trucking: support full-price control arm in coupon A/B (pct 0 = no code)

CAMPAIGN_COUPON_AB_PCTS="20,30,0" now means 20% / 30% / full-price. The 0 arm
mints no code; pick_coupon_for_email returns ("","") so it renders identically
to a normal-price send, while carriers are still deterministically hash-bucketed
into it (re-hash a converter's email to recover their arm). Even ~33/33/33 split
incl. the control verified over 30k. Adds test_full_price_control_arm; 8/8 pass.

scripts/build_trucking_campaigns.py

@@ -275,9 +275,12 @@ COUPON_PCT = int(os.getenv("CAMPAIGN_COUPON_PCT", "40"))
 # arm, getting that arm's own daily code. Because each code stores its own
 # percent in discount_codes, the discount the email advertises always matches the
 # discount checkout actually applies, and redemptions are measurable per code
-# (description marker campaign-daily:<date>:<pct>). Empty/unset = single-arm test
-# at COUPON_PCT (legacy behavior). The split is even and stable per carrier, so a
-# given carrier always sees the same percent across re-sends (no arm-hopping).
+# (description marker campaign-daily:<date>:<pct>). A percent of 0 is a valid
+# FULL-PRICE control arm (e.g. "20,30,0"): no code is minted and the carrier sees
+# the normal price, but they're still hash-bucketed so the control is measurable.
+# Empty/unset = single-arm test at COUPON_PCT (legacy behavior). The split is even
+# and stable per carrier, so a given carrier always sees the same arm across
+# re-sends (no arm-hopping).
 COUPON_AB_PCTS = tuple(
     int(p.strip())
     for p in os.getenv("CAMPAIGN_COUPON_AB_PCTS", "").split(",")
@@ -445,28 +448,43 @@ def get_or_create_daily_coupons(conn, send_date: date) -> dict[int, str]:
     Returns a mapping of percent -> code. With no A/B test configured this is a
     single arm {COUPON_PCT: code}; with CAMPAIGN_COUPON_AB_PCTS="20,30,40" it
     returns one code per percent so recipients can be split across arms.
+
+    A percent of 0 is a valid FULL-PRICE control arm: no code is minted (the map
+    value is ""), so carriers bucketed into it see the normal price and no coupon,
+    while still being deterministically assigned (and thus measurable) by email
+    hash. Example: CAMPAIGN_COUPON_AB_PCTS="20,30,0".
     """
     pcts = list(COUPON_AB_PCTS) if COUPON_AB_PCTS else [COUPON_PCT]
-    return {pct: get_or_create_daily_coupon(conn, send_date, pct) for pct in pcts}
+    out: dict[int, str] = {}
+    for pct in pcts:
+        out[pct] = "" if pct <= 0 else get_or_create_daily_coupon(conn, send_date, pct)
+    return out
 
 
 def pick_coupon_for_email(email: str, daily_coupons: dict[int, str] | None) -> tuple[str, str]:
     """Deterministically assign a carrier to one coupon arm by a stable hash of
-    their email. Returns (code, pct_str); ("", "") when coupons are off.
+    their email. Returns (code, pct_str); ("", "") when coupons are off OR when
+    the carrier is bucketed into a full-price control arm (pct 0, code "").
 
     The hash makes the split even and *stable*: the same carrier always lands in
     the same arm across re-sends, so an A/B comparison isn't polluted by a carrier
-    seeing 20% one day and 40% the next.
+    seeing 20% one day and 40% the next. A full-price (0%) arm is returned as
+    ("", "") so the email renders the normal-price branch, identical to a no-coupon
+    send — but the carrier is still deterministically in that arm, so re-hashing
+    a converter's email recovers which arm they were in for attribution.
     """
     if not daily_coupons:
         return "", ""
     pcts = sorted(daily_coupons.keys())
     if len(pcts) == 1:
         pct = pcts[0]
-        return daily_coupons[pct], str(pct)
-    h = hashlib.sha256((email or "").strip().lower().encode()).hexdigest()
-    pct = pcts[int(h, 16) % len(pcts)]
-    return daily_coupons[pct], str(pct)
+    else:
+        h = hashlib.sha256((email or "").strip().lower().encode()).hexdigest()
+        pct = pcts[int(h, 16) % len(pcts)]
+    code = daily_coupons[pct]
+    if not code:                      # full-price control arm: no code, no deal
+        return "", ""
+    return code, str(pct)
 
 
 def coupon_attribs(coupon_code: str | None, coupon_pct: str | None = None) -> dict:

scripts/tests/test_coupon_ab.py

@@ -59,6 +59,36 @@ def test_single_arm():
     assert btc.pick_coupon_for_email("a@b.com", {40: "ZZZZZ"}) == ("ZZZZZ", "40")
 
 
+def test_full_price_control_arm():
+    """A 0% arm (code "") is a full-price control: returned as ("", "") so the
+    email renders the normal-price branch, but carriers are still hash-bucketed
+    evenly across all three arms (so the control is measurable)."""
+    coupons = {20: "AAAAA", 30: "BBBBB", 0: ""}  # 0 = full-price control
+    counts: Counter = Counter()
+    arm_for: dict = {}
+    for i in range(30000):
+        e = f"user{i}@carrier{i % 500}.com"
+        code, pct = btc.pick_coupon_for_email(e, coupons)
+        # The control arm presents identically to a no-coupon send.
+        if code == "":
+            assert pct == ""
+        else:
+            assert code in ("AAAAA", "BBBBB") and pct in ("20", "30")
+        # Bucket label: recover arm even for the silent control via re-hash.
+        h = __import__("hashlib").sha256(e.encode()).hexdigest()
+        arm = sorted(coupons)[int(h, 16) % 3]
+        arm_for[e] = arm
+        counts[arm] += 1
+        # Stable across re-calls.
+        assert btc.pick_coupon_for_email(e, coupons) == (code, pct)
+    total = sum(counts.values())
+    for arm in (0, 20, 30):
+        share = counts[arm] / total
+        assert 0.31 <= share <= 0.353, (arm, share)
+    # The price helper yields no number for the control arm (blank pct).
+    assert btc.discounted_price_attribs("mcs150", None, "")["coupon_priceable"] == ""
+
+
 def test_coupon_attribs_reflects_pct():
     a = btc.coupon_attribs("BBBBB", "30")
     assert a["coupon_code"] == "BBBBB"