django-gandalf

django-gandalf lets you declare multi-step, tree-shaped Django form flows as readable, composable code.

You build a flow with a small, immutable builder — .step() to add a form, .branch() to fork on an answer, .expand() to grow steps from an answer — and mount it as an ordinary Django view. Gandalf handles the per-step URLs, the session state, back-navigation, editing, file uploads, and running your completion logic exactly once.

It is built for the point where a journey stops being a straight line and starts branching: business vs individual, domestic vs international, nested compliance sub-flows, optional setup paths, and path fragments reused across journeys. Instead of scattered step conditions and navigation overrides, you describe the flow as one explicit tree.

from gandalf.wizard import Wizard, condition

onboarding = (
    Wizard()
    .step(AccountTypeForm, name="account_type")
    .branch(
        condition(is_business_account, Wizard().step(BusinessDetailsForm, name="business")),
        default=Wizard().step(PersonalDetailsForm, name="personal"),
    )
    .step(ReviewForm, name="review")
)

The only dependency is Django. Coming from django-formtools? See Coming from django-formtools at the end for a declaration-by-declaration mapping.

Installation & setup

pip install django-gandalf   # or: uv add django-gandalf

Gandalf ships no models or migrations, but it does rely on a few pieces of standard Django plumbing:

# settings.py
INSTALLED_APPS = [
    # ...
    "django.contrib.sessions",   # wizard state lives in the session
    "gandalf",
]

MIDDLEWARE = [
    "django.contrib.sessions.middleware.SessionMiddleware",  # required
    "django.middleware.csrf.CsrfViewMiddleware",
    # ...
]

TEMPLATES = [
    {
        # ...
        "OPTIONS": {
            "context_processors": [
                # required so `request.wizard` is reachable in step templates
                "django.template.context_processors.request",
            ],
        },
    }
]

Requires Python 3.10+ and Django 4.2+.

Try the examples locally

Every worked example in this README is a real, runnable wizard bundled with the repository. Boot the demo app with:

just serve

That starts Django at http://127.0.0.1:8000/, whose index page links to the bundled example wizards. Each section below ends with a ▶ Try it live link to that example's start URL, e.g. http://127.0.0.1:8000/readme/signup/. These are local URLs — they only resolve while just serve is running.

The code for these examples lives in tests/testapp/readme_examples.py, and tests/functional/test_readme_examples.py drives each one through the Django test client — so the snippets below are checked in CI, not just prose.

Quickstart: a linear wizard

The shortest end-to-end flow is a linear wizard that collects a couple of forms and does something with the combined result when it finishes.

from django import forms
from django.http import HttpResponse
from django.urls import include, path

from gandalf.viewsets import WizardViewSet
from gandalf.wizard import MergeCleanedData, Wizard


class NameForm(forms.Form):
    name = forms.CharField()


class EmailForm(forms.Form):
    email = forms.EmailField()


class SignupWizardViewSet(WizardViewSet):
    url_name = "signup"
    template_name = "signup/step.html"
    wizard = (
        Wizard()
        .step(NameForm, name="name")
        .step(EmailForm, name="email")
    )

    def done(self, bound_wizard):
        payload = MergeCleanedData().reduce(bound_wizard.path)
        create_account(**payload)          # runs exactly once
        return HttpResponse("Thanks!")

Mount it with a single include:

urlpatterns = [
    path("signup/", include(SignupWizardViewSet.urls())),
]

The step template is a plain Django form — no management form, no wizard-specific markup, because Gandalf keeps position in the session rather than in the POST body:

<form method="post">
  {% csrf_token %}
  {{ form.as_p }}
  <button type="submit">Continue</button>
</form>

That is the whole thing: two forms, a viewset, one URL include.

▶ Try it live: http://127.0.0.1:8000/readme/signup/  ·  Source: readme_examples.py

How it works

A few ideas carry the rest of the library.

