Class getters and setters

[ymf]

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

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:

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

[jlm699]

What I dont get is what these code blocks do:

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

>>> 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.

[jrcagle]

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:

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:

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