Question

I am programming a simulations for single neurons. Therefore I have to handle a lot of Parameters. Now the Idea is that I have two classes, one for a SingleParameter and a Collection of parameters. I use property() to access the parameter value easy and to make the code more readable. This works perfect for a sinlge parameter but I don’t know how to implement it for the collection as I want to name the property in Collection after the SingleParameter. Here an example:

class SingleParameter(object):   def __init__(self, name, default_value=0, unit='not specified'):     self.name = name     self.default_value = default_value     self.unit = unit     self.set(default_value)   def get(self):     return self._v   def set(self, value):     self._v = value   v = property(fget=get, fset=set, doc='value of parameter')  par1 = SingleParameter(name='par1', default_value=10, unit='mV') par2 = SingleParameter(name='par2', default_value=20, unit='mA')  # par1 and par2 I can access perfectly via 'p1.v = ...' # or get its value with 'p1.v'  class Collection(object):   def __init__(self):     self.dict = {}   def __getitem__(self, name):     return self.dict[name] # get the whole object     # to get the value instead:     # return self.dict[name].v   def add(self, parameter):     self.dict[parameter.name] = parameter     # now comes the part that I don't know how to implement with property():     # It shoule be something like     # self.__dict__[parameter.name] = property(...) ?  col = Collection() col.add(par1) col.add(par2) col['par1'] # gives the whole object  # Now here is what I would like to get: # col.par1 -> should result like col['par1'].v # col.par1 = 5 -> should result like col['par1'].v = 5 

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Other questions that I put to understand property():

• Why do managed attributes just work for class attributes and not for instance attributes in python?
• How can I assign a new class attribute via __dict__ in python?

Answer 1

Using the same get/set functions for both classes forces you into an ugly hack with the argument list. Very sketchy, this is how I would do it:

In class SingleParameter, define get and set as usual:

def get(self):   return self._s def set(self, value):   self._s = value 

In class Collection, you cannot know the information until you create the property, so you define the metaset/metaget function and particularize them only later with a lambda function:

def metaget(self, par):   return par.s def metaset(self, value, par):   par.s = value def add(self, par):   self[par.name] = par   setattr(Collection, par.name,     property(       fget=lambda x : Collection.metaget(x, par),       fset=lambda x, y : Collection.metaset(x,y, par))