The builder is immutable. Every .step() / .branch() / .expand() returns a new Wizard, like Django QuerySet chaining — nothing mutates in place. That makes reusable bases safe:

base = Wizard().step(AccountForm, name="account")

staff_wizard = base.step(InternalReviewForm, name="internal_review")
customer_wizard = base.step(ProfileForm, name="profile")
# `base` still contains only AccountForm.

Every step is named, and every step gets its own URL. name="email" is shorthand for context={"step_name": "email"}. From url_name, urls() publishes three patterns — the start URL, the bare run URL (signup/<run_id>/), and the step URL (signup/<run_id>/email/). A step URL is a claim: it either renders that step or redirects to wherever the run actually is, so a stale link can never land an answer on the wrong step.

A run re-proves itself on every request. Gandalf stores raw submissions, not "how far you got". On each request it replays the stored answers through their forms up to the first missing or no-longer-valid one — that is what makes position, branch selection, editing, and completion all fall out of a single walk, and what makes stale state impossible. (The cost of that replay is covered in What replaying costs.)

done(self, bound_wizard) receives the run, not a list of forms, so it can read the answers however it needs:

• bound_wizard.path — the resolved route: the answered steps in order, iterable, each a RuntimeStep exposing .form.cleaned_data, .data (raw submission), and .files.
• MergeCleanedData().reduce(bound_wizard.path) — folds every step's cleaned_data into one dict (last-write-wins). Subclass it for a different merge policy.
• bound_wizard.path.find_step(name=...) / path.filter_steps(...) — look a step up by name or any context key. These live on path, so they only ever see steps actually on the resolved route — prior answers, never the current (unanswered) step or a step not yet reached.
• bound_wizard.runtime_tree — the head of the walked tree (.next to the following step).
• bound_wizard.get_state() / get_run_data() — the raw stored JSON.

Plain Form or full FormView. Pass a plain Form and Gandalf generates the step's FormView for you, rendered with the viewset's template_name. Pass your own FormView when a step needs get_initial(), get_form_kwargs(), a per-step template, or other view-level behavior — it keeps its own configuration and can be reused as a standalone view outside the wizard. Inside the wizard the step still sees self.request.wizard, so it can inspect run state when useful. See Step views for what such a view must provide and what it sees when it reads run state.

Branching

.branch() forks the flow on a prior answer. Each arm is a sub-Wizard (or None for "nothing extra here"); a condition(predicate, arm) pairs a predicate(request) with the arm it selects. Selection is first-match-wins, falling back to default.

from gandalf.wizard import Wizard, condition


def is_business_account(request):
    account_step = request.wizard.path.find_step(name="account_type")
    return account_step.form.cleaned_data["account_type"] == "business"


class BranchingWizardViewSet(WizardViewSet):
    url_name = "onboarding"
    template_name = "onboarding/step.html"
    wizard = (
        Wizard()
        .step(AccountTypeForm, name="account_type")
        .branch(
            condition(
                is_business_account,
                Wizard().step(BusinessDetailsForm, name="business"),
            ),
            default=Wizard().step(PersonalDetailsForm, name="personal"),
        )
        .step(ReviewForm, name="review")
    )

    def done(self, bound_wizard):
        payload = MergeCleanedData().reduce(bound_wizard.path)
        ...

A predicate always runs behind a fully-validated prefix — every step before the branch has already validated on this same walk — so it can dereference path.find_step(...).form.cleaned_data unconditionally without guarding for missing answers.

Because arms are sub-Wizards, they compose: define a subflow once and drop it into several branches. And a de-selected arm's answers are not thrown away — see Dormant memory below.

