R fitdistr for Beta distribution: which starting parameters?
===

I need to fit my data into a Beta distribution and retrieve the alpha parameter. I'm trying to use R from python (rpy2) and my code looks like:

    from rpy2 import *
    from rpy2.robjects.packages import importr
    MASS = importr('MASS') #myVector is a Numpy array with values between 0 and 1
    MASS.fitdistr(myVector,"beta")


But I get this error:

    Error in function (x, densfun, start, ...)  : 
      'start' must be a named list
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python2.7/dist-packages/rpy2/robjects/functions.py", line 82, in __call__
        return super(SignatureTranslatedFunction, self).__call__(*args, **kwargs)
      File "/usr/lib/python2.7/dist-packages/rpy2/robjects/functions.py", line 34, in __call__
        res = super(Function, self).__call__(*new_args, **new_kwargs)
    rpy2.rinterface.RRuntimeError: Error in function (x, densfun, start, ...)  : 
      'start' must be a named list

I can't seem to find any good documentation for R with detailed examples, so I only found [this][1]:

> ***start*** 	A named list giving the parameters to be optimized with initial values. This can be omitted for some of the named
> distributions (see Details). ... 	Additional parameters, either for
> densfun or for optim. In particular, it can be used to specify bounds
> via lower or upper or both. If arguments of densfun (or the density
> function corresponding to a character-string specification) are
> included they will be held fixed.

I really have no clue as to:

 - what to put as a starting parameters and how that will affect my estimation
 - what syntax to use in Python, since `start=list(shape1=0.5, shape2=0.5)` won't do the trick

 
Any hint?

  [1]: http://stat.ethz.ch/R-manual/R-patched/library/stats/html/Beta.html