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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_subnet_dict():
 """Returns dictionary of "availability zone" -> subnet for current VPC."""

  subnet_dict = {}
  vpc = get_vpc()
  for subnet in vpc.subnets.all():
    zone = subnet.availability_zone
    assert zone not in subnet_dict, "More than one subnet in %s, why?" % (zone,)
    subnet_dict[zone] = subnet
  return subnet_dict