Although I used Python a long time and OOP, I never really dwelled into the reasons that someone would use super instead of other ways in Python. Usually I would use other ways in an effort do avoid confusing words and those ugly underscores. Sometimes however it is worth making something a bit less readable and such a case is super.
Why should you learn to use super though? For a single reason.. super equals less headaches.
Calling a parent class’ initializer
Remember that with initializer I merely mean the __init__ method of a class. So let’s take for example the class A below.
class A(object):
def __init__(self):
print('This is A')
def hello(self):
print('Hello, this is a method from A')
When we instantiate this class, the initializer runs and thus we get printed ‘This is A’. Now we want B to inherit A:
class B(A):
def __init__(self):
print('This is B')
The result is a hybrid class – half A, half B. The problem is that both classes have an initializer and in such cases the hybrid’s methods, variables, etc. are preferred. So in practice B has the method hello that it inherited from A but will only run its own initializer.
In real life we tend to run the initializer for every parent class. This is simply because of how program designs tend to be. In our simple example a way to solve this is to explicitly call the initializer of A:
class B(A):
def __init__(self):
A.__init__(self)
print('This is B')
The self always makes me dizzy so I will explain a bit on it. Namely why can’t we just have A.__init__()? The reason is that A is not an instance but a class. self however is an instance and that’s why we use it. As you might have noticed though, it is an instance of B and still we pass it to A as if it was an instance of A. So why the hell does it work?
The reason it works is that as we said B is a hybrid – half A, half B. This is very similar to having a double citizenship. A half Greek, half Norwegian can be recognized in both Greece and Norway. In the same way A and B can be recognized as either A or B. Logical, aye?
The bless of not knowing
The above example works fine. But what if one changes the name of A into G? For a simple example like ours, we could just change every occurence of A into G. However if you are dealing with medium to large projects you might have many classes that inherit from A and way many files. Furthermore if you have tests, you probably have all sort of test classes that inherit as well.
The point is that in such cases a little change somewhere can invoke havoc. The programmer will need to keep track of every single place where we inherit class A which just is not practical. That’s where super comes into play.
With super we can call A’s initializer without ever typing the name of the class:
class B(A):
def __init__(self):
super(B, self).__init__() # notice we type B, not A
print('This is B')
Now, no matter if you rename A to G or V, you won’t have to make any changes to classes that inherit from that class!
The bless of caring even less
So you saw how super takes away the problem of having to keep track of class names we inherit from. I think all this makes much more sense when we inherit from multiple classes.
Say we have classes X and Y:
class X(object):
def __init__(self):
print('This is X')
class Y(object):
def __init__(self):
print('This is Y')
Now if B inherit from everyone else, in the no-super way it will look like this:
class B(A, X, Y):
def __init__(self):
A.__init__(self)
X.__init__(self)
Y.__init__(self)
print('This is B')
With super we can minimize it to:
class B(A):
def __init__(self):
super(B, self).__init__(self)
print('This is B')
At first glance this looks like we merely minimize the code to a single line. The real benefit however is that if we did not use super, now our class B would be much more prone to mistakes since either A, X, or Y might change name somewhere (more classes – higher probability of a rename).
I hope all this makes it very apparent that in big OOP projects where you have a lot of interaction between objects, classes, etc. Using super is just a simple trick that adds a huge gain for the programmer. So whenever you need to call an initializer (or any other method) from a parent class, please save yourself some trouble and use super!
Python 2 issues
You might have noticed that I use object in every parent class in the examples above. In Python 3 you don’t have to do this.
class A(object):
..
This merely makes a class inherit from object. The problem in Python 2 you see is that not everything is an object. As such we have to explicitly state it. In Python 3 all classes inherit from object be default so the code becomes much cleaner. Notice that even super is much cleaner in Python 3:
class A:
..
class B(A):
def __init__(self):
super().__init__() # no self pollution
..
This is a tutorial on hex which is very useful if you are ever going to read low-level code or program low-level things like network protocols or microcontrollers. I use a real project that I worked on to showcase all this, namely a matrix of 9 LEDs.
You should be able and understand why people put hex in the code instead of raw binary (if it exists for that programming language). There are very specific reasons for doing this and since converting from hex to binary is so damn easy, there is no excuse for you to not be able and do it in your brain.