▶ Try it live: http://127.0.0.1:8000/readme/branching/  ·  Source: readme_examples.py

Dynamic wizards: get_wizard()

When the shape of the flow depends on request context — tenant, plan, permissions, locale, feature flags — override get_wizard(self, bound_wizard) instead of setting a class-level wizard. It is called per request, so the same view can build a different flow for each caller. Here the plan is captured in the URL, and the team plan gets an extra company step:

class OnboardingWizardViewSet(WizardViewSet):
    url_name = "onboarding"
    template_name = "onboarding/step.html"

    def get_wizard(self, bound_wizard):
        wizard = Wizard().step(NameForm, name="name")
        if self.kwargs["plan"] == "team":
            wizard = wizard.step(CompanyForm, name="company")
        return wizard.step(EmailForm, name="email")

urlpatterns = [
    path("onboarding/<slug:plan>/", include(OnboardingWizardViewSet.urls())),
]

The mount prefix can capture kwargs of its own; the default URL hooks forward them into every redirect, so self.kwargs["plan"] is available on each request of the run. Reach for get_wizard() when the shape depends on the request; when it depends on a prior answer the user just gave, reach for .expand() instead — it grows the tree in a single walk without re-reading stored state.

▶ Try it live: http://127.0.0.1:8000/readme/onboarding/solo/ or /team/  ·  Source: readme_examples.py

.expand(): grow the wizard from a prior answer

A branch chooses between subflows you declared up front. Sometimes the shape of the flow is not known until a prior answer supplies it — N item steps for a count the user just typed. .expand() grows the tree during the walk from a builder you provide:

def build_item_steps(request):
    count = int(request.wizard.path.find_step(name="count").form.cleaned_data["count"])
    steps = Wizard()
    for index in range(count):
        steps = steps.step(ItemForm, name=f"item-{index}")
    return steps


class ExpandWizardViewSet(WizardViewSet):
    url_name = "collect-items"
    template_name = "collect/step.html"
    wizard = (
        Wizard()
        .step(ItemCountForm, name="count")
        .expand(build_item_steps)
        .step(ReviewForm, name="review")
    )

The builder runs mid-walk, behind the validated count, and its steps are spliced in where .expand() sits. That is the difference from a state-reading get_wizard(): answering the count parks the user on the first grown step in a single request, where get_wizard() has to walk twice (once to notice its own submission changed the shape).

Good to know: the builder reaches back to prior answers by name, so renaming an upstream step can break it; grown answers store positionally, so raising a count keeps the answers already given and lowering it drops the trailing ones; and every grown step must be routable (carry a name).

▶ Try it live: http://127.0.0.1:8000/readme/expand/  ·  Source: readme_examples.py

Back-navigation: editing earlier steps

Because every step has its own URL, an "edit" affordance is just a link. GET a completed step's URL to render it pre-filled; POST the changed answer back to it to place it there. Editing is not a separate operation — putting an answer at a step works the same whether or not it already had one.

A review template wires per-step edit links from the runtime path:

<h1>Review your details</h1>
<ul>
  {% for step in request.wizard.path %}
    <li>
      <a href="../{{ step.declaration.context.step_name }}/">
        Edit {{ step.declaration.context.step_name }}
      </a>
    </li>
  {% endfor %}
</ul>
<form method="post">
  {% csrf_token %}
  {{ form.as_p }}
  <button type="submit">Confirm</button>
</form>

The promise is that changing an answer costs the user only as much of the wizard as the change actually invalidates — usually nothing. A trivial edit lands straight back on the summary; an edit that flips a branch parks only at the steps that now need attention, then fast-forwards through every still-valid answer. Nothing downstream is lost to a typo, because an invalid edit is kept and re-rendered with its errors while the sealed tail is carried verbatim.

For an explicit in-page back link, any step template can reach request.wizard.back_url (the previous step's URL, branch-aware; None on the first step) and request.wizard.run_url (a "return to where I was" link):

{% if request.wizard.back_url %}
  <a href="{{ request.wizard.back_url }}">Back</a>
{% endif %}

▶ Try it live: http://127.0.0.1:8000/readme/editing/  ·  Source: readme_examples.py

Dormant memory: flipping a branch and back

Editing an answer that flips a branch does not discard the arm you leave. A de-selected arm's answers are kept as dormant memory, re-validated and restored if you flip back — so the user never re-types an answer they already gave.

