Fix AttributeError in WizardView.post() when get_form_list() is overridden

[TMBeaver]

The problem

A wizard that overrides get_form_list() without calling super() raises AttributeError and returns HTTP 500 on every successful step submission:

  File "formtools/wizard/views.py", line 317, in post
    del self._resolved_form_list
AttributeError: _resolved_form_list

Overriding get_form_list() is a supported extension point — the test suite already ships TestWizardWithCustomGetFormList for exactly this pattern:

class TestWizardWithCustomGetFormList(TestWizard):
    form_list = [Step1]

    def get_form_list(self):
        return {'start': Step1, 'step2': Step2}

The wizard renders fine on GET, but is unusable end-to-end: submitting step 1 crashes instead of advancing.

Root cause

The condition_dict caching added in 2.6 (#291) memoises on two attributes:

self._resolved_form_list = form_list
self._cache_signature = cache_signature

These are set only by the default WizardView.get_form_list(). post() then invalidates them with bare del statements, which raise when the attributes were never created:

# Clear caches as changed step data could affect conditions
del self._resolved_form_list
del self._cache_signature

The reader a few lines up already anticipates their absence:

if (hasattr(self, '_resolved_form_list') and
    self._cache_signature == cache_signature):

so the guard is simply missing on the invalidation side. This is a regression against 2.5.1, where post() had no cache-clearing code.

test_get_form_list_custom only issues a GET, which is why the suite stayed green.

The fix

Discard the caches only if they were set, matching the hasattr() guard the reader already uses. No behaviour change on the default path.

Tests

Added test_get_form_list_custom_post, a POST counterpart to the existing GET-only test_get_form_list_custom. It fails on master with the AttributeError above and passes with the fix.

Full suite: 100 passing before, 101 passing after.

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