Getting Started

The tutorial on these pages is meant for those who are interested in developing widgets in Orange. Orange Widgets are components in Orange's visual programming environment. They are wrappers around some data analysis code that provide graphical user interface (GUI). Widgets communicate, and pass tokens through communication channels to interact with other widgets. While simplest widgets consist of even less than 100 lines of code, those more complex that often implement some fancy graphical display of data and allow for some really nice interaction may be over 1000 lines long.

When we have started to write this tutorial, we have been working on widgets for quite a while. There are now (now being in the very time this page has been crafted) about 50 widgets available, and we have pretty much defined how widgets and their interfaces should look like. We have also made some libraries that help set up GUI with only a few lines of code, and some mechanisms that one may found useful and user friendly, like progress bars and alike.

On this page, we will start with some simple essentials, and then show how to build a simple widget that will be ready to run within Orange Canvas, our visual programming environment.


Each Orange widget belongs to a category and within a category has an associated priority. Opening Orange Canvas, a visual programming environment that comes with Orange, widgets are listed in toolbox on the top of the window:

By default, Orange is installed in site-packages directory of Python libraries. Widgets are all put in the subdirectories of OrangeWidget directory; these subdirectories define widget categories. For instance, under windows and default settings, a directory that stores all the widgets displayed in the Evaluate pane is C:\Python23\Lib\site-packages\orange\OrangeWidgets\Evaluate. Figure above shows that at the time of writing of this text there were five widgets for evaluation of classifiers, and this is how my Evaluate directory looked like:

Notice that there are a number of files in Evaluate directory, so how does Orange Canvas distinguish those that define widgets? Well, widgets are Python script files that start with a header. Here is a header for

""" <name>Test Learners</name> <description>Estimates the predictive performance of learners on a data set.</description> <icon>icons/TestLearners.png</icon> <priority>200</priority> """

OWTestLearners is a Python script, so the header information we show about lies within the comment block, with triple quote opening and closing the comment. Header defines the name of the widget, its description, the name of the picture file the widget will use for an icon, and a number expressing the priority of the widget. The name of the widget as given in the header will be the one that will be used throughout in Orange Canvas. As for naming, the actual file name of the widget is not important. The description of the widget is shown once mouse rests on an toolbar icon representing the widget. And for the priority: this determines the order in which widgets appear in the toolbox. The one shown above for Evaluate groups has widget named Test Learners with priority 200, Classifications with 300, ROC Analysis with 1010, Lift Curve with 1020 and Calibration Plot with 1030. Notice that every time the priority number crosses a multiplier of a 1000, there is a gap in the toolbox between the widgets; in this way, a subgroups of the widgets within the same group can be imposed.

Widgets communicate. They use typed channels, and exchange tokens. Each widget would define its input and output channels in something like:

self.inputs = [("Test Data Set", ExampleTable, self.cdata), ("Learner", orange.Learner, self.learner, 0)] self.outputs = [("Evaluation Results", orngTest.ExperimentResults)]

Above two lines are for Test Learners widget, so hovering with your mouse over its icon in the widget toolbox would yield:

We will go over the syntax of channel definitions later, but for now the following is important:

  1. Widgets are defined in a Python files.
  2. For Orange and Orange canvas to find them, they reside in subdirectories in OrangeWidgets directory of Orange installation. The name of the subdirectory matters, as this is the name of the widget category. Widgets in the same directory will be grouped in the same pane of widget toolbox in Orange Canvas.
  3. A file describing a widget starts with a header. This, given in sort of XMLish style, tells about the name, short description, location of an icon and priority of the widget.
  4. The sole role of priority is to specify the placement (order) of widgets in the Orange Canvas toolbox.
  5. Somewhere in the code (where we will learn later) there are two lines which tell which channels the widgets uses for communication. These, together with the header information, completely specify the widget as it is seen from the outside.

Oh, by the way. Orange caches widget descriptions to achieve a faster startup, but this cache is automatically refreshed at startup if any change is detected in widgets' files.

Let's Start

Now that we went through some of the more boring stuff, let us now have some fun and write a widget. We will start with a very simple one, that will receive a data set on the input and will output a data set with 10% of the data instances. Not to mess with other widgets, we will create a Test directory within OrangeWidgets directory, and write the widget in a file called OWDataSamplerA: OW for Orange Widget, DataSampler since this is what widget will be doing, and A since we prototype a number of this widgets in our tutorial.

The script defining the OWDataSamplerA widget starts with a follwing header:

<name>Data Sampler</name> <description>Randomly selects a subset of instances from the data set</description> <icon>icons/DataSamplerA.png</icon> <priority>10</priority>

This should all be clear now, perhaps just a remark on an icon. We can put any name here, and if Orange Canvas won't find the corresponding file, it will use a file called Unknown.png (an icon with a question mark).

