在实例方法的返回值上调用的__str__方法

Question

I am developing a module that draws lines and finds their midpoints. For purposes of testing, I want to create some string outputs from the relevant classes.

class Line:
  def __init__(self, endpoints):
    self.start = endpoints[0]
    self.end = endpoints[1]

  def midpoint():
    x = (start.getX + end.getX) / 2.0
    y = (start.getY + end.getY) / 2.0
    return Point(x, y)

  def __str__(self):
    return "line from " + `self.start` + " to " + `self.end` + "."

class Point:
  def __init__(self, x, y):
    self.x = x
    self.y = y

  def getX():
    return x

  def getY():
    return y

  def __str__(self):
    return "[" + str(self.x) + ", " + str(self.y) + "]"

  __repr__ = __str__

point1 = Point(4,5)
point2 = Point(0,0)
line1 = Line([point1, point2])

print line1
print line1.midpoint

Expected output:

line from [4, 5] to [0, 0]
[2.0, 2.5]

Instead I get:

line from [4, 5] to [0, 0]
<bound method Line.midpoint of <__main__.Line instance of 0x105064e18>>

How can I get the expected string representation of the midpoint, which is being returned as an instance of the Point class?

Answer 1

You are printing the method itself, not the returned value of the method. Change your last line to this:

print line1.midpoint()

Also, the first definition line of your method should use self as the only parameter, like so:

def midpoint(self):

The same applies to the rest of the methods, they should have self as a parameter (Point.getX and Point.getY).

Answer 2

In the midpoint method, you should have start.getX(), start.getY(), end.getX(), and end.getY() should have "self." in front of it. You should also have "self" as a parameter for the method for every method in a class.

midpoint method

I will paste the entire code below to show you exactly what I have done.

entire code