Place replicas and explain failure domains

from dataclasses import dataclass
import ipaddress
import re

class PlanError(ValueError):
    pass

def bounded_text(value, label):
    if not isinstance(value,str) or not value.strip() or len(value)>200:
        raise PlanError(label)
    return value

def integer(value, low, high):
    if type(value) is not int or not low<=value<=high:
        raise PlanError("integer outside plan bounds")
    return value

@dataclass(frozen=True)
class Replica:
    service: str
    region: str
    zone: str
    healthy: bool = True


def available_services(replicas, failed_regions=(), failed_zones=()):
    """Placement availability only; not capacity, health-check routing or failover."""
    if not isinstance(replicas,(list,tuple)) or not 1<=len(replicas)<=20:
        raise PlanError("bounded replicas required")
    if not isinstance(failed_regions,(list,tuple)) or not isinstance(failed_zones,(list,tuple)):
        raise PlanError("failure collections required")
    for region in failed_regions:bounded_text(region,"failed region")
    for pair in failed_zones:
        if not isinstance(pair,(list,tuple)) or len(pair)!=2:raise PlanError("zone needs region and name")
        bounded_text(pair[0],"zone region");bounded_text(pair[1],"zone name")
    failed=set(tuple(pair) for pair in failed_zones)
    available=set()
    for replica in replicas:
        if not isinstance(replica,Replica) or type(replica.healthy) is not bool:raise PlanError("typed replica required")
        for value in (replica.service,replica.region,replica.zone):bounded_text(value,"replica field")
        if replica.healthy and replica.region not in failed_regions and (replica.region,replica.zone) not in failed:
            available.add(replica.service)
    return tuple(sorted(available))

replicas=[Replica("api","r1","a"),Replica("api","r1","b"),Replica("database","r1","a")]
remaining=available_services(replicas,failed_zones=(("r1","a"),))
print(remaining)
print("complete API path", {"api","database"} <= set(remaining))

placements=[("api","a"),("api","b"),("database","a")]
for failed in ("a","b"):
    remaining={service for service,zone in placements if zone!=failed}
    print("lost",failed,"remaining",sorted(remaining))