Orange Widgets are all derived from the class OWWidget. The name of the class should be match the file name, so the lines following the header in our Data Sampler widget should look something like:

from OWWidget import * import OWGUI class OWDataSamplerA(OWWidget): def __init__(self, parent=None, signalManager=None): OWWidget.__init__(self, parent, signalManager, 'SampleDataA') self.inputs = [("Data", ExampleTable,] self.outputs = [("Sampled Data", ExampleTable)] # GUI box = OWGUI.widgetBox(self.controlArea, "Info") self.infoa = OWGUI.widgetLabel(box, 'No data on input yet, waiting to get something.') self.infob = OWGUI.widgetLabel(box, '') self.resize(100,50)

In initialization, the widget calls the init function of a base class, passing the name 'SampleData' which will, essentially, be used for nothing else than a stem of a file for saving the parameters of the widgets (we will regress on these somehow latter in tutorial). Widget then defines inputs and outputs. For input, widget defines a "Data" channel, accepting tokens of the type orange.ExampleTable and specifying that data function will be used to handle them. For now, we will use a single output channel called "Sampled Data", which will be of the same type (orange.ExampleTable).

Notice that the types of the channels are specified by a class name; you can use any classes here, but if your widgets need to talk with other widgets in Orange, you will need to check which classes are used there. Luckily, and as one of the main design principles, there are just a few channel types that current Orange widgets are using.

The next four lines specify the GUI of our widget. This will be simple, and will include only two lines of text of which, if nothing will happen, the first line will report on "no data yet", and second line will be empty. By (another) design principles, in an interface Orange widgets are most often split to control and main area. Control area appears on the left and should include any controls for settings or options that your widget will use. Main are would most often include a graph, table or some drawing that will be based on the inputs to the widget and current options/setting in the control area. OWWidget make these two areas available through its attributes self.controlArea and self.mainArea. Notice that while it would be nice for all widgets to have this common visual look, you can use these areas in any way you want to, even disregarding one and composing your widget completely unlike the others in Orange.

As our widget won't display anything apart from some info, we will place the two labels in the control area and surround it with the box "Info".

In order to complete our widget, we now need to define how will it handle the input data. This is done in a function called data (remember, we did introduce this name in the specification of the input channel):

def data(self, dataset): if dataset: self.infoa.setText('%d instances in input data set' % len(dataset)) indices = orange.MakeRandomIndices2(p0=0.1) ind = indices(dataset) sample =, 0) self.infob.setText('%d sampled instances' % len(sample)) self.send("Sampled Data", sample) else: self.infoa.setText('No data on input yet, waiting to get something.') self.infob.setText('') self.send("Sampled Data", None)

The function is defined within a class definition, so its first argument has to be self. The second argument called dataset is the token sent through the input channel which our function needs to handle.

To handle the non-empty token, the widget updates the interface reporting on number of data items on the input, then does the data sampling using Orange's routines for these (see chapter on Random Sampling in Orange Reference Guide for more), and updates the interface reporting on the number of sampled instances. Finally, the sampled data is sent as a token to the output channel with a name "Sampled Data".

Notice that the token can be empty (dataset==None), resulting from either the sending widget to which we have connected intentionally emptying the channel, or when the link between the two widgets is removed. In any case, it is important that we always write token handlers that appropriately handle the empty tokens. In our implementation, we took care of empty input data set by appropriately setting the GUI of a widget and sending an empty token to the output channel.

Although our widget is now ready to test, for a final touch, let's design an icon for our widget. As specified in the widget header, we will call it DataSamplerA.png and will put it in icons subdirectory of OrangeWidgets directory (together with all other icons of other widgets).

For a test, we now open Orange Canvas. There should be a new pane in a widget toolbox called Test (this is the name of the directory we have used to put in our widget). If we click on this pane, it displays an icon of our widget. Try to hoover on it to see if the header and channel info was processed correctly:

Now for the real test. We put the File widget on the schema (from Data pane), read data set (or any that comes handy, if you can find none, download iris from Orange's data set repository). We also put our Data Sampler widget on the pane and open it (double click on the icon, or right-click and choose Open):

Drag this window off the window with the widget schema of Orange Canvas, and connect File and Data Sampler widget (click on an ouput connector - green box - of the File widget, and drag the line to the input connector of the Data Sampler). If everything is ok, as soon as you release the mouse the connection is established and, the token that was waiting on the output of the file widget was sent to the Data Sampler widget, which in turn updated its window:

To see if the Data Sampler indeed sent some data to the output, connect it to the Data Table widget:

Try opening different data files (the change should propagate through your widgets and with Data Table window open, you should immediately see the result of sampling). Try also removing the connection between File and Data Sampler (right click on the connection, choose Remove). What happens to the data displayed in the Data Table?

Testing Your Widget Outside Orange Canvas

When prototyping a single widget, for a fast test I often get bored of running Orange Canvas, setting the schema and clicking on icons to get widget windows. There are two options to bypass this. The first one is to add a testing script at the end of your widget. To do this, we finished Data Sampler with:

if __name__=="__main__": appl = QApplication(sys.argv) ow = OWDataSamplerA() dataset = orange.ExampleTable('') appl.exec_()

These are essentially some calls to Qt routines that run GUI for our widgets. At the core, however, notice that instead of sending the token to the input channel, we directly called the routine for token handling (data).

To test your widget in more complex environment, that for instance requires to set a complex schema in which your widget collaborates, use Orange Canvas to set the schema and then either 1) save the schema to be opened every time you run Orange Canvas, or 2) save this schema (File menu) as an application within a single file you will need to run each time you will test your widget.