def find_field(network_value,
               field_name):
  """Looks up the value(s) of a given field name across a network.

  Args:
    network_value: A `NetworkValue`; see `Network`.
    field_name: The name of a `Value` field.

  Returns:
    A mapping, from each variable name in `network_value` whose `Value` has a
    field named `field_name`, to the value of that field. This could be empty.
  """
  findings = {}
  for name, value in network_value.items():
    if field_name in value.as_dict:
      findings[name] = value.get(field_name)
  return findings