Binary and LED patterns
I was building a trivial LED matrix the other day for an MBED microcontroller (think Arduino-like). The usual problem is that my brain is faulty so I do all sorts of things in the wrong way. I take this blog as the opportunity to make up for what I learn just to make sure that I won’t forget (and ofcourse to teach others if they are interested).
So my task was to achieve some patterns with 9 LEDs. Notice that it doesn’t matter how the microcontroller was connected etc. since here I am only dealing with how bits and bytes fit into low level programming. My first pattern was a rotating star that you can see below:
This star is made out of two LED patterns: a star and a cross.
The 1s and 0s simply mean if the LED at a position should be turned on or off. Now, when we’re dealing with low level things the minimum unit of information that can be sent is 1 byte (= 8 bits). In our case we have 9 LEDs so we need however a minimum of 2 bytes (= 16 bits). The above examples become the below binaries:
Star: 0000000101010101
Cross: 0000000010111010
Now, the problem is that when we deal with low level programming, most low level languages (C, C++ etc.) don’t let you write numbers as binary in your code. You can’t write printf(0b101) for example. You need separate libraries if you want to do that and that would be fine for our case. But imagine if there was a matrix of 100 LEDs. Someone reading printf(001001010101101010101010101010101110101001011100101) would just get lost in the 0s and 1s. That’s one of the big reasons hex is used – it’s super minimal.
Binaries as integers
At first when I wanted to create a star, I simply converted each binary to an integer and just put it in my code. Below you can see a simplified version of my code.
..
#define STAR 341
#define CROSS 186
int main() {
while (1) {
leds.write(CROSS)
sleep(1)
leds.write(STAR)
sleep(1)
}
}
The way I found those integers was by simply using Python. It is a rather trivial procedure:
>>> 0b101010101
341
>>> 0b10111010
186
Notice that I omit the extra 0s since they don’t add any value just like 000150 is always going to be 150 no matter how many zeros you add at front.
Binaries as hex
The code I used, worked fine. The problem with this solution is that it’s impossible to have a clue what an integer is in binary – and when we deal with low-level programming that matters most of the times. In our case for example each single 1 and 0 controls one LED. Being able to figure out fast the binary of a number in this case is very important.
For example say you find the code snippet below somewhere:
#define STAR1 341
#define STAR2 186
How can you tell if it’s the STAR1 or STAR2 that looks like an ‘X’? It’s just impossible. And what if there were many more stars or if the LED matrix was huge? Then it would be a nightmare to understand the code. That’s where hex comes in handy.
The good thing with hex is that someone can see the hex and decode it to binary in his brain. So say we had the above example but instead of integers had hex values:
#define STAR1 0x155
#define STAR2 0xba
A skilled programmer would directly see 0001 0101 0101 and 0000 1011 1010 with no effort. And he wouldn’t either need to decode the whole number to find out. Watching just the last hex digit of each STAR would give him (or us) a hint about which STAR is which.
It’s time we become that skilled programmer, don’t you think?
Hex to binary in da brain
Fortunately it is very simple to convert hex to binary in the brain. You simply have to understand that each hex number is made out of 4 bits since we need a max of 4 bits to represent the largest number in base 16 (which is the character ‘F’). So 0xF is 0b1111. (Notice that putting 0x in front denotes that the number is in hexadecimal represation and putting 0b denotes the binary representation accordingly.)
The procedure of binarizing a hex is simple:
Find the binary of each hex character
Place 0s in front of each binary (from above) so we always have 4 digits
Squeeze them all together as if they were strings
So for example:
F is 1111
5 is 0110
FF is 1111 1111
55 is 0110 0110
5F is 0110 1111
F5 is 1111 0110
Hopefully you get the hang of it. The question is.. what happens if we have 0x102? This might seem tricky since we get three very simple binaries: 1, 0 and 10. But as I said, if you add the 0s in front before you squeeze them together, you should get the correct value – in this case 1 0000 0010!
Also you need to memorise a few things to be able and do all this. I have written the bare minimum below:
Binary Decimal
1 = 1
10 = 2
100 = 4
1000 = 8
1010 = A
1111 = F
Then it’s quite easy to find in brain all the rest. For example to find the binary of B we can simply think that A is 1010, and that since B is just one step ahead, we add 1 to it and thus get 1011. Or to find 5 we simply have to add 1 to 4 which becomes 100+1=101. And so on.
This should also make it clear what the command chmod 777 in Linux does.
