# Python view-model cleanup (M5 prep — Python side only)

Scoped-down precursor to M5 of the Go rewrite. See:
- [2026-05-03-2349-go-backend-rewrite.md](2026-05-03-2349-go-backend-rewrite.md)
- [2026-05-03-2349-go-backend-rewrite-progress.md](2026-05-03-2349-go-backend-rewrite-progress.md)

## Context

The Python app is still production. Before adding `/api/X` shadow
routes, JSON DTOs, parity tooling, and Go handlers, get the Python
side into a clean shape. Today the `/adults` and `/juniors` routes
each carry 150–200 lines of inline view-model construction inside
the Flask handler:

- [adults_view](app.py#L179) — 167 LOC, computes per-row
  status/cell_text/balance/credits/debts inline.
- [juniors_view](app.py#L347) — 205 LOC, same plus `"?"` sentinel
  branching and `:NJ,MA` breakdown.
- [payments](app.py#L553) — 35 LOC, lighter but still mixes IO and
  grouping.

Pulling that into pure builder functions:
- shrinks `app.py` and makes each route handler do only what a route
  handler should (IO + cache + step timing + render);
- gives us **already-tested** pure functions that future M5 work can
  call from a `/api/X` shadow endpoint with one line of `jsonify(...)`;
- has zero behavioural change (existing tests in
  [test_app.py](tests/test_app.py) act as the regression guard).

This is a Python-only change. No Go work, no JSON contract, no shadow
routes — those come after.

## Approach

Three pure builder functions in a new `scripts/views.py` module. Each
takes already-loaded, deserialized inputs (no Flask globals, no IO,
no cache calls) and returns the exact dict that today's route passes
to `render_template`.

```python
def build_adults_view_model(
    members, sorted_months, transactions, exceptions, current_month,
    *, attendance_url, payments_url, bank_account,
) -> dict: ...

def build_juniors_view_model(
    junior_members, sorted_months, transactions, exceptions, current_month,
    *, attendance_url, payments_url, bank_account,
) -> dict: ...

def build_payments_view_model(
    transactions, member_names,
    *, attendance_url, payments_url,
) -> dict: ...
```

The route handlers shrink to: load data (with `get_cached_data` and
`record_step` calls staying in `app.py`), call the builder, render.

```python
@app.route("/adults")
def adults_view():
    members, sorted_months = ... # cached loads + record_step calls
    transactions = ...
    exceptions = ...
    result_meta = ...
    record_step("process_data")
    vm = build_adults_view_model(
        members, sorted_months, transactions, exceptions,
        datetime.now().strftime("%Y-%m"),
        attendance_url=..., payments_url=..., bank_account=BANK_ACCOUNT,
    )
    return render_template("adults.html", **vm)
```

## Decisions

1. **Pure builders, no Flask state.** No `record_step`, no `g.*`, no
   `get_cached_data` inside builders. They take plain args, return
   plain dicts. This is what makes them trivially unit-testable and,
   later, trivially reusable from `/api/X`.
2. **Preserve byte-equal behaviour.** The dicts returned must match
   today's `render_template(...)` kwargs key-for-key, value-for-value
   — including the existing `json.dumps(member_data)` /
   `json.dumps(month_labels_json)` / `json.dumps(raw_payments_json)`
   wrappers. Those wrappers exist for inline JS in templates; the
   refactor doesn't touch them. (When `/api/X` lands later, that route
   will produce a sibling dict with the wrappers stripped, but that's
   future work.)
3. **`reconcile()` stays inside the route, not the builder.** It
   crosses the IO/cache boundary in spirit (its inputs come from
   cache); but it's also pure-domain and called by both adults and
   juniors. Keep the call site in the route so `record_step("reconcile")`
   timing isn't lost. Builder takes `result` as an argument.
4. **Shared helpers stay in `app.py`** for now —
   [get_month_labels](app.py#L41) and
   [group_payments_by_person](app.py#L60) are already module-level
   pure functions, used by routes and now by builders. Either leave
   them in `app.py` and import into `scripts/views.py`, or move them
   into `scripts/views.py`. **Choose: move into `scripts/views.py`**
   — they're view-model concerns, not Flask concerns, and `app.py`
   should keep shrinking.
5. **No new test file needed.** Existing
   [tests/test_app.py](tests/test_app.py) tests
   `test_adults_route`, `test_juniors_route`, `test_payments_route`
   exercise the rendered HTML end-to-end. If they pass after the
   refactor, behaviour is preserved. Adding builder-level unit tests
   is a *nice-to-have* but not required for this iteration.

## Tasks

### 1. Create `scripts/views.py` with the three builders + shared helpers

- Move `get_month_labels` and `group_payments_by_person` from
  [app.py:41–77](app.py#L41-L77) into `scripts/views.py`. Update the
  import in `app.py`.
- Implement `build_adults_view_model` by extracting
  [app.py:200–344](app.py#L200-L344) (everything between `result =
  reconcile(...)` and `return render_template(...)`). Take `result`
  as a parameter; emit the same dict that's currently passed as
  `**kwargs` to `render_template`.
- Implement `build_juniors_view_model` by extracting
  [app.py:370–550](app.py#L370-L550). Same shape — including the
  `adapted_members` adapter loop, the junior `?`-sentinel branches,
  and the `:NJ,MA` breakdown.
- Implement `build_payments_view_model` by extracting
  [app.py:570–586](app.py#L570-L586) (the `group_payments_by_person`
  call + `Unmatched / Unknown` bucket + sort).

### 2. Slim down the route handlers in `app.py`

Each handler keeps:
- `attendance_url`/`payments_url` URL building
- `get_cached_data` calls (with `record_step` between)
- `reconcile(...)` call (adults/juniors) with `record_step("reconcile")`
- `record_step("process_data")` after the builder call
- `return render_template("X.html", **view_model)`

Each handler **drops** all the per-row computation, totals
formatting, credits/debts sorting, `raw_payments_by_person` building,
and `import json` lines.

Target post-refactor LOC for each route handler: ~25–30 lines
(currently 167 / 205 / 35).

### 3. Run the existing test suite + manual smoke

```
make test                # all tests green
make web-py              # browse /adults /juniors /payments visually
```

## Critical files

- [app.py](app.py) — routes shrink dramatically; `get_month_labels`
  and `group_payments_by_person` get moved out.
- New: `scripts/views.py` — three builders + the two helpers.
- [tests/test_app.py](tests/test_app.py) — unchanged; serves as the
  regression guard.

## Verification

1. `make test` — all four `TestWebApp` tests pass unchanged.
2. `make web-py` and visit `/adults`, `/juniors`, `/payments` — each
   renders identically to before (same table contents, same totals,
   same credits/debts, same `?` rendering on juniors).
3. `git diff app.py` shows substantial deletions (route bodies
   shrink) and only thin glue calling the new builders.
4. Optional sanity check: temporarily add `print(repr(view_model))`
   in the route before `render_template` on `main` and on the branch,
   diff for one fixture run — should be byte-identical dicts.

## Out of scope (future iterations, not this plan)

- `/api/<route>` shadow endpoints in Flask
- Go `internal/web/api/` DTOs and JSON Schemas
- Go `internal/services/membership/views.go` aggregators
- Go HTTP handlers for `/api/X`
- `cmd/parity/main.go` and `make parity` target
- Junior breakdown sidecar work in
  `go/internal/services/membership/sources.go`

These are the original M5 tasks (M5.1–M5.4 in the progress tracker).
They become much easier once today's refactor lands, because the
shadow `/api/X` routes will be one-liners over the new builders.