Take the branching wizard: pick a business account, fill in the company name, then edit the account type to personal. The business arm is now inactive, but its answer is not gone. Flip the account type back to business and the company name is already there — the run fast-forwards past it to the summary instead of asking again.

class FlipFlopWizardViewSet(WizardViewSet):
    url_name = "flip-flop"
    template_name = "onboarding/editing.html"
    wizard = (
        Wizard()
        .step(AccountTypeForm, name="account_type")
        .branch(
            condition(is_business_account, Wizard().step(CompanyForm, name="company")),
            default=Wizard().step(PersonalForm, name="preferred_name"),
        )
        .step(ReviewForm, name="review")
    )

Dormant arms live in the session until the run completes, and arm identity is positional (declaration order) — so a dynamic get_wizard() that reorders branch arms between requests would misattribute the memory, the same positional-alignment rule that applies to steps.

▶ Try it live: http://127.0.0.1:8000/readme/flip-flop/ (choose business, fill the company name, then edit the account type to personal and back)  ·  Source: readme_examples.py

File uploads

Steps may declare forms.FileField. Uploaded bytes cannot live in the session, so Gandalf persists them through a companion WizardFileStorage; the session carries only a small ref (storage key plus original name/content-type/size). The step template just needs the usual enctype:

<form method="post" enctype="multipart/form-data">
  {% csrf_token %}
  {{ form.as_p }}
  <button type="submit">Continue</button>
</form>

class FileUploadWizardViewSet(WizardViewSet):
    url_name = "profile"
    template_name = "profile/step.html"
    wizard = (
        Wizard()
        .step(ProfilePhotoForm, name="photo")
        .step(NameForm, name="name")
    )

    def done(self, bound_wizard):
        photo_step = bound_wizard.path.find_step(name="photo")
        filename = photo_step.files["photo"]["name"]
        ...

On replay, Gandalf reopens each stored file and re-injects it into request.FILES before re-validating the step, so validators that inspect the upload see the same value they saw originally. Editing respects keep-vs-replace per field. After done() returns, the run's files are cleaned up automatically.

The default storage writes under a gandalf/<run_id>/ prefix of Django's default storage; point it elsewhere (S3, a per-tenant location) by subclassing WizardFileStorage and passing it to .configure(file_storage_class=...).

▶ Try it live: http://127.0.0.1:8000/readme/file-upload/  ·  Source: readme_examples.py

Step views: bringing your own FormView

Pass a plain Form and Gandalf generates the step's view. Pass your own FormView when the step needs view-level behavior — a per-step template, get_initial(), get_form_kwargs(), a custom form_valid().

A step view is dispatched as an ordinary Django view, so it needs the two things any standalone FormView needs. Gandalf supplies neither for a view you bring yourself:

• template_name (or get_template_names()). A step with its own view does not inherit the viewset's template_name — that default only reaches the views Gandalf generates. Without one, rendering raises ImproperlyConfigured.
• get_success_url() (or success_url). Gandalf reads only the status code of the step's response — a 3xx means "this answer stands, carry on" — and discards the response, so the URL is never followed. self.request.path is the idiomatic no-op, and is what the generated views use. Without one, a valid POST raises ImproperlyConfigured.

from django.views.generic.edit import FormView


class BillingStepView(FormView):
    form_class = BillingForm
    template_name = "billing/step.html"

    def get_success_url(self):
        return self.request.path

    def get_initial(self):
        initial = super().get_initial()
        account = self.request.wizard.path.find_step(name="account")
        initial["company"] = account.form.cleaned_data["email"].partition("@")[2]
        return initial


class FormViewStepWizardViewSet(WizardViewSet):
    url_name = "billing"
    template_name = "billing/step.html"
    wizard = (
        Wizard()
        .step(EmailForm, name="account")
        .step(BillingStepView, name="billing")
        .step(ReviewForm, name="confirm")
    )

    def done(self, bound_wizard):
        payload = MergeCleanedData().reduce(bound_wizard.path)
        return HttpResponse(f"Billing {payload['company']} ({payload['country']})")

Mixing the two styles is the normal case: billing brings its own view, while account and confirm stay plain Forms and get theirs generated with the viewset's template_name. Nothing else changes — the step is still named, still gets its own URL, and still contributes its cleaned_data to bound_wizard.path.