Big fat hex stars
The below is more like an exercise to test what we’ve learned. It should be rather simple to find the hex of the star below.
It might seem overwhelming, but the only thing you need to do is go in groups of 4s and write down each hex value.
Grouping in 4bit groups:
Decoding the above becomes 8388A08388A0 which is WRONG.
This was actually a trap to teach you the hard way that we should always start from the last digit. In this case in the end we are in a situation where we have an orphan digit 1. We can’t work with that since we need 4 digits to make a hex number.
The right way is to always start from the end. This is for all numbers no matter if they are represented in binary, octal, hex, decimal or whatever – as long as they are numbers, always start from the last digit and you’ll do fine. The reason is that when you finally get to the last number you can add as many zeros as you like (or need) without altering the value of the whole thing.
So the correct grouping is this (starting from bottom-right):
And then we just start from the bottom and get 1051141051141! Notice that in the end we again have a single 1 (at the top left this time), but this time we can add as many zeroes as we want since adding zeros in front of a number doesn’t change its vallue.
I spent quite some time lately trying to do all of sort of things with USB and Python. I will put out what I’ve learned through this course. Ofcourse I could use Pyusb or some other library but where’s the learning in that? As the Brits say: give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime.
This tutorial starts with a brief overview and then goes into general usage of Dbus and lastly using Dbus with UDisks2 to get notified if a USB storage device has been inserted.
Update:
I initially wrote this article with the title “Python detection of USB storage device”. After some time of working with UDisks2 and Dbus however I found out that the code tends to be very ugly and unexpected behaviours occur (not sure if it’s because of Dbus or UDisks2). So if you want to control an input/output device like a USB device or get events, I recommmend you use pyudev straight away! I will probably write an article on how to use that too. If you want to use DBus for educational purposes or want to use it maybe with some other software, please continue reading.
The daemons checkin’ out your USB
There are two main daemons running on Ubuntu (and probably all major distros) that deal with connecting/disconnecting devices.
The one is udisks which deals with storage devices like USB sticks and the like. The second is udev, a daemon that deals with all kind of devices from PCI boards to the keyboard and mouse (including everything that udisks deals with).
Let’s prove that they exist and that they are running on our system:
Now, depending on how old your Linux distribution is you might have udisks or udisks2 (fourth process) and then you have the udev daemon (second process). But what do these daemons do? Well let’s check for ourselves, shall we?
Let’s monitor our USB
Open two terminals. In the first type:
udisksctl monitor
And on the second one type:
udevadm monitor
These are front-ends for the daemons udisks and udev respectively.
Now, while looking at the two terminals we do the below:
First remove or put in a non-storage device like a mouse or a keyboard (USB mouse/keyboard works fine).
Now put in or remove a storage device (USB stick, external hard disk, etc).
You will notice that for the first action, udev prints a bunch of stuff while udisks does nothing. For the second case where we mingle with a storage device, both services print out stuff.
For example when I insert a USB stick I get this on the udisk terminal:
manos@box:~$ udisksctl monitor
Monitoring the udisks daemon. Press Ctrl+C to exit.
11:31:00.793: The udisks-daemon is running (name-owner :1.76).
11:31:05.850: Added /org/freedesktop/UDisks2/drives/Samsung_1040d8f1f9b92b56e6d1af1d7a0dd4c82781
org.freedesktop.UDisks2.Drive:
CanPowerOff: true
Configuration: {}
ConnectionBus: usb
Ejectable: true
..
and this on the udev terminal:
manos@box:~$ udevadm monitor
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent
KERNEL[2569.327243] add /devices/pci0000:00/0000:00:14.0/usb2/2-3/2-3.4/2-3.4.2 (usb)
KERNEL[2569.328960] add /devices/pci0000:00/0000:00:14.0/usb2/2-3/2-3.4/2-3.4.2/2-3.4.2:1.0 (usb)
KERNEL[2569.329026] add /devices/pci0000:00/0000:00:14.0/usb2/2-3/2-3.4/2-3.4.2/2-3.4.2:1.0/host7 (scsi)
..
This makes it clear that udisks records only changes to storage devices like USB sticks and hard disks. udev on the other hand monitors any device that can connect/disconnect to your PC.
On a sidenote (in case you’re wondering) the KERNEL[blah blah] messages that appeared on the udev terminal, are Netlink messages sent from the kernel to udev.
