Add support for avoiding shifts on specific dates for workers

- Introduced AvoidShiftOnDates model to define constraints for workers.
- Enhanced Worker class to register avoid-shift rules with the rota builder.
- Implemented logic in RotaBuilder to enforce these constraints during shift assignment.
- Added tests to verify functionality for avoiding shifts on single dates and date ranges.

rota_generator/shifts.py

@@ -300,10 +300,18 @@ class MaxShiftsPerWeekBlockConstraint(BaseShiftConstraint):
     max_shifts: int | None = None
 
 def _pydantic_to_dict(obj):
+    # Recursively convert pydantic models, lists and dicts to JSON-serializable structures
+    if obj is None:
+        return None
     if isinstance(obj, list):
         return [_pydantic_to_dict(x) for x in obj]
+    if isinstance(obj, dict):
+        return {k: _pydantic_to_dict(v) for k, v in obj.items()}
+    # datetime/date -> ISO string
+    if isinstance(obj, (datetime.date, datetime.datetime)):
+        return obj.isoformat()
     if hasattr(obj, "model_dump"):
-        return obj.model_dump()
+        return _pydantic_to_dict(obj.model_dump())
     return obj
 
 
@@ -582,6 +590,9 @@ class RotaConstraintOptions(BaseModel):
     # These are lists of dicts with keys like {"grades": [...], "weeks": [...], "shifts": [...]}
     avoid_shifts_by_grades: List[dict] = Field(default_factory=list, description="List of rules to avoid assigning grades to shifts")
     avoid_shifts_by_worker_names: List[dict] = Field(default_factory=list, description="List of rules to avoid assigning specific workers to shifts")
+    # Rules to avoid assigning specific workers to particular shifts on specific dates.
+    # Each entry is a dict with keys: {"names": [...], "shifts": [...], "start_date": date|None, "end_date": date|None, "dates": [date,...]|None}
+    avoid_shifts_by_worker_dates: List[dict] = Field(default_factory=list, description="List of rules to avoid assigning specific workers to shifts on particular dates or date ranges")
     allocate_locum_shifts: bool = Field(True, description="Allow allocation of locum shifts")
     distribute_locum_shifts: bool = Field(True, description="Distribute locum shifts among available workers")
     balance_locum_shifts: bool = Field(False, description="Balance locum shifts similarly to normal shifts")
@@ -2311,6 +2322,61 @@ class RotaBuilder(object):
                     )
                     pass
 
+            # Date-based per-worker per-shift avoidance rules
+            for constraint_dict in self.constraint_options_model.avoid_shifts_by_worker_dates:
+                # validate shift names
+                if invalid_shifts := set(constraint_dict["shifts"]).difference(set(self.get_shift_names())):
+                    message = f"avoid shifts by worker/dates constraint contains a non existent shift: {invalid_shifts}"
+                    if self.ignore_valid_shifts:
+                        self.add_warning("Non existent shift", message)
+                        continue
+                    else:
+                        raise InvalidShift(message)
+
+                if worker.name not in constraint_dict["names"]:
+                    continue
+
+                # Build set of allowed (week,day) pairs that fall within the date selector
+                selected_weeks_days = set()
+
+                # explicit dates take precedence
+                if constraint_dict.get("dates"):
+                    date_set = set(constraint_dict["dates"])
+                    for wk, d in self.get_week_day_combinations():
+                        date = self.get_date_by_week_day(wk, d)
+                        if date in date_set:
+                            selected_weeks_days.add((wk, d))
+                else:
+                    sd = constraint_dict.get("start_date")
+                    ed = constraint_dict.get("end_date")
+                    # If neither provided, apply to whole rota
+                    if sd is None and ed is None:
+                        for wk, d in self.get_week_day_combinations():
+                            selected_weeks_days.add((wk, d))
+                    else:
+                        if sd is None:
+                            sd = self.start_date
+                        if ed is None:
+                            ed = self.rota_end_date
+                        for wk, d in self.get_week_day_combinations():
+                            date = self.get_date_by_week_day(wk, d)
+                            if sd <= date <= ed:
+                                selected_weeks_days.add((wk, d))
+
+                if not selected_weeks_days:
+                    # nothing to constrain
+                    continue
+
+                # Add constraint that forbids the given shifts for this worker on the selected dates
+                self.model.constraints.add(
+                    0
+                    == sum(
+                        self.model.works[worker.id, wk, d, shift]
+                        for wk, d, shift in self.get_all_shiftname_combinations()
+                        if (wk, d) in selected_weeks_days and shift in constraint_dict["shifts"]
+                    )
+                )
+
             for constraint_dict in self.constraint_options_model.avoid_shifts_by_worker_names:
                 if invalid_shifts := set(constraint_dict["shifts"]).difference(
                     set(self.get_shift_names())
@@ -4092,6 +4158,44 @@ class RotaBuilder(object):
             {"names": names, "weeks": weeks, "shifts": shifts}
         )
 
+    def add_avoid_shifts_for_workers_on_dates(
+        self,
+        names: Iterable[str],
+        shifts: Iterable[str],
+        start_date: datetime.date | str | None = None,
+        end_date: datetime.date | str | None = None,
+        dates: Iterable[datetime.date] | None = None,
+    ):
+        """Add a rule preventing the given workers from being assigned the given shifts
+        on the provided date range or explicit dates.
+
+        - `names`: iterable of worker names to which the rule applies.
+        - `shifts`: iterable of shift names to avoid.
+        - `start_date`/`end_date`: inclusive date range (accepts date or string "%d/%m/%Y" or "%d/%m/%y").
+        - `dates`: optional explicit iterable of `datetime.date` objects.
+        """
+        def _coerce_date(d):
+            if d is None:
+                return None
+            if isinstance(d, datetime.date):
+                return d
+            if isinstance(d, str):
+                for fmt in ("%d/%m/%Y", "%d/%m/%y", "%Y-%m-%d"):
+                    try:
+                        return datetime.datetime.strptime(d, fmt).date()
+                    except ValueError:
+                        continue
+            raise ValueError(f"Cannot parse date: {d}")
+
+        sd = _coerce_date(start_date)
+        ed = _coerce_date(end_date)
+
+        entry = {"names": list(names), "shifts": list(shifts), "start_date": sd, "end_date": ed, "dates": None}
+        if dates is not None:
+            entry["dates"] = [d if isinstance(d, datetime.date) else _coerce_date(d) for d in dates]
+
+        self.constraint_options_model.avoid_shifts_by_worker_dates.append(entry)
+
     def add_grade_constraint_by_week(
         self, grades: Iterable[int], weeks: Iterable[int], shifts
     ):
@@ -4900,6 +5004,22 @@ class RotaBuilder(object):
                     timetable[worker.name][week][day] = assigned_shifts
         return timetable
 
+    def get_assigned_shifts_for_worker_on_day(self, worker_id: int, week: int, day: DayStr) -> list:
+        """Return a list of assigned SingleShift objects for the given worker on a specific week/day.
+
+        This is a small compatibility helper used by tests that expect objects rather than shift-name strings.
+        """
+        works = self.model.works
+        assigned = []
+        for shift_name in self.get_shift_names_by_week_day(week, day, return_empty_if_week_day_not_found=True):
+            try:
+                if works[worker_id, week, day, shift_name].value > 0.5:
+                    assigned.append(self.get_shift_by_name(shift_name))
+            except Exception:
+                # If variable missing or access error, skip
+                continue
+        return assigned
+
     def get_worker_timetable_brief(
         self, show_prefs=False, marker_every=30, show_unavailable=False
     ):