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Re: Method default argument whose type is the class not yet defined

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On 11/10/2012 2:33 PM, Jennie wrote:
> What is the best solution to solve the following problem in Python 3.3?
>
> import math
>  >>> class Point:
> ...     def __init__(self, x=0, y=0):
> ...         self.x = x
> ...         self.y = y
> ...     def __sub__(self, other):
> ...         return Point(self.x - other.x, self.y - other.y)
> ...     def distance(self, point=Point()):
> ...         """Return the distance from `point`."""
> ...         return math.sqrt((self - point).x ** 2 + (self - point).y ** 2)
> ...
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<stdin>", line 5, in Point
> NameError: name 'Point' is not defined
>
> I propose three solutions. The first one:
>
>  >>> class Point:
> ...     def __init__(self, x=0, y=0):
> ...         self.x = x
> ...         self.y = y
> ...     def __sub__(self, other):
> ...         return Point(self.x - other.x, self.y - other.y)
> ...     def distance(self, point=None):
> ...         p = point if point else Point()
> ...         return math.sqrt((self - p).x ** 2 + (self - p).y ** 2)
> ...
>  >>> p = Point()
>  >>> p.distance()
> 0.0
>  >>> p.distance(Point(3, 4))
> 5.0

What I do not like about this one is that it creates a new 0 point each 
time one is needed. Two solutions:

change Point() to point0 in the distance function and create
point0 = Point()
after the class.

-or-
instead of p = line,
px,py = point.x, point.y if point else 0.0, 0.0


> The second one:
>
>  >>> class Point:
> ...     def __init__(self, x=0, y=0):
> ...         self.x = x
> ...         self.y = y
> ...     def __sub__(self, other):
> ...         return Point(self.x - other.x, self.y - other.y)
> ...
>  >>> def distance(self, point=Point()):
> ...     return math.sqrt((self - point).x ** 2 + (self - point).y ** 2)
> ...
>  >>> Point.distance = distance
>  >>> p = Point()
>  >>> p.distance(Point(3, 4))
> 5.0

my first thought


> The last one:
>
>  >>> class Point:
> ...     def __init__(self, x=0, y=0):
> ...         self.x = x
> ...         self.y = y
> ...         Point.distance = distance
> ...     def __sub__(self, other):
> ...         return Point(self.x - other.x, self.y - other.y)
> ...
>  >>> def distance(self, point=Point()):
> ...     return math.sqrt((self - point).x ** 2 + (self - point).y ** 2)
> ...
>  >>> p = Point()
>  >>> p.distance(Point(3, 4))
> 5.0
>
> Is there a better solution?


-- 
Terry Jan Reedy

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Method default argument whose type is the class not yet defined Jennie <nameDOTportua@gmail.com> - 2012-11-10 20:33 +0100
  Re: Method default argument whose type is the class not yet defined Chris Angelico <rosuav@gmail.com> - 2012-11-11 06:56 +1100
  Re: Method default argument whose type is the class not yet defined Terry Reedy <tjreedy@udel.edu> - 2012-11-10 15:29 -0500
    Re: Method default argument whose type is the class not yet defined Jennie <nameDOTportua@gmail.com> - 2012-11-10 21:51 +0100
      Re: Method default argument whose type is the class not yet defined Dave Angel <d@davea.name> - 2012-11-10 17:30 -0500
    Re: Method default argument whose type is the class not yet defined Jennie <marco.buttu@gmail.com> - 2012-11-10 21:51 +0100
  Re: Method default argument whose type is the class not yet defined Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-11 01:13 +0000
    Re: Method default argument whose type is the class not yet defined Chris Angelico <rosuav@gmail.com> - 2012-11-11 13:13 +1100
    Re: Method default argument whose type is the class not yet defined Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-10 19:43 -0700
      Re: Method default argument whose type is the class not yet defined Roy Smith <roy@panix.com> - 2012-11-10 21:53 -0500
        Re: Method default argument whose type is the class not yet defined Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-10 23:43 -0700
        Re: Method default argument whose type is the class not yet defined Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-10 23:45 -0700
    Re: Method default argument whose type is the class not yet defined Chris Angelico <rosuav@gmail.com> - 2012-11-11 13:47 +1100
    Re: Method default argument whose type is the class not yet defined Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-11 14:21 +0000
      Re: Method default argument whose type is the class not yet defined Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-11 22:31 +0000
        Re: Method default argument whose type is the class not yet defined Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-12 00:31 +0000
          Re: Method default argument whose type is the class not yet defined Steve Howell <showell30@yahoo.com> - 2012-11-11 16:56 -0800
          Re: Method default argument whose type is the class not yet defined Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-12 04:46 +0000
        Re: Method default argument whose type is the class not yet defined Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-12 01:10 +0000
          Re: Method default argument whose type is the class not yet defined Roy Smith <roy@panix.com> - 2012-11-11 20:15 -0500
            Re: Method default argument whose type is the class not yet defined Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-12 01:35 +0000
        Re: Method default argument whose type is the class not yet defined Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-12 01:18 +0000
          Re: Method default argument whose type is the class not yet defined Roy Smith <roy@panix.com> - 2012-11-11 20:34 -0500
        Re: Method default argument whose type is the class not yet defined Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-12 01:29 +0000
        Re: Method default argument whose type is the class not yet defined Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-12 01:50 +0000
  Re: Method default argument whose type is the class not yet defined Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-11 01:23 +0000
  Re: Method default argument whose type is the class not yet defined Steve Howell <showell30@yahoo.com> - 2012-11-11 16:32 -0800

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