The bigger picture
Now, we know that udisks is just like udev but with the difference that udisks plays with storage devices.
It turns out that udisks actually uses udev itself:
DEVICE INFORMATION
udisks relies on recent versions of udev(7) and the Linux kernel.
So what does udev do then? Well udev’s main work is to populate the /dev folder on your root directory. So it’s kind of a discovering daemon that tells the kernel what devices are connected and where. That’s all!
Below you can see the bigger picture of how everything works together.
udev, udisks and dbus, they are all daemons running in the background from the time you start your computer.
Netlink and Dbus Netlink and Dbus are two different protocols used for processes to talk with each other. In the first case the kernel communicates with udev and in the second case udisks with the dbus daemon. Many applications use the dbus protocol like Unity, NetworkManager, Skype, and pretty much all Gnome apps.
Notice that the dbus-daemon that is running all the time is part of the dbus protocol. I talk more about how dbus works, later so you get an understanding as to why a daemon is needed.
What’s udev?
The kernel uses the Netlink protocol (essentially UNIX sockets) to send messages to udev daemon. These messages are the ones we saw earlier when we typed udevadm monitor in the terminal. The way this works is by udev setting up some specified rules that the kernel reads at bootup. Then whenener a device is connected to your computer, a message to udev is sent from the kernel.
udisks uses the udev library and thus has access to all this. If you can use udev then you can pretty much access the kernel messages sent to udev.
And what about that dbus thingy?
The dbus daemon is not specific to udisks only. dbus daemon runs always and is a generic solution for processes to exchange information. NetworkManager for example uses this, so someone could create a program that automatically gets notified if an ethernet card is inserted by simply using dbus. All programs that use dbus, have access to each other. Luckily for us, udisks uses dbus 🙂
Choosing between udev, dbus and direct kernel messaging
From all the above it should become apparent that there are three main ways we can get notified about a USB insertion:
Talk directly with the kernel with Netlink messages. In this case we essentially place ourselves in udev’s position.
Use the udev library like udisks does. In python this is done with using pyudev.
Use the Dbus protocol to talk with udisks directly.
There are pros and cons with all these three approaches. I will explain a bit on all three starting from the lowest level to the highest level.
Kernel
The kernel way is probably the best but is probably a hell to get working since it’s so low-level. You need to deal with raw sockets since that’s how Netlink communication is done. The good thing with this approach is that you are not dependent on anything more than the kernel itself. So it’s pretty hard that something will break and you can always be sure that your solution will work for any distribution.
udev
udev is probably the best practical solution. It handles the messages sent from the kernel so we don’t need to handle them. We only use a udev library and that’s all. From what I am aware all major Linux distributions use udev so you’re not going wrong with this solution. The only bad thing (and this is my own opinion) is that the udev library bindings for Python don’t come with Python so you need to install them on your system on your own.
Dbus
Dbus uses itself udev as we mentioned earlier so it has already a dependency. However as we sayied, pretty much all distros use udev so that shouldn’t be a problem. The main good things with Dbus is that the dbus library comes with Python (both 2 and 3) so you can start coding directly without having to install a bunch of stuff. An extra good thing is that maaaaaaany programs use the Dbus protocol so once you learn it, you can potentially access a lot of things. The bad thing with dbus is its ugly design (my opinion again).
In this tutorial I will talk about dbus since it’s one of the things that you learn once and can use in many different situations in the future.
How dbus works
Choosing Dbus means we will have to learn a bit on how Dbus protocol works and then try and find out how udisks uses dbus. I will try to make this as easy as possible even if it’s a quite hard task considering how sparse information is and how ugly some things in dbus are.
So what exactly is dbus? How does it connect things together? I think the easiest way to explain, is by showing you a diagram.
The dbus-daemon keeps a sort of a freeway which all programs can access. In our diagram, udisks, NetworkManager and our python program can share information between them. The dotted yellow rectangles in the diagram are representing the messages exchanged between the programs.
There are actually two freeways (buses)
Dbus uses two different freeways like the one I mentioned above. That also means there are two dbus-daemon running. One is a system-wide freeway and the other is a session-wide freeway. In this context, session is the duration of time that a user is logged in.
