#19189: FormWizard done method can't revisit forms
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Reporter: kenth | Owner: nobody
Type: New feature | Status: reopened
Component: contrib.formtools | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by kenth):
* status: closed => reopened
* has_patch: 0 => 1
* resolution: needsinfo =>
Comment:
Thanks for reviewing the ticket. I found the need to perform
"revalidation" in the `done` step in an e-commerce project incorporating
validation using stripe.com which separates validation (performed in the
form's `clean` method) and charging (which I perform in `done`). However,
I think the problem is more general than this specific case.
I had rejected performing a 'charge' in the clean of a confirmation step,
instead preferring the `done` method for two reasons:
1. The wizard validates each form at least twice (when the form is
initially validated and again in `render_done`). To ensure an action is
performed only once, the `clean` method needs to leave a token to not
repeat the action. This can be fragile.
2. The wizard does not revalidate earlier steps when first validating a
form. Thus, if a confirmation step's `clean` charges when first validated,
and an earlier step's form fails revalidation in `render_done` (say stock
on an item is no longer available, or a discount window has expired), the
charge has already occurred and the `done` step will not be processed. The
transaction is a mess at that point.
An alternate solution would be to have earlier forms check if the charge
has occurred and always return `valid()` in that case. I'm not sure how
that would interact with tampering detection and the like.
Accordingly actions which must not be duplicated are best performed in the
`done` method. The `done` method guarantees all forms validate & the
wizard state is deleted upon completion. The current wizard allows for
forms which do not revalidate, but does not allow for a `done` method
which may not complete successfully. The proposed exception handles this
case.
An implementation I currently use defines the `Exception` in my wizard
subclass & then overrides `render_done` to catch the exception and step
back into the `FormWizard` as needed. Django being open-source and Python,
the workaround is quite functional. However I thought the problem and
solution general enough for inclusion.
I have attached a patch w/ tests. I can work up the docs if it would help.
Thanks. Kent
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Ticket URL: <https://code.djangoproject.com/ticket/19189#comment:3>
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