How to call functions/methods inside objects in python

Question

Here is my code.

How do I execute the print_point line? Edited to work. Thank you

class Point:
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def distance_from_origin(self):
        return ((self.x **2) + (self.y **2)) **.5
    def print_point(p):
        print '(%s, %s)' % (str(p.x), str(p.y))

Here is what I type into the shell after running module:

p = Point(5,2)

p.x

    5

p.y

    2

p.distance_from_origin()

    5.385164807134504

p.print_point(p)

    Traceback (most recent call last):
    File "<pyshell#196>", line 1, in <module>
    p.print_point(p)
    TypeError: print_point() takes exactly 1 argument (2 given)

You got self right in distance_from_origin, but not print_point. (Also, check your indentation: the defs should be indented.) — Andrew Jaffe
– Andrew Jaffe, Commented Dec 11, 2011 at 21:38

Andrew Jaffe · Accepted Answer · 2011-12-11 23:08:10Z

2

It's not clear what you want, since you've got the indentation wrong. Is print_point part of the class?

If so, you got self right in distance_from_origin, but perhaps not in print_point:

def print_point(self):
    print '(%f, %f)' % (self.x, self.y)

It is legal, but a bad idea, to use a different name for the self parameter such as your p.

(And note that the very act of printing will do the str conversion for you...)

Or did you want print_point to be out of the class, in which case you should have it as is but with (p.x, p.y)? Or, as suggested by other answers, a static or class method?

edited Dec 11, 2011 at 23:08

answered Dec 11, 2011 at 21:43

Andrew Jaffe

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2 Comments

SwimBikeRun Over a year ago

Yes this is what I wanted. Thanks. I also typed commas instead of periods which made it harder. p,y instead of p.y.

ovgolovin Over a year ago

Actually, in the original code p played the role of self.

David Neale · Accepted Answer · 2011-12-12 07:35:24Z

0

Add self as the first argument or print_point. You'll also need to reference x and y using the self prefix because those fields belong to the instance.

def print_point(self):
     print '(%s, %s)' % (str(self.x), str(self.y))

edited Dec 12, 2011 at 7:35

answered Dec 11, 2011 at 21:38

David Neale

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1 Comment

ovgolovin Over a year ago

What does str(p, self.x) mean? Actually, in his code p played the role of self.

ovgolovin · Accepted Answer · 2011-12-11 22:12:21Z

You used self as a parameter in the function distance_from_origin.

But you chose p as a parameter in the function print_point.

Actually, it works with p:

>>> p = Point(5,2)
>>> p.print_point()
(5, 2)

But there is a convention to use self for passing the instance of the class to the method. Otherwise everything gets very cumbersome.

Also, %s converts the object to string using str method. So, there is no need to convert in manually to string.

Taking it all into account, the function print_point will look like:

def print_point(self):
    print '(%s, %s)' % (self.x, self.y)

Point coordinates are represented by numbers so you may want to use special method for number output.

There is a new format method for string. It definitely exists in Python 2.7. Just look at the examples and choose the most satisfactory output format for your case. For example you may do this:

def print_point(self):
    print('({self.x}, {self.y})'.format(self=self))

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