These dbus daemons are running:
ps ax | grep dbus-daemon
844 ? Ss 0:00 dbus-daemon --system --fork
2451 ? Ss 0:00 dbus-daemon --fork --session --address=unix:abstract=/tmp/dbus-IP8uQnHdYR
2545 ? S 0:00 /bin/dbus-daemon --config-file=/etc/at-spi2/accessibility.conf --nofork --print-address 3
As you see there are two dbus-daemon running, or three in this case. Omit the third one (no idea what that is). The two main daemons are the first two. Notice the --system in the first one and the --session in the second. These are exactly what you might guess they are.. The first one keeps the system-wide freeway and the second keeps the session-wide freeway. I think it’s time we drop the ‘freeway’ word and use bus instead.
Now each freeway has its own rules. Furthermore, each program using dbus, can setup its own rules. All these rules are of the type “root can send this type of message” and “any user can access this type of message but not send this type”, etc.
An important thing to notice is that surprisingly there is a dbus-daemon running on your system. This daemon is used to put all programs using dbus communication together. So when we will try to connect to program A via dbus, we will actually connect to dbus-daemon which acts as a mediator.
So how do we use this module? I give below two examples. First I try to access NetworkManager and then udisks. I do this to show you how the workflow is and as a proof that you can use dbus for more than merely USB insertion scenarios.
Listing programs connected to dbus
The simplest way to find which programs are connected to dbus is from Python.::
That will list all programs connected to the system bus. In practice, this means that we can connect to any of these programs (as long as we connect to the right bus ofcourse)!
In the same fashion we can list programs connected to the session bus (these are probably much more):
Notice that in some cases we get domain looking strings and in some cases we get weird numbers like ‘:1.83’ and ‘:1.128’. These are pretty much the same thing as domains and IP on the internet. In this case however instead of the domain/IP pointing to a host, they point to a program on our computer. Ofcourse the number “:1.128” is quite cryptic and there’s actually no way to know which program it resolves to. But that is fine since the programs that are meant to be accessed, are given domain-like names just so that we can find them easily.
The right tools
Now that we know which program we want to connect to, we have to somehow find out how and what we can access from it. For this I will use a tool called d-tree. This tool gives you an overview of all programs connected to dbus and also give you a list of things that you can access.
On Ubuntu you can install this program directly with:
sudo apt-get install d-feet
On a side-note, you might feel tempted to access all this via Python or by using the terminal. Do your self a favour and don’t. I already took that path and there are just too many issues you might run into that you’ll never be able and truly understand how things run without getting in the low-level source code.
Running a program’s method
Whenever you want to access a program’s method or property, you need exactly four things:
The program’s domain name
Object path
Interface
The method or property (signals are a special case so will talk about them separately)
All these things are pretty random and make little sense. However that’s the API that we have for DBus so if you want it you have to bare with me. You just have to learn how to find each one of them and then how to use them in your Python program to access the property or method you want. You will be using d-feet to locate all these four things!
As an example I will take NetworkManager. Just look below..
The method I want to run in this case is GetDevices(). However in order to run it I have to locate all the other things: the program, the interface and the object path. Just looking at the snapshot it’s easy to see where all these are located. As said, you just have to get used to finding them for any program.
Translating all this to code looks like this:
import dbus
bus = dbus.SystemBus()
obj = bus.get_object('org.freedesktop.NetworkManager', '/org/freedesktop/NetworkManager')
obj.GetDevices()
Where ‘org.freedesktop.NetworkManager’ is the program’s domain and ‘/org/freedesktop/NetworkManager’ is the so called object path. There is no real logic as to what an object path is. It’s just some bad implementation in the core, where someone tried to bring object oriented coding into Dbus which is written in C. What came out of this effort is this monster of illogical terms that just don’t fit together and just complicate things. However we have to learn all this if we are to use Dbus.
Now if you are a person that spots things, you probably noticed that we didn’t enter the Interface (which happens to look exactly like the program name) anywhere in our code. So how come the code works when I just stated that we always need 4 things? Well.. it’s one of those bad designs. The reason it works is because the dbus library tries to “guess” which interface you want to use. In this case it was right. However many times it doesn’t work and you will run into problems that are really hard to debug. So a friendly advice is to ALWAYS give the 4 things mentioned. FOUR IS THE NEW FIVE.
Same output as before. But use this convention and you will never run into debugging nightmares. After all, explicit is better than implicit and that is especially true in this case.
Listing devices with udisks
Building on top of what we did with NetworkManager we continue but this time with UDisks2.