Because the view keeps its own configuration, the same class can also be mounted as an ordinary standalone view outside the wizard — one place for the form's behavior across "create in wizard" and "edit later" screens. Give the standalone subclass a real get_success_url(); only the in-wizard one wants the self.request.path no-op.

Reading run state from a step view

The step runs on a wizard-shaped request, so self.request.wizard is the same BoundWizard the rest of the flow sees — path for the resolved route, path.find_step(name=...) to address a prior answer, as get_initial() does above. That works from anywhere in the view: get_initial(), get_form_kwargs(), get_context_data(), form_valid().

What a step view sees is the prefix before it — the answers the walk has already validated on this request, never the step's own answer and nothing after it. That is the same contract a branch predicate gets, and it holds whether the step is being rendered or replayed behind the cursor. A step is replayed on every later request, so its reads run again each time; keep them cheap for the same reason you keep clean() cheap (see What replaying costs).

find_step() returns None for a step the run cannot see, so guard the lookup when the step you want is not unconditionally upstream of this one. The example above does not guard, because account always precedes billing.

▶ Try it live: http://127.0.0.1:8000/readme/form-view/  ·  Source: readme_examples.py

Escaping the wizard

Sometimes an answer means the user should not be in the wizard any more — an email lookup finds an existing account, so the right destination is the login page, not the next step. A step says so by raising an escape, an ordinary exception in the spirit of Http404:

from django.contrib.auth.models import User
from django.urls import reverse

from gandalf.escapes import Park


class EmailLookupForm(forms.Form):
    email = forms.EmailField()

    def clean(self):
        cleaned_data = super().clean()
        if User.objects.filter(email=cleaned_data.get("email")).exists():
            raise Park(reverse("login"))
        return cleaned_data

All three escapes take the same arguments as django.shortcuts.redirect (a URL, a named route, or a model with get_absolute_url()); which one you raise decides what the user comes back to:

Escapes can also be raised from a FormView's form_valid() when the decision needs the view. Escape is the base class, so except Escape catches all three.

▶ Try it live: http://127.0.0.1:8000/readme/escape/  ·  Source: readme_examples.py   (enter existing@example.com to trigger the park)

Completion and storage

done() runs exactly once. A run finishes the first time it is walked and every step is satisfied; done() is called, its files are cleaned up, and the run is retired — its answers are dropped and a small completion marker takes their place. After that, every request for it (bare run URL or any step URL, GET or POST) is answered by run_unavailable() without reaching the wizard. So a stale tab cannot finalize twice, and a refreshed completion page cannot re-charge a card. Put side effects in done() and they happen once, full stop.

The marker is written after done() returns, so a done() that raises leaves the run intact and resumable.

run_unavailable(self, bound_wizard, reason) answers everything that cannot be run — reason is "completed" (finished) or "unknown" (never started, obliterated, or a lost session). The default redirects to the start URL; override it to say something more specific:

class CheckoutWizardViewSet(WizardViewSet):
    def run_unavailable(self, bound_wizard, reason):
        if reason == "completed":
            return redirect("order-thanks")
        raise Http404("That checkout has expired.")

Storage is session-backed. Gandalf ships SessionStorage, which keeps plain JSON in request.session (Django's session backend handles persistence). It is the one touch point set on the viewset rather than via .configure(...), because it must exist before the wizard does — a dynamic get_wizard() reads stored state to shape itself:

class SignupWizardViewSet(WizardViewSet):
    storage_class = CustomSessionStorage

Retired runs are pruned to the most recent SessionStorage.max_completed_runs (25 by default), so completed runs cannot grow a session without bound.

Configuration

Declaring steps is usually all you need; .configure(...) overrides a runtime default when you want one. It is optional — a WizardViewSet configures a plain Wizard with defaults automatically.

signup_wizard = (
    Wizard()
    .step(AccountForm, name="account")
    .configure(file_storage_class=TenantFileStorage)
)

The same keyword pattern applies to every touch point on the configured wizard — form_view_factory, cursor_walker_class, step_dispatcher_class, state_serializer_class, and step_router_class. Each has a sensible default, so you only configure what you need. For a custom URL scheme, subclass StepNameRouter (routing on a different context key) and pass it as step_router_class, or write the URL patterns yourself and override get_wizard_url() / get_step_url() on the viewset.

