将类作为其方法的默认参数传递

Question

Here is the minimal Point class needed for this question.

 class Point:
      def __init__(self, x = 0, y = 0):
           self.x = x
           self.y = y
      def dist_from(self, point = Point(0, 0)):
           return ((self.x-point.x)**2 + (self.y-point.y)**2)**0.5

I want the dist_from method to return the distance from origin if no point is passed. And since method overloading is not possible, I decided to pass a Point object (the origin) as the default value.

But I am missing something very fundamental.

Traceback (most recent call last):
  File "C:\Users\Naveen\Desktop\temp.py", line 26, in <module>
    class Point:
  File "C:\Users\Naveen\Desktop\temp.py", line 32, in Point
    def dist_from(self, point = Point(0, 0)):
NameError: name 'Point' is not defined

Answer 1

Default values are expressions that get evaluated when the function object is compiled. The look-up for Point will fail since the class named Point hasn't been created at that point.

This can be side stepped by supplying a default value of None for the default and act on that fact inside the method body (where the name Point has been defined:

def dist_from(self, point = None):
      if point is None:
          point = type(self)(0, 0)  # Could do Point(0, 0)
      return (self.x*point.x + self.y*point.y)**0.5

I'm using type(self) here instead of Point so as to not hard-code the class name in there.