Dynamically add fields to Form objects in Django

I tried the content written in the digression of Dynamic specification of queryset of ModelChoiceField in Django, so memo memo.

Last homework

By playing with Form.fields, you can change various attributes of other fields after the form is generated. Is it possible to add / remove the field itself?

I actually tried it

Launch Django's shell command and try it on an interactive shell

Create an empty Form object

>>> from django import forms
>>> f = forms.Form()
>>> f.as_p()
''
>>> f.fields
OrderedDict()

No fields are defined at this point, so calling ʻas_p ()does not render anything Form.fields only shows empty ʻOrderdDict objects

… For the first time, we know that the type of Form.fields is a class called OrderedDict.

Add fields to Form.fields

>>> f.fields['name'] = forms.CharField(label='name')
>>> f.as_p()
'<p><label for="id_name">name:</label> <input id="id_name" name="name" type="text" /></p>'

>>> f.fields
OrderedDict([('name', <django.forms.fields.CharField object at 0x00000000045E5B388>)])

Add CharField with the key'name'. Rendered properly with ʻas_p (). The contents of Form.fields also contain the object of CharField` with the key'name'.

Calling ʻis_valid () and checking ʻerrors, cleaned_data

>>> f.is_valid()
Flase

>>> f.errors
{}

>>> f.cleaned_data
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'Form' object has no attribute 'cleaned_data'

Since the added name field is required = True (default value), it is expected that ʻis_valid ()will be False, but for some reasonForm.errors` is empty.

When I looked it up, if the Form object didn't change the value of each field, or didn't receive a data as an argument, it would just create an empty ʻerrors and handle full_clean () Seems to finish (cleaned_data` is not generated either).

There is no help for it, so pass the data argument to the Form object and regenerate it. I'll give it a try.

>>> f = forms.Form(data={})
>>> f.fields['name'] = forms.CharField(label='name')
>>> f.is_valid()
False

>>> f.errors
{'name': ['This field is required.']}

>>> f.cleaned_data
{}

As expected, the content of ʻerrorscontained a mandatory check error for'name'. An empty dict is also generated forcleaned_data`.

Detour: Behavior of ʻis_valid () and ʻerrors

At first, I tried to directly assign an empty dict to Form.data to deal with the problem of ↑, but the result did not change. When I followed the source of django, it was because of the following processing at the beginning of __init __ () of the BaseForm class, which is the parent of the Form class.

forms.py

        self.is_bound = data is not None or files is not None

So, the implementation of ʻis_valid` is only one line below

forms.py

    def is_valid(self):
        """
        Returns True if the form has no errors. Otherwise, False. If errors are
        being ignored, returns False.
        """
        return self.is_bound and not bool(self.errors)

In addition, errors are defined as methods with property decorators rather than fields.

forms.py

    @property
    def errors(self):
        "Returns an ErrorDict for the data provided for the form"
        if self._errors is None:
            self.full_clean()
        return self._errors

So, you couldn't just edit data directly, you had to do the following two things.

• Set the value of ʻis_bound` to True
• Call full_clean ()

Well, if you think about using it as a normal web application, you shouldn't have to change the parameters and redo the validation while processing one request ... It is natural that validation (call of full_clean ()) is not executed every time ʻis_valid () or ʻerrors is accessed, but only the first time.

Change data so that validation is True

>>> f.data = {'name':'hoge'}
>>> f.full_clean()
>>> f.is_valid()
True

>>> f.errors
{}

>>> f.cleaned_data
{'name':'hoge'}

Try adding more fields

>>> f.fields['date'] = forms.DateField(label='date',required=False)

>>> f.as_p()
'<ul class="errorlist"><li>This field is required.</li></ul>\n<p><label for="id_name">name:</label> <input id="id_name" name="name" type="text" /></p>\n<p><label for="id_date">date:</label> <input id="id_date" name="date" type="text" /></p>'

>>> f.fields
OrderedDict([('name', <django.forms.fields.CharField object at 0x00000000045E5B38>), ('date', <django.forms.fields.DateField object at 0x0000000002A3C828>)])

>>> f.full_clean()
>>> f.is_valid()
True

>>> f.errors
{}

>>> f.cleaned_data
{'date':None, 'name':'hoge'}

>>> f.data['date'] = '2014-12-04'
>>> f.full_clean()
>>> f.is_valid()
True

>>> f.errors
{}

>>> f.cleaned_data
{'date':datetime.date(2014, 12, 4), 'name':'hoge'}

Remove fields from Form.fields

>>> f.fields.popitem()
('date', <django.forms.fields.DateField object at 0x0000000002A3C828>)

>>> f.as_p()
'<p><label for="id_name">name:</label> <input id="id_name" name="name" type="text" value="hoge" /></p>'

>>> f.fields
OrderedDict([('name', <django.forms.fields.CharField object at 0x00000000045E5B38>)])

>>> f.full_clean()
>>> f.cleaned_data
{'name': 'hoge'}

>>> f.data
{'date': '2014-12-04', 'name': 'hoge'}

Form.fields deletes the item with ʻOrderedDict.popitem. If the argument last` is True (default), it will be LIFO, and if it is False, it will be LILO. This time it's True, so the date field I added later was removed.

Summary

So, Django's Form object can be added / removed later in the field itself. I think it can respond to requests such as "Allow you to freely add screen input items in the master settings!"