Detailed explanation of Django's Form mechanism

Environment: Python 2.7.5 + Django 1.6

Using Django, we can define a Form in a declarative way, as follows:

# -*- coding: utf-8 -*-
from django import forms
class SimpleForm(forms.Form):
    field_a = forms.CharField(max_length=100) 
    field_b = forms.CharField(max_length=100)

It is very comfortable to write, but the problem comes when I initialize this Form After that, for example:

from polls.forms import SimpleForm

sf = SimpleForm({'field_a':'value of field_a', 'field_b':'value of field_b'})

Then execute dir in python shell (sf), it is found that this instance does not have the two attributes field_a and field_b. Obviously we cannot reference the fields on sf like sf.field_a. But obviously we can reference form fields in the template in the form of {{ form_name.field_name }}. What's going on?

After some investigation, I found that the implementation mechanism behind it is quite tortuous. First of all, if we want to reference the fields in the form, how should we write it? It should be written like this: sf['field_a']

Why should it be written like this? Above code, __getitem__ method of django.forms.BaseForm:

def __getitem__(self, name):
    "Returns a BoundField with the given name."
    try:
        field = self.fields[name]
    except KeyError:
        raise KeyError('Key %r not found in Form' % name)
    return BoundField(self, field, name)

This turns BaseForm into a dict-like container, so you can use the above syntax to reference fields in the form.

A new question comes again, why can the form fields be referenced in the template in the form of {{ form_name.field_name }}? See Django's official documentation: https://docs.djangoproject.com/en/1.6/topics/templates/#variables. It turns out that when Django's template engine encounters an expression like {{ form_name.field_name }}, it will run a dictionary lookup on the form_name object, so when the template engine evaluates {{ sf.field_a }} it actually runs sf['field_a '], the truth came out.

In addition, the type of SimpleForm above is actually django.forms.DeclarativeFieldsMetaclass. This metaclass actually converts all the fields declared with declarative syntax in SimpleForm (including declarative fields in the parent class) into a dict through the get_declared_fields method, and assigns the value of the dict to the class to be generated. base_fields attribute, and then generate a new class based on SimpleForm.