As you might notice, udisks doesn’t have a straighforward method that we can call to list all devices. So someone has to dig through the things, read UDisks2 dbus API or simply google. Luckily for you, I did the digging.
From the d-feet snapshot we can see directly the four things we need: the program domain (org.freedesktop.UDisks2), the interface (org.freedesktop.DBus.ObjectManager), the object path (/org/freedesktop/UDisks2) and lastly the method (GetManagedObjects()).
You should get a blob of stuff including any mounted drives. Indeed you still have to dig through things but at least you shouldn’t be totally lost by now.
Getting properties/attributes
Building on the above, we will try to access some properties of a device. But first have a look at d-feet and pay attention to the fact that if you scroll down you will see the storage devices connected to your computer.
Then we can also see that the device I choose has a bunch of properties.
Now the logical thing to access these properties would be to do as we did earlier with the methods. Instead of methods we would just call the properties. And that my friend.. just won’t work! (Ugly Dbus, I hate you.)
So in order to access any property of any object we need to use a very specific interface: ‘org.freedesktp.DBus.Properties’.
So to not make this too tedious I will give the whole code again:
import dbus
bus = dbus.SystemBus()
obj = bus.get_object('org.freedesktop.UDisks2', '/org/freedesktop/UDisks2/drives/MBED_microcontrolleur_10105a42e87da33c103dccfb6bc235360a97')
iface = dbus.Interface(obj, 'org.freedesktop.DBus.Properties') # Here we use this 'magic' interface
iface.GetAll('org.freedesktop.UDisks2.Drive')
Notice that the object path ‘/org/freedesktop/UDisks2/drives/MBED_microcontrolleur_10105a42e87da33c103dccfb6bc235360a97’ has to be replaced with one that actually exists on your computer.
Then we open the correct interface and call GetAll(). The argument to GetAll() is the interface name that you want to get the properties of. As I told many times.. Dbus is so damn confusing! Anyway, the good thing is that the ‘org.freedesktp.DBus.Properties’ interface has only three methods we need to learn: Get(), Set(), GetAll(). So things can’t get more complicated than this.
If we want to access a very specific property we just use Get() instead of GetAll() and pass the interface name followed by the name of the property we want:
import dbus
bus = dbus.SystemBus()
obj = bus.get_object('org.freedesktop.UDisks2', '/org/freedesktop/UDisks2/drives/MBED_microcontrolleur_10105a42e87da33c103dccfb6bc235360a97')
iface = dbus.Interface(obj, 'org.freedesktp.DBus.Properties')
iface.Get('org.freedesktop.UDisks2.Drive', 'Id') # Only difference
How about that USB insertion notification? (signals)
We saw how we can call methods but what about the interesting things like getting notified about an event like USB insertion? For this we can use signals.
All a signal is, is an incoming message from UDisks2 telling us that a mountable device has been inserted. From our part this signal will run a function in our code, a so called callback function. If you check in d-feet you will see many types of signals. The one that interests us is InterfacesAdded() from org.freedesktop.DBus.ObjectManager.
So how do we do this? I will get you straight the code for this one and do the explaining afterwards.
import dbus
from dbus.mainloop.glib import DBusGMainLoop
DBusGMainLoop(set_as_default=True)
bus = dbus.SystemBus()
# Function which will run when signal is received
def callback_function(*args):
print('Received something .. ', args)
# Which signal to have an eye for
iface = 'org.freedesktop.DBus.ObjectManager'
signal = 'InterfacesAdded'
bus.add_signal_receiver(callback_function, signal, iface)
# Let's start the loop
import gobject
loop = gobject.MainLoop()
loop.run()
If you run the above code, you will get printed things for every disconnection or connection of a storage device. Try plugging in your USB stick and see for yourself.
There are two things worth noticing in the code with signals:
We import and use a bunch of weird things like gobject and DBusGMainLoop
We don’t need the four things like earlier. We only need to know the interface and the signal (oh and both are just strings)
For our use the only thing we need from these libraries are the so called loops. The loops are needed since someone has to be looping somewhere waiting for the signal to arrive. We could probably create our own loop but I am not so sure how easy it would be. You could take that route if you want, may Zoidberg Jesus guide you the way. Below I mention a bit on what excactly Glib and Gobject is, just in case you’re curious and bored to google.
Glib
Glib is a library that originally was developed to be used with Gnome’s GTK. However later it split from it so that it could be used on any Linux machine. Many programs use the Glib library since it’s it provides so many things that is hard to find (in C) in a general-purpose library. Some things offered are data-structures like hash tables and linked lists (just keep in mind next time you will need any of this).
