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