Getting Started

Orange Widgets are components in Orange Canvas, a visual programming environment of Orange. They represent some self contained functionalities and provide a graphical user interface (GUI). Widgets communicate with each other and pass objects through communication channels to interact with other widgets.

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


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


Each widget has a name description and a set of input/outputs (referred to as the widget’s meta description).

This meta data is discovered at Orange Canvas application startup leveraging setuptools/distribute and its entry points protocol. Orange Canvas looks for widgets using an orange.widgets entry point.

Defining a widget

OWBaseWidget is the base class of a widget in the Orange Canvas workflow.

Every widget in the canvas framework needs to define its meta data. This includes the widget’s name and text descriptions and more importantly its input/output specification. This is done by defining constants in the widget’s class namespace.

We will start with a very simple example. A widget that will output a single integer specified by the user.

from orangewidget.widget import OWBaseWidget, Output
from orangewidget.settings import Setting
from orangewidget import gui

class IntNumber(OWBaseWidget):
    # Widget's name as displayed in the canvas
    name = "Integer Number"
    # Short widget description
    description = "Lets the user input a number"

    # An icon resource file path for this widget
    # (a path relative to the module where this widget is defined)
    icon = "icons/number.svg"

    # Widget's outputs; here, a single output named "Number", of type int
    class Outputs:
        number = Output("Number", int)

By design principle, Orange widgets in an interface 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 area 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. OWBaseWidget makes 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, even disregarding one and composing your widget completely unlike the others in Orange.

We specify the default layout with class attribute flags. Here we will only be using a single column (controlArea) GUI.

# Basic (convenience) GUI definition:
#   a simple 'single column' GUI layout
want_main_area = False
#   with a fixed non resizable geometry.
resizing_enabled = False

We want the current number entered by the user to be saved and restored when saving/loading a workflow. We can achieve this by declaring a special property/member in the widget’s class definition like so:

number = Setting(42)

And finally the actual code to define the GUI and the associated widget functionality:

def __init__(self):

    from AnyQt.QtGui import QIntValidator
    gui.lineEdit(self.controlArea, self, "number", "Enter a number",
                 valueType=int, validator=QIntValidator())

def number_changed(self):
    # Send the entered number on "Number" output

By itself this widget is useless because no widget accepts its output. So let us define a widget that displays a number.

from orangewidget.widget import OWBaseWidget, Input
from orangewidget import gui

class Print(OWBaseWidget):
    name = "Print"
    description = "Print out a number"
    icon = "icons/print.svg"

    class Inputs:
        number = Input("Number", int)

    want_main_area = False

    def __init__(self):
        self.number = None

        self.label = gui.widgetLabel(self.controlArea, "The number is: ??")

    def set_number(self, number):
        """Set the input number."""
        self.number = number
        if self.number is None:
            self.label.setText("The number is: ??")
            self.label.setText("The number is {}".format(self.number))

We define inputs with a class Inputs, just like outputs are defined by Outputs. However, each input must be handled by a class methods. We mark the handlers by decorating them; in above case by putting @Inputs.number before the method’s definition.

Notice how in the set_number method we check whether the number is None. None is sent to the widget when a connection between the widgets is removed or if the sending widget to which we are connected intentionally emptied the channel.

Now we can use one widget to input a number and another to display it.

One more:

from orangewidget.widget import OWBaseWidget, Input, Output

class Adder(OWBaseWidget):
    name = "Add two integers"
    description = "Add two numbers"
    icon = "icons/add.svg"

    class Inputs:
        a = Input("A", int)
        b = Input("B", int)

    class Outputs:
        sum = Output("A + B", int)

    want_main_area = False

    def __init__(self):
        self.a = None
        self.b = None

    def set_A(self, a):
        """Set input 'A'."""
        self.a = a

    def set_B(self, b):
        """Set input 'B'."""
        self.b = b

    def handleNewSignals(self):
        """Reimplemeted from OWBaseWidget."""
        if self.a is not None and self.b is not None:
            self.Outputs.sum.send(self.a + self.b)
            # Clear the channel by sending `None`