Gobject
Gobject is built on top of Glib. What it does is merely allow object oriented programming with the Glib. Since Glib is a C library, it doesn’t have object-oriented things lik objects, inheritance, classes etc. For that reason Gobject was developed. In our case, the reason we use it is to easily be able and change things on the loop without having to dig inside the C code.
Unblocking the loop
Now the loop above seems to work fine. There is a problem though. Your whole program blocks in this loop. Even if you add 1000 threads, they will all block for some weird reason and only the loop will run.
In order to stop the loop you have to:
Add the code with the signal on a Python thread
Initialize internally a thread for gobject (no clue what that does but won’t work if you don’t do it)
So in practice we just put the whole code above in a separate thread and then we make sure that gobject is calling threads.init() before we run() the loop.
And here is the code with the loop running on its own thread:
import threading, time
def start_listening():
import dbus
from dbus.mainloop.glib import DBusGMainLoop
DBusGMainLoop(set_as_default=True)
bus = dbus.SystemBus()
# Function which will run when signal is received
def callback_function(*args):
print('Received something .. ', args)
# Which signal to have an eye for
iface = 'org.freedesktop.DBus.ObjectManager'
signal = 'InterfacesAdded'
bus.add_signal_receiver(callback_function, signal, iface)
# Let's start the loop
import gobject
gobject.threads_init() # Without this, we will be stuck in the glib loop
loop = gobject.MainLoop()
loop.run()
# Our thread will run start_listening
thread=threading.Thread(target=start_listening)
thread.daemon=True # This makes sure that CTRL+C works
thread.start()
# And our program will continue in this pointless loop
while True:
time.sleep(1)
print("tralala")
This is a good starting point for your program. Feel free to copy/paste it 🙂
Digging further
All this should give you an idea on how to get started. Now the main thing is probably that you have to filter out some signal events since InterfacesAdded() might run several times for a single unplug or plug-in of a device. Or depending on your case maybe you want to wait for a different signal. Using d-feet this should be rather easy to figure out.
The easiest way to do things from my experience is to use d-free and the Python interpreter directly to test things. However if you feel comfortable with ugly complicated interfaces then the UDisks2 dbus API might be of benefit for you.
This guide will show you how to run a GUI application headless in a Docker container and even more specific scenarios involving running Firefox and Chrome. If you are not interested about those then you can just stop in the middle of this tutorial.
What the hell is X?
X is a program that sits on a Linux machine with a monitor (so servers usually don’t use X). X’s job is to talk to the Linux kernel in behalf of GUI programs. So if you are playing a game for example, the game (that is, the application) is constantly sending drawing commands to the X server like “draw me a rectangle here”. X forwards all this to the Kernel which will further forward the information to the GPU to render it on the monitor.
X can even receive commands from the keyboard or mouse. When you click to shoot on your game for example, the command “click at 466,333” is sent from your mouse to the kernel, from the kernel to the X and from X to the game. That way the game can have a clue on what is happening!
You will often hear X being called a server and the reason for that is simply because the way the applications send commands to it is through sockets. For that reason the applications are also referred to as clients many times.
If you are reading this then the X is running on your PC. Let’s prove it:
We can see that X is running as root and has PID 1436. An other important thing is to notice the :0 which is called display in X jargon. A display is essentially:
A monitor
A mouse
A keyboard
And this is the bigger picture of how it all looks together:
Now there is a variable in Linux that is used whenever we run a GUI program. That variable is surprisingly called DISPLAY. The syntax of the DISPLAY variable is
<hostname>:<display>.<monitor>
. Let’s check the DISPLAY on our computer:
> echo $DISPLAY
:0
I get :0, which means we use display 0. Notice however that this says nothing about which monitor we use. This makes sense since if you are running 2 or more monitors on your Linux you still have the same environment variables in both of them. It wouldn’t make sense that an environment variable changes just because you echo it from a different screen, would it? For that reason we get the display and not the monitor so that we get the same output on both. As about the hostname, since there is no info about it, the local host is assumed.
On a notice, if you have multiple monitors you can still specify which monitor to run an application by simply typing the full display variable you want. So if you have a monitor 0 and a monitor 1 on the current display, I can run firefox on monitor 1 with:
DISPLAY=:0.1 firefox
Creating a virtual monitor
Instead of running X, we can run a different version of it that can create virtual displays. Xvfb (virtual framebuffer – whatever the hell that means) will create a virtual monitor for us.
