Gaphas is the diagramming widget library for Python.
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Gaphas is the diagramming widget library for Python.

Gaphas Demo

Gaphas is a library that provides the user interface component (widget) for drawing diagrams. Diagrams can be drawn to screen and then easily exported to a variety of formats, including SVG and PDF. Want to build an app with chart-like diagrams? Then Gaphas is for you! Use this library to build a tree, network, flowchart, or other diagrams.

This library is currently being used by Gaphor for UML drawing, RAFCON for state-machine based robot control, and ASCEND for solving mathematical models.

📑 Table of Contents

📜 Background

Gaphas was built to provide the foundational diagramming portions of Gaphor. Since Gaphor is built on GTK+ and cairo, PyGObject provides access to the GUI toolkit, and PyCairo to the 2D graphics library. However, there wasn't a project that abstracted these technologies to easily create a diagramming tool.

Here is how it works:

  • Items (canvas items) can be added to a Canvas.
  • The canvas maintains the tree structure (parent-child relationships between items).
  • A constraint solver is used to maintain item constraints and inter-item constraints.
  • The item (and user) should not be bothered with things like bounding-box calculations.
  • Very modular: e.g. handle support could be swapped in and swapped out.
  • Rendering using Cairo.

The main portions of the library include:

  • canvas - The main canvas class (container for Items).
  • items - Objects placed on a Canvas.
  • solver - A constraint solver to define the layout and connection of items.
  • view - Responsible for the calculation of bounding boxes which is stored in a quadtree data structure for fast access.
  • gtkview - A view to be used in GTK+ applications that interacts with users with tools.
  • painters - The workers used to paint items.
  • tools - Tools are used to handle user events (such as mouse movement and button presses).
  • aspects - Provides an intermediate step between tools and items.

💾 Install

To install Gaphas, simply use pip:

$ pip install gaphas

Use of a virtual environment is highly recommended.


To setup a development environment with Linux:

$ sudo apt-get install -y python3-dev python3-gi python3-gi-cairo
    gir1.2-gtk-3.0 libgirepository1.0-dev libcairo2-dev
$ pip install poetry
$ poetry install

🔦 Usage

import gi
gi.require_version("Gtk", "3.0")
from gi.repository import Gtk

from gaphas import Canvas, GtkView
from gaphas.examples import Box
from gaphas.painter import DefaultPainter
from gaphas.item import Line
from gaphas.segment import Segment

def create_canvas(canvas, title):
    # Setup drawing window
    view = GtkView()
    view.painter = DefaultPainter()
    view.canvas = canvas
    window = Gtk.Window()
    window.set_default_size(400, 400)
    win_box = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL)
    win_box.pack_start(view, True, True, 0)

    # Draw first gaphas box
    b1 = Box(60, 60)
    b1.matrix = (1.0, 0.0, 0.0, 1, 10, 10)

    # Draw second gaphas box
    b2 = Box(60, 60)
    b2.min_width = 40
    b2.min_height = 50
    b2.matrix.translate(170, 170)

    # Draw gaphas line
    line = Line()
    line.matrix.translate(100, 60)
    line.handles()[1].pos = (30, 30)
    segment = Segment(line, view=None)

    window.connect("destroy", Gtk.main_quit)

c = Canvas()
create_canvas(c, "Simple Gaphas App")


The Canvas class (from acts as a container for Item's (from The item's parent/child relationships are maintained here (not in the Item!).

An Item can have a set of Handles (from which can be used to manipulate the item (although this is not necessary). Each item has its own coordinate system with a x and y position, for example a (0, 0) point. Item.matrix is the transformation relative to the parent item of the Item, as defined in the Canvas.

Handles can connect to Ports. A Port is a location (line or point) where a handle is allowed to connect on another item. The process of connecting depends on the case at hand, but most often involves the creation of some sort of constraint between the Handle and the item it is connecting to (see doc/ports.txt).

The Canvas also contains a constraint Solver (from that can be used to solve mathematical dependencies between items (such as Handles that should be aligned). The constraint solver can also be used to keep constraints contained within the item true, for example to make sure a box maintains its rectangular shape.

View (from is used to visualize a canvas. On a View, a Tool (from can be applied, which will handle user input like button and key presses. Painters (from are used to do the actual drawing. This module also makes it easy to draw to other media other than a screen, such as a printer or PDF document.

