can add a model with the value "modulea.moduleb.MyClass" in the field and then it works.

BUT I have an issue with the detail page of a model.

I get a TypeError because my field has a class reference (which is callable) and the code below then instances it!

• c:\Users\Mike\Dropbox\development\tools\virtualenvs\django-1.4\lib\site-packages\django\forms\forms.py in as_widget

  451.             else:

  452.                 attrs['id'] = self.html_initial_id

  454.         if not only_initial:

  455.             name = self.html_name

  456.         else:

  457.             name = self.html_initial_name

  458.         return widget.render(name, self.value(), attrs=attrs)

       ...

  460.     def as_text(self, attrs=None, **kwargs):

  461.         """

  462.         Returns a string of HTML for representing this as an <input type="text">.

  463.         """

  464.         return self.as_widget(TextInput(), attrs, **kwargs)

  ▼ Local vars

  Variable	Value
  auto_id	u'id_mapper_class'
  widget	<xmlmapping.fieldsbis.PickleWidget object at 0x05B13290>
  name	'mapper_class'
  self	<django.forms.forms.BoundField object at 0x05B1A9F0>
  only_initial	False
  attrs	{'id': u'id_mapper_class'}

• c:\Users\Mike\Dropbox\development\tools\virtualenvs\django-1.4\lib\site-packages\django\forms\forms.py in value

  484.         """

  485.         Returns the value for this BoundField, using the initial value if

  486.         the form is not bound or the data otherwise.

  487.         """

  488.         if not self.form.is_bound:

  489.             data = self.form.initial.get(self.name, self.field.initial)

  490.             if callable(data):

  491.                 data = data()

       ...

  492.         else:

  493.             data = self.field.bound_data(

  494.                 self.data, self.form.initial.get(self.name, self.field.initial)

  495.             )

  496.         return self.field.prepare_value(data)

  ▼ Local vars

  Variable	Value
  self	<django.forms.forms.BoundField object at 0x05B1A9F0>
  data	<class 'xmlmapping.mappers.MapperWithNoLog'>

I overrided the render method of the widget but the errors comes from self.value() of the BoundField.

How can I fix that? override that?

Any other idea?

Thanks.

[Michael Palumbo]

Maybe using a Model like Kurtis mentioned will be better (and easier) than making a custom field... ?