Working with Libvirt using Python

Today I decide to toy around with a KVM server we have running in house for testing. I started by using the libvirt Documentation, but it was very limited and I had to do some trial and error. I will go over a few basic things I learned using the libvirt Python module:

Normally we us virt-manager to connect to our KVM instances, and the authentication is tunnled over SSH (using SSH Keys) and QEMU. First things first I had to learn how to connect with the same method we used in virt-manager:

>>> import libvirt
>>> conn = libvirt.open("qemu+ssh://root@192.168.1.2/system")
>>> conn
<libvirt.virConnect instance at 0x2b518c0>

Using the open function I was able to successfully connect to our Hypervisor:

>>> conn.getHostname()
'kvm01.company.com'

From here I need only see our running virtual machines, and how to interact with them. I noticed two separate function the listDefinedDomains() and listDomainsID().

These two functions do two separate things, first lets go over listDefinedDomains(), this function will return a list of offline guest by name:

>>> conn.listDefinedDomains()
['rawhide', 'el4-i386', 'el5-64', 'el6-64', ....]

Where as listDomainsID() will return a list of online devices by ID:

>>> conn.listDomainsID()
[1, 2, 10, 3, 17, 7, 9, 8, 5, 6]

Once connected to the Domain you can get its name:

>>> for i in conn.listDomainsID():
...     print conn.lookupByID(i).name()
...
...
build01-dev
data01-dev
build02-dev
CentOS6-x86_64-dev
..

And as you would expect you can connect to either offline or online domains by using their name:

>>> dom = conn.lookupByName('build01-dev')
>>> dom
<libvirt.virDomain instance at 0x2b82c68>
>>> dom.name()
'build01-dev'

Now that we understand the process of connecting to our Hypervisor and parsing our Domains, lets go over how to start and stop these Domains, or as libvirt calls it create and destroy.

Lets start out with a offline (or destroyed) domain:

>>> dom = conn.lookupByName('rawhide')
>>> dom.isActive()
0

To bring this Domain online we need to use the create() function:

>>> dom.create()
0
>>> dom.isActive()
1

And to power it down you would destroy() it:

>>> dom.destroy()
0
>>> dom.isActive()
0

I hope this helped out a few of you out there, I know this was rather confusion when I first started messing around with it.

Line Equations in Python

The other day I was playing with some Python challenges found on a popular sites, this challenge worked with xy coordinates on a 2D plane.

I wanted to show some of the code I wrote and how they work.

The Plane

Being the data is a tuple of x, y coordinates we will use the Cartesian Coordinate System.

I started my code off as a simple Python class:

class Line(object):

    def __init__(self, data):
            self.first, self.second = data

Using this Class I can input two points in a tuple like this:

data = ((1,1), (2,3))

These coordinates would look like this on a 2D plane:

plane1

The Line

The coordinates ((1,1), (2,3)) holds quite a bit of data when it comes to the terms of Algebra.

Slope

The Slope of a line will tell us how steep it is, and can be calculated with the change in Y / change int X.

def slope(self):
        '''Get the slope of a line segment'''
        (x1, y1), (x2, y2) = self.first, self.second
        try:
                return (float(y2)-y1)/(float(x2)-x1)
        except ZeroDivisionError:
                # line is vertical
                return None

Using the slope method tells us this line has a slope of 2.

>>> data = ((1,1), (2,3))

>>> line = Line(data)

>>> m = line.slope()

>>> print m
2.0

Y Intercept

The Y Intercept tells us at what point a line will meet the Y axis. To get a Y Intercept we use the equation b = y – mx where m is our slope.

def yintercept(self, slope):
        '''Get the y intercept of a line segment'''
        if slope != None:
                x, y = self.first
                return y - slope * x
        else:
                return None

And if we plug all our data back in we get a intercept of -1:

>>> b = line.yintercept(slope)

>>> print b
-1.0

Solve for Y

Now that we know the slope of our line, and where our line meets the Y Axis we can plug in any X coordinate and solve for where Y will be:

def solve_for_y(self, x, slope, yintercept):
        '''Solve for Y cord using line equation'''
        if slope != None and yintercept != None:
                return float(slope) * x + float(yintercept)
        else:
                raise Exception('Can not solve on a vertical line')

And just like that we can when X is equal to 3 our Y will be 5, just look at the graph above and imagaine.

>>> line.solve_for_y(3, m, b)
5.0

Solve for X

And lastly using our slope and Y intercept we can solve for X when Y is some value:

def solve_for_x(self, y, slope, yintercept):
        '''Solve for X cord using line equatio'''
        if slope != 0 and slope:
                return float((y - float(yintercept))) / float(slope)
        else:
                raise Exception('Can not solve on a horizontal line')

And we can do the reverse of above to verify they are both working, when X is equal to 5 our Y should be 3:

>>> line.solve_for_x(5, m, b)
3.0

Put it all together

class Line(object):

    def __init__(self, data):
            self.first, self.second = data

    def slope(self):
            '''Get the slope of a line segment'''
            (x1, y1), (x2, y2) = self.first, self.second
            try:
                    return (float(y2)-y1)/(float(x2)-x1)
            except ZeroDivisionError:
                    # line is vertical
                    return None

    def yintercept(self, slope):
            '''Get the y intercept of a line segment'''
            if slope != None:
                    x, y = self.first
                    return y - slope * x
            else:
                    return None

    def solve_for_y(self, x, slope, yintercept):
            '''Solve for Y cord using line equation'''
            if slope != None and yintercept != None:
                    return float(slope) * x + float(yintercept)
            else:
                    raise Exception('Can not solve on a vertical line')

    def solve_for_x(self, y, slope, yintercept):
            '''Solve for X cord using line equatio'''
            if slope != 0 and slope:
                    return float((y - float(yintercept))) / float(slope)
            else:
                    raise Exception('Can not solve on a horizontal line')