Class getters and setters

•

What is DaniWeb IT Discussion Community?

You're currently browsing the Python section within the Software Development category of DaniWeb, a massive community of 403,516 software developers, web developers, Internet marketers, and tech gurus who are all enthusiastic about making contacts, networking, and learning from each other. In fact, there are 3,867 IT professionals currently interacting right now! Registration is free, only takes a minute and lets you enjoy all of the interactive features of the site.

Join our community!

Please support our Python advertiser: Programming Forums

Thread Tools Display Modes

Views: 304 | Replies: 2

•

Join Date: May 2008

Posts: 14

Reputation: ymf is an unknown quantity at this point

Rep Power: 1

Solved Threads: 0

ymf

Offline

Newbie Poster

Class getters and setters

Jul 2nd, 2008

Hi,
I have a code that creates a class 'Person' :

Help with Code Tags

(Toggle Plain Text)

class Person:
    def __init__(self, fname, lname):
        self._fname = fname
        self._lname = lname

    def fname(self):
        return self._fname

    def set_fname(self, fname):
        self._fname = fname

    def lname(self):
        return self._lname

    def set_lname(self, lname):
        self._lname = lname

    def name(self):
        return self._fname + ' ' + self._lname

What I dont get is what these code blocks do:

Help with Code Tags

(Toggle Plain Text)

def set_fname(self, fname):
        self._fname = fname

 def set_lname(self, lname):
        self._lname = lname

What exactly does the set_fname and set_lname do? and how come the self._fname and self._lname are inaccessible?

Thanks in advance,
ymf

•

Join Date: Jul 2008

Location: Durham, NC

Posts: 139

Reputation: jlm699 is an unknown quantity at this point

Rep Power: 1

Solved Threads: 21

jlm699

Offline

Junior Poster

Re: Class getters and setters

Jul 9th, 2008

•

Originally Posted by ymf

What I dont get is what these code blocks do:
Help with Code Tags

(Toggle Plain Text)
def set_fname(self, fname):
        self._fname = fname

 def set_lname(self, lname):
        self._lname = lname
def set_fname(self, fname): self._fname = fname def set_lname(self, lname): self._lname = lname
What exactly does the set_fname and set_lname do? and how come the self._fname and self._lname are inaccessible?

Those blocks of code reassign the value of _lname and _fname. self._fname and self._lname are not technically inaccessible, they are just simply hidden to the common user. If you typ

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
>>> p1 = Person('James', 'Matthews')
>>> p1
<__main__.Person instance at 0x01C84FD0>
>>> p1.fname()
'James'
>>> p1._fname
'James'
>>> 
>>> p1 = Person('James', 'Matthews')
>>> p1
<__main__.Person instance at 0x01C84FD0>
>>> p1.fname()
'James'
>>> p1._fname
'James'
>>> 

As you can see, I can still access those objects. However the helper function fname() makes it easier on the user.

Let's Go Pens!

•

Join Date: Jul 2006

Posts: 562

Reputation: jrcagle is on a distinguished road

Rep Power: 4

Solved Threads: 72

jrcagle

Offline

Posting Pro

Re: Class getters and setters

Jul 9th, 2008

Just to jlm's good points above, the getter and setter functions allow the object to control access to its own data members.

For instance, suppose you have a sprite that can rotate. Clearly, if the rotation is greater than 360 degrees, then you want to reduce the rotation to its reference angle between 0 and 360. So you would write:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def set_angle(self, angle):
   self._angle = angle % 360
def set_angle(self, angle):
   self._angle = angle % 360

Now, set_angle controls access to self._angle so that it always has a valid value.

Hence, the data member self._angle is private and is only ever accessed by the object itself. That guarantees that some outside client can't dork up _angle by setting it to an illegal value.

But the method to set it, set_angle, is public.

This is VERY important concept in object-oriented programming: data members should be private; methods should be public (there are exceptions, but that's the good rule-of-thumb).

One more thing: having defined your getter and setter functions, you can now create a public data member that looks like a regular data member, but restricts access as described above. Here's how:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def get_angle(self):
    return self._angle
 
def set_angle(self, angle):
   self._angle = angle % 360
 
angle = property(get_angle, set_angle)
def get_angle(self):
    return self._angle

def set_angle(self, angle):
   self._angle = angle % 360

angle = property(get_angle, set_angle)

Now, you can make statements like this:

self.angle = 450

and the setter function will be automagically called just as if you had written

self.set_angle(450)

So the real prize in writing getter and setter functions is that you can protect your object from illegal values; from there, public properties allow you to set values without the awkward method calls.

Hope that helps,
Jeff