str method called on return values of instance methods

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?

You don't call line1.midpoint, you're just referencing it. BTW, I thought that noone used the backticks in Python code anymore. — Matthias
– Matthias, Commented Dec 6, 2016 at 16:31
either call your midpoint (add brackets d'oh) or make it a property... — user3012759
– user3012759, Commented Dec 6, 2016 at 16:32
The backticks call repr so it would be repr(self.start) or you could use the str(self.start). FYI: The backticks are removed in Python 3. — Matthias
– Matthias, Commented Dec 6, 2016 at 16:37
@pgblu You can use repr(self.start) instead of the backticks or use: return 'line from {!r} to {!r}'.format(self.start, self.end) — Anthon
– Anthon, Commented Dec 6, 2016 at 16:37
Apart from backticks being removed as @Matthias already indicated, print as a statement is gone in Python 3 as well. Start adding from __future__ import print_function to the top of your code and use print() — Anthon
– Anthon, Commented Dec 6, 2016 at 16:39

Martinbaste · Accepted Answer · 2016-12-06 16:34:01Z

3

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).

edited Dec 6, 2016 at 16:34

answered Dec 6, 2016 at 16:30

Martinbaste

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4 Comments

pgblu Over a year ago

I've tried that. But when I add the parentheses, I get a TypeError: midpoint() takes no arguments (1 given)

Chris Over a year ago

@pgblu You need to add self as a parameter. There are also several other errors I see in your code. I suggest you go through it and clean it up.

Matthias Over a year ago

But wouldn't print line1.midpoint() call repr on the returned value? In that case we would need to use print str(line1.midpoint()).

Chris Over a year ago

@Matthias Your correct. I belive the OP is confused about what __repr__ and what __str__ are for. They're used to give a visual representation of a given object.

D. Ross · Accepted Answer · 2016-12-06 19:19:43Z

1

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

answered Dec 6, 2016 at 19:19

D. Ross

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