mutually exclusive arguments to a constructor

[Roy Smith]

In article <g6k1t8xg0a.ln2 at news.ducksburg.com>,
 Adam Funk <a24061 at ducksburg.com> wrote:

> (Warning: this question obviously reflects the fact that I am more
> accustomed to using Java than Python.)
> 
> Suppose I'm creating a class that represents a bearing or azimuth,
> created either from a string of traditional bearing notation
> ("N24d30mE") or from a number indicating the angle in degrees as
> usually measured in trigonometry (65.5, measured counter-clockwise
> from the x-axis).

There's two ways to do this.

One would be to have the __init__ method switch on the type of its 
argument:

def __init__(self, bearing_or_azimuth):
   if isinstance(bearing_or_azimuth, basestring):
      # do the bearing thing
   else:
      # do the azimuth thing

I suspect many people would consider that unpythonic.  The other way 
would be what, in the C++/Java world, would be called the "named 
constructor idiom".  Just write two factory functions:

class DirectionIndicatingThingie:
   @staticmethod
   def from_bearing(cls, bearing):
      dit = DirectionIndicatingThingie()
      dit.direction = whatever
      return dit

and likewise for from_azimuth()

"But!", some C++/Java type bondage addicts might cry, "there's nothing 
to prevent somebody from creating a DirectionIndicatingThingie directly, 
bypassing the factory functions.  There's no way to make the constructor 
private!".  To which the free-willed pythonistas would respond, "If it 
hurts when you do that, don't do that".