Method default argument whose type is the class not yet defined
Terry Reedy
tjreedy at udel.edu
Sat Nov 10 15:29:39 EST 2012
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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