Question

From the source code, you can see that Django 1.4’s Form class has a has_changed() method and changed_data property which seem rather useful, but are undocumented. My question is: do these work as expected, i.e.:

• In Form.clean(), Form.has_changed() returns True if any any form data has changed, otherwise False
• In Form.clean(), Form.changed_data is a list of field names whose values have changed.

If so, are there any specific reasons not to use them, apart from the usual caveats/dangers about using undocumented features (i.e. subject to change, not supported, etc.)?

NOTE 1: For these to work with custom widgets, those widgets need to have a _has_changed() method, which is defined for built in widgets.

NOTE 2: Interestingly, the documentation does include an offhand mention of the Formset.has_changed() method, but not of Form.has_changed().

Answer 1

After studying the Django source further, with helpful hints from Florian’s answer, I can report that has_changed and changed_data do work as described in my question as long as the form has a way to get the initial data to compare the new data against.

So the question is, how does a form created from POST data know what the initial values of the GET form were? The short answer is it doesn’t — unless you tell it somehow. There are two ways to tell it:

1. Via the initial keyword argument to the form and/or the initial keyword arguments to the fields, exactly the same way you tell the GET form the initial values. NOTE: If you do this, it’s up to you to make sure you use the same values for your GET and POST forms. This is the only truly reliable way to do it, since you directly control what the initial values are.

2. You let Django do the work of remembering the initial values from your GET by setting the show_hidden_initial keyword argument to True for each applicable field. For these fields, Django renders a hidden input element with the initial value into the GET form’s HTML. Later, when you call has_changed or changed_data on the POST form, for any field with show_hidden_initial as True Django will automatically get the initial values from the hidden input elements in the POST data (superseding any initial values from initial form or field arguments). NOTE: as with anything relying on POST data, this approach is ultimately unreliable, since the values for the hidden inputs could still be changed despite being hidden.