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Re: mutually exclusive arguments to a constructor

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In article <g6k1t8xg0a.ln2@news.ducksburg.com>,
 Adam Funk <a24061@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".

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mutually exclusive arguments to a constructor Adam Funk <a24061@ducksburg.com> - 2011-12-30 20:40 +0000
  Re: mutually exclusive arguments to a constructor "Günther Dietrich" <gd.usenet@spamfence.net> - 2011-12-30 22:00 +0100
    Re: mutually exclusive arguments to a constructor Adam Funk <a24061@ducksburg.com> - 2011-12-31 20:55 +0000
  Re: mutually exclusive arguments to a constructor Mel Wilson <mwilson@the-wire.com> - 2011-12-30 16:08 -0500
  Re: mutually exclusive arguments to a constructor Arnaud Delobelle <arnodel@gmail.com> - 2011-12-30 21:14 +0000
  Re: mutually exclusive arguments to a constructor Jason Friedman <jason@powerpull.net> - 2011-12-30 21:18 +0000
    Re: mutually exclusive arguments to a constructor Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-30 22:21 +0000
  Re: mutually exclusive arguments to a constructor Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-30 22:13 +0000
  Re: mutually exclusive arguments to a constructor Roy Smith <roy@panix.com> - 2011-12-30 18:24 -0500
    Re: mutually exclusive arguments to a constructor Chris Angelico <rosuav@gmail.com> - 2011-12-31 10:36 +1100
      Re: mutually exclusive arguments to a constructor Roy Smith <roy@panix.com> - 2011-12-30 18:39 -0500
        Re: mutually exclusive arguments to a constructor Chris Angelico <rosuav@gmail.com> - 2011-12-31 10:47 +1100
    Re: mutually exclusive arguments to a constructor Adam Funk <a24061@ducksburg.com> - 2011-12-31 20:59 +0000

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