Fields & Widgets

## What binding means A Django `Form` instance is in one of two states, decided entirely by its constructor arguments: - **Bound**: `data` or `files` is not `None`. Django sets `form.is_bound = True` and treats `data` as the user's submission. - **Unbound**: both are `None`. The form can only be rendered, empty or pre-filled from `initial`. The check is a literal `is not None` test. `SignupForm(data={})` is therefore **bound** (an empty submission that will fail every required field), while `SignupForm(initial={...})` is **unbound** however much initial data it carries. | Call | `is_bound` | What `is_valid()` does | |---|---|---| | `SignupForm()` | False | returns False, runs nothing | | `SignupForm(initial={'email': '[email protected]'})` | False | returns False, runs nothing | | `SignupForm(data={})` | True | validates; required fields fail | | `SignupForm(request.POST, request.FILES)` | True | validates the submission | ## When cleaned_data appears `cleaned_data` is not an attribute of a freshly built form. It is created by `full_clean()`, which Django runs lazily the first time you call `is_valid()` or read `form.errors`: 1. `full_clean()` creates an empty error dictionary. For an unbound form it stops there, so an unbound form never gains `cleaned_data` and reading it raises `AttributeError`. 2. For a bound form it creates `cleaned_data = {}` and cleans each field: the widget extracts the raw value from `data` or `files`, the field coerces it into a Python object, and the result is stored under the field's name. 3. Form-wide cleaning and an internal hook that `ModelForm` uses run afterwards (their order is the validation topic's material). What ends up in `cleaned_data` is **typed, normalised data**: a `datetime.date` for a `DateField`, a `bool` for a `BooleanField`, a whitespace-stripped string for a `CharField` (`strip=True` is the default). A field that raised a `ValidationError` is **not** in `cleaned_data`; its message sits in `form.errors`. Hence the working rule: read `cleaned_data` only after `is_valid()` returned True, and never go back to `request.POST` for a value the form has already cleaned. ## data versus initial The two dictionaries look alike and do different jobs: - **`data`** is what the user submitted. It is validated and becomes `cleaned_data`. - **`initial`** is what an unbound form displays. It is never validated. You can pass it to the form or declare it on the field; the form-level dictionary wins, and a callable such as `datetime.date.today` is called each time it is needed. - In a bound form, an ordinary enabled field that is missing from `data` is treated as **empty**, not filled from `initial`. A `CharField(required=False, initial='weekly')` absent from the submission cleans to `''`. - `initial` still matters to a bound form in narrow places: `has_changed()` and `changed_data` compare the submission against it, a `disabled=True` field cleans its initial value instead of submitted data, and a `FileField` with no new upload keeps its initial file. ## The request.POST or None idiom Many function views write `form = SignupForm(request.POST or None)` to share one code path for GET and POST. It works because an empty `QueryDict` is falsy, so a GET builds an unbound form. The edge is a POST whose body carries no fields at all: it also builds an unbound form, and the view silently re-renders without errors instead of reporting the required fields. In a normal template form the CSRF token keeps the POST non-empty, but branching on `request.method == 'POST'` states the intent without relying on truthiness. ## A newsletter sign-up example ```python from django import forms class NewsletterSignupForm(forms.Form): email = forms.EmailField() first_name = forms.CharField(required=False, max_length=50) f = NewsletterSignupForm(data={'email': ' [email protected] '}) f.is_valid() # True f.cleaned_data # {'email': '[email protected]', 'first_name': ''} ``` The address arrives padded with spaces and leaves stripped; the missing optional name cleans to the `CharField`'s empty value, the empty string. ## What interviewers listen for - Binding is decided by `data` and `files`, not by the HTTP method and not by `initial`. - `is_valid()` on an unbound form returns False without raising. - `cleaned_data` holds coerced Python values, only for the fields that passed. - `initial` is display data, not a default for missing input, apart from disabled fields and kept files.