For a runtime-level view of how the pieces fit together, see ARCHITECTURE.md.

What replaying costs

Gandalf re-proves stored submissions rather than trusting a recorded position. The rule is small enough to keep in your head:

A form's clean() runs once per completed step per HTTP request.

So with k answers stored, a request costs k replays, and a POST costs one more for the answer being submitted; completing an N-step run costs N² validations end to end, spread over 2N requests. The number that matters is not N, it is how many of your steps are expensive — each completed step is validated once per request whether the user is on step 5 or step 29, so N² only bites when most steps do real work in clean().

Measured on a 2023 laptop with just bench, for a linear wizard:

Gandalf's own share is about a millisecond per request at 30 steps; everything else is your forms. If expensive clean() becomes a problem, move the work into done() (where it runs once), store a cheaply-recheckable token, or accept that some checks belong only at submission time. just bench measures your own shapes, and tests/functional/test_walk_cost.py pins the counts so they cannot regress unnoticed.

Coming from django-formtools

Gandalf neither forks nor depends on django-formtools — the storage shape, the URL model, and the re-proving walk all differ, so there is no drop-in replacement. What maps cleanly is the declaration: a form_list becomes chained .step(...) calls, and a condition_dict becomes .branch(condition(predicate, subflow)). The predicates are the same idea — a callable given the request — but a Gandalf predicate runs behind a fully-validated prefix, so it reads prior answers with path.find_step(...).form.cleaned_data unconditionally.

Linear wizard

# formtools
class CheckoutWizard(SessionWizardView):
    form_list = [CustomerForm, AddressForm, ConfirmForm]

# gandalf
checkout_wizard = (
    Wizard()
    .step(CustomerForm, name="customer")
    .step(AddressForm, name="address")
    .step(ConfirmForm, name="confirm")
)

Conditional step inclusion

# formtools — a condition_dict keyed by step name
def needs_vat(wizard):
    cleaned = wizard.get_cleaned_data_for_step("company") or {}
    return cleaned.get("is_business")

class CompanyWizard(SessionWizardView):
    form_list = [("company", CompanyForm), ("vat", VATForm), ("summary", SummaryForm)]
    condition_dict = {"vat": needs_vat}

# gandalf — the condition lives next to the step it guards
def needs_vat(request):
    company_step = request.wizard.path.find_step(name="company")
    return company_step.form.cleaned_data.get("is_business")

company_wizard = (
    Wizard()
    .step(CompanyForm, name="company")
    .branch(
        condition(needs_vat, Wizard().step(VATForm, name="vat")),
        default=None,  # skip VAT when the condition is false
    )
    .step(SummaryForm, name="summary")
)

Tree-like branching with reusable subflows

# formtools — branching lives in imperative get_next_step() logic
class OnboardingWizard(SessionWizardView):
    form_list = [AccountTypeForm, BizAForm, BizBForm, PersonAForm, FinalForm]

    def get_next_step(self, step=None):
        ...  # custom, dynamic next-step logic

# gandalf — the shape is the declaration
business_flow = Wizard().step(BizAForm, name="biz_a").step(BizBForm, name="biz_b")
personal_flow = Wizard().step(PersonAForm, name="person_a")

onboarding_wizard = (
    Wizard()
    .step(AccountTypeForm, name="account_type")
    .branch(
        condition(is_business_account, business_flow),
        default=personal_flow,
    )
    .step(FinalForm, name="final")
)

The payoff for tree-shaped journeys: branch condition and target stay together, arms are reusable sub-wizards, and the whole flow is visible in one declaration instead of growing bespoke navigation plumbing as branches multiply.

Contributing

See CONTRIBUTING.md for local setup, workflow expectations, separated unit and functional test commands, and commit message conventions.