Django Forms

Philosophy

• Prefer ModelForm for model-backed forms

• Keep validation logic in forms, not views

• Always handle and display form errors

• Use commit=False when you need to modify the instance before saving

Validation

Field-level (clean_<field> ):

• Validate and transform a single field

• Return the cleaned value or raise ValidationError

• Use for: format checks, uniqueness, normalization

Cross-field (clean ):

• Call super().clean() first

• Access multiple fields via cleaned_data

• Use self.add_error(field, message) for field-specific errors

• Use for: password confirmation, conditional requirements

View Integration

• Check request.method explicitly

• Instantiate form with request.POST for POST, empty for GET

• Use form.save(commit=False) to set additional fields (e.g., author)

• Return redirect on success, re-render with form on error

HTMX handling:

• Check request.headers.get("HX-Request") for HTMX requests

• Return partial template on success/error for HTMX

• Use HX-Trigger header to notify other components

Templates

• Display form.non_field_errors for cross-field errors

• Display field.errors for each field

• Use partial templates (_form.html ) for HTMX responses

• Include loading indicator with hx-indicator

Widgets

• Override in Meta.widgets dict

• Set HTML attributes via attrs parameter

• Common: Textarea(attrs={"rows": 5}) , DateTimeInput(attrs={"type": "datetime-local"})

Formsets

• Use inlineformset_factory for related model collections

• Validate both form and formset: form.is_valid() and formset.is_valid()

• Pass instance for editing existing parent objects

Pitfalls

• Validating in views instead of forms

• Silently redirecting without checking is_valid()

• Forgetting commit=False when setting related fields

• Not displaying form errors to users