How to avoid circular dependencies when setting Properties?

Question

This is a design principle question for classes dealing with mathematical/physical equations where the user is allowed to set any parameter upon which the remaining are being ca

Answer 1

I would recommend to teach your application what can be derived from what. For example, a typical case is that you have a set of n variables, and any one of them can be derived from the rest. (You can model more complicated cases as well, of course, but I wouldn't do it until you actually run into such cases).

This can be modeled like this:

# variable_derivations is a dictionary: variable_id -> function
# each function produces this variable's value given all the other variables as kwargs
class SimpleDependency:
  _registry = {}
  def __init__(self, variable_derivations):
    unknown_variable_ids = variable_derivations.keys() - self._registry.keys():
      raise UnknownVariable(next(iter(unknown_variable_ids)))
    self.variable_derivations = variable_derivations

  def register_variable(self, variable, variable_id):
    if variable_id in self._registry:
      raise DuplicateVariable(variable_id)
    self._registry[variable_id] = variable

  def update(self, updated_variable_id, new_value):
    if updated_variable_id not in self.variable_ids:
      raise UnknownVariable(updated_variable_id)
    self._registry[updated_variable_id].assign(new_value)
    other_variable_ids = self.variable_ids.keys() - {updated_variable_id}
    for variable_id in other_variable_ids:
      function = self.variable_derivations[variable_id]
      arguments = {var_id : self._registry[var_id] for var_id in other_variable_ids}
      self._registry[variable_id].assign(function(**arguments))

class FloatVariable(numbers.Real):
  def __init__(self, variable_id, variable_value = 0):
    self.variable_id = variable_id
    self.value = variable_value
  def assign(self, value):
    self.value = value
  def __float__(self):
    return self.value

This is just a sketch, I didn't test or think through every possible issue.