So let’s make a new monitor (I assume you have installed xvfb):
Xvfb :1 -screen 0 1024x768x16
This will start the Xvfb server with a display 1 and a virtual screen(monitor) 0. We can access this by simply typing DISPLAY=:1.0 before running our graphical program. In this case the program will start in the virtual screen instead of our monitor.
We see we have the normal display 0. (A way to tell it is the default screen is to see that it runs as root.) We can also see the second display :1 and screen 0 with resolution 1024×768. So what if we want to use it?
Open a new terminal and type:
> DISPLAY=:1.0 firefox
..
This will start firefox at the given display. The reason I use the DISPLAY at the same line is to make sure that the subprocess inherits the variable DISPLAY. An other way to do this is to type:
> DISPLAY=:1.0
> export DISPLAY
> firefox
..
Run a GUI program in a Docker container
We will now create a virtual screen inside a docker container.
So now we are sure that we are running the virtual screen. Let’s access it and run something graphical on it. In this case I will run Firefox and Python+Selenium just as a proof of concept of what is happening.
First I put my display variable and use export to assure that any sub-shells or sub-processes use the same display (with export, they inherit the variable DISPLAY!):
root@660ddd5cc806:/# firefox
(process:14967): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Xlib: extension "RANDR" missing on display ":99.0".
(firefox:14967): GConf-WARNING **: Client failed to connect to the D-BUS daemon:
//bin/dbus-launch terminated abnormally without any error message
..
The errors don’t mean anything. But we can’t be sure, can we? I mean, since we can’t see what’s happening it’s really hard to tell. There are two things we can do, either use ImageMagick to take a snapshot and send it to our host via a socket or we can simply use Selenium. I will do that since most people probably want to achieve all this for testing purposes anyway.
If you get a bunch of HTML, then we have succeeded!
The Chrome issue
If you try and run Chrome in a Docker container, it won’t work even if you have setup everything correctly. The reason is that Chrome uses something called sandboxing. Reading this I could not let but notice the word jail. Apparently it seems that Chrome uses Linux containers (the same that Docker uses). For this reason you have to put a bit of extra effort to solve this issue since because of technical difficulties it’s not possible to run containers in containers.
I have now installed Selenium, Chrome and Xvfb. Now I am going to make make a virtual monitor and run Chrome:
root@7dd2c07cb8cb:/# Xvfb :99 -screen 0 1024x768x16 &> xvfb.log &
[1] 6729
root@7dd2c07cb8cb:/# DISPLAY=:99.0
root@7dd2c07cb8cb:/# export DISPLAY
root@7dd2c07cb8cb:/# google-chrome
Xlib: extension "RANDR" missing on display ":99.0".
Xlib: extension "RANDR" missing on display ":99.0".
[6736:6736:1017/143449:ERROR:desktop_window_tree_host_x11.cc(802)] Not implemented reached in virtual void views::DesktopWindowTreeHostX11::InitModalType(ui::ModalType)
ATTENTION: default value of option force_s3tc_enable overridden by environment.
failed to create drawable
[6775:6775:1017/143449:ERROR:gl_surface_glx.cc(633)] glXCreatePbuffer failed.
[6775:6775:1017/143449:ERROR:gpu_info_collector.cc(27)] gfx::GLContext::CreateOffscreenGLSurface failed
[6775:6775:1017/143449:ERROR:gpu_info_collector.cc(89)] Could not create surface for info collection.
[6775:6775:1017/143449:ERROR:gpu_main.cc(402)] gpu::CollectGraphicsInfo failed (fatal).
[6775:6775:1017/143449:ERROR:sandbox_linux.cc(305)] InitializeSandbox() called with multiple threads in process gpu-process
[6775:6775:1017/143449:ERROR:gpu_child_thread.cc(143)] Exiting GPU process due to errors during initialization
[6736:6736:1017/143449:ERROR:gpu_process_transport_factory.cc(418)] Failed to establish GPU channel.
It seems that it works. It’s normal that we get the gpu errors since we don’t have a gpu! However I don’t like gambling so we will take it a step further to check that the browser actually works. However for this I will need to download the webdriver for Google Chrome..
LinuxMeerkat 