Updating item state

If an items needs updating, it sends out an update request on the Canvas (Canvas.request_update()). The canvas performs an update by performing the following steps:

  1. Pre-update using Item.pre_update(context) for each item marked for update.
  2. Update the Canvas-to-Item matrices, for fast transformation of coordinates from the canvas' to the items' coordinate system. The c2i matrix is stored on the Item as Item._matrix_c2i.
  3. Solve the constraints.
  4. Normalize the items by setting the coordinates of the first handle to (0, 0).
  5. Update the Canvas-to-Item matrices for items that have been changed by normalization.
  6. Post-update using Item.post_update(context) for each item marked for update, including items that have been marked during the constraint solving step.

Gaphas attempts to do as much updating as possible in the {pre|post}_update() methods, since they are called when the application is not handling user input.

The context contains a CairoContext. This can be used, for example, to calculate the dimensions of text. One thing to keep in mind is that updating is done from the canvas. Items should not update sub-items. After the update steps are complete, the Item should be ready to be drawn.

Constraint solving

Constraint solving is one of the big features of this library. The Solver is able to mathematically solve these constraint rules that are applied to an item or between items. Constraints are applied to items through Variables owned by the item. An example of applying a constraint to an item is that Element items use constraints to maintain their rectangular shape. An example of applying constraints between items is to apply a constraint between a line and a box in order to connect them.

Constraints that apply to one item are pretty straight forward, as all variables live in the same coordinate system of the item. The variables, like the Handle's x and y coordinate can simply be put in a constraint.

When two items are connected to each other and constraints are created, a problem shows up: variables live in separate coordinate systems. In order to overcome this problem, a Projection (from has been defined. With a Projection instance, a variable can be "projected" on another coordinate system. In this case, the Canvas' coordinate system is used when two items are connected to each other.


Drawing is done by the View. All items marked for redraw, the items that have been updated, will be drawn in the order in which they reside in the Canvas. The order starts with the first root item, then its children, then second root item, etc.

The view context passed to the Items draw() method has the following properties:

  • view - The view we're drawing to.
  • cairo - The CairoContext to draw to.
  • selected - True if the item is actually selected in the view.
  • focused - True if the item has the focus
  • hovered - True if the mouse pointer if over the item. Only the top-most item is marked as hovered.
  • dropzone - The item is marked as the drop zone. This happens then an item is dragged over the item, and if it is dropped, it will become a child of this item.
  • draw_all - True if everything drawable on the item should be drawn, for example when calculating the bounding boxes of an item.

The View automatically calculates the bounding box for the item, based on the items drawn in the draw(context) function (this is only done when really necessary, e.g. after an update of the item). The bounding box is in viewport coordinates.

The actual drawing is done by Painters ( A series of Painters have been defined: one for handles, one for items, etc.


Behaviour is added to the canvas(-view) by tools. Tools can be chained together in order to provide more complex behaviour.

To make it easy, a DefaultTool has been defined which is a ToolChain instance with the tools added as follows:

  • ToolChain - Delegates to a set of individual tools and keeps track of which tool has grabbed the focus. This normally happens when the user presses a mouse button. Once this happens, the tool requests a "grab" and all events, like motion or button release, are sent directly to the focused tool.

  • HoverTool - Makes the item under the mouse button the "hovered item". When such an item is drawn, its context.hovered_item flag will be set to True.

  • HandleTool - Allows for handles to be dragged around and focuses the item when its handle is clicked on.

  • ItemTool - Selects items and enables dragging items around.

  • TextEditTool - A demo tool that features a text edit pop-up.

  • RubberbandTool - Invoked when the mouse button is pressed on a section of the view where no items or handles are present. It allows the user to select items using a "rubber band" selection box.


Interaction with the canvas view (visual component) is handled by tools. Although the default tools do a fair amount of work, in most cases you'll desire to create some custom connection behavior. In order to implement these, HandleTool provides hooks including connect, disconnect and glue.

One of the challenges you'll likely face is what to do when an item is removed from the canvas and there are other items (lines) connected to it. Gaphas provides a solution to this by providing a disconnect handler to the handle instance once it is connected. A function can be assigned to this disconnect handler, which is then called when the item it is connected to is removed from the canvas.


Gaphas has a simple built-in system for registering changes in its classes and notifying the application. This code resides in

There is also a "reverter" framework in place. This system is notified when objects change their state, and it will figure out the reverse operation that has to be applied in order to undo the operation.


The API can be separated into a Model-View-Controller with these three parts:

  1. The Model, including the canvas and items
  2. The View, called view
  3. The Controller, called tools

Canvas and Items

Class: gaphas.canvas.Canvