Merge pull request #186 from Kenneth-T-Moore/p092_option_pre_process

Poem 092 - User-defined function hook for pre-processing option setva…

POEM_092.md

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+POEM ID: 092  
+Title: User-defined function hook for pre-processing option set.  
+authors: Kenneth-T-Moore (Ken Moore)  
+Competing POEMs:  
+Related POEMs:  
+Associated implementation PR: [PR xxx](https://github.com/OpenMDAO/OpenMDAO/pull/xxx)  
+
+Status:
+
+- [ ] Active
+- [x] Requesting decision
+- [ ] Accepted
+- [ ] Rejected
+- [ ] Integrated
+
+## Motivation
+
+OpenMDAO's component options feature is sometimes used to define constant quantities with units. When coupled with
+the top-level setting of system options, this creates as situation where an option might be specified in the
+wrong units. As a general solution, this POEM proposes the addition of a pre-set function hook that the user
+can define to pre-process a value and compute a new value before setting the option in a component. This hook
+could be used to convert units, or it could be used for other processing.
+
+## Proposed Solution
+
+The only API change from this POEM is an additional argument to the `declare` method of OptionDictionary. The
+new argument is "set_function", which takes a function whose input parameters are a dictionary that contains
+the option's metadata and a value, and whose return is a new value.
+
+
+## Example
+
+```language=python
+import openmdao.api as om
+from openmdao.utils.units import convert_units
+
+
+def units_setter(opt_meta, value):
+    """
+    Check and convert new units tuple into
+
+    Parameters
+    ----------
+    opt_meta : dict
+        Dictionary of entries for the option.
+    value : any
+        New value for the option.
+
+    Returns
+    -------
+    any
+        Post processed value to set into the option.
+    """
+    new_val, new_units = value
+    old_val, units = opt_meta['val']
+
+    converted_val = convert_units(new_val, new_units, units)
+    return (converted_val, units)
+
+
+class AviaryComp(om.ExplicitComponent):
+
+    def setup(self):
+
+        self.add_input('x', 3.0)
+        self.add_output('y', 3.0)
+
+    def initialize(self):
+        self.options.declare('length', default=(12.0, 'inch'),
+                             set_function=units_setter)
+
+    def compute(self, inputs, outputs):
+        length = self.options['length'][0]
+
+        x = inputs['x']
+        outputs['y'] = length * x
+
+
+class Fakeviary(om.Group):
+
+    def setup(self):
+        self.add_subsystem('mass', AviaryComp())
+
+
+prob = om.Problem()
+model = prob.model
+
+model.add_subsystem('statics', Fakeviary())
+
+prob.model_options['*'] = {'length': (2.0, 'ft')}
+prob.setup()
+
+prob.run_model()
+print('The following should be 72 if the units convert correctly.')
+print(prob.get_val('statics.mass.y'))
+print('done')
+```
+
+
+## Notes