# Method default argument whose type is the class not yet defined

Jennie nameDOTportua at gmail.com
Sat Nov 10 20:33:05 CET 2012

```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

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

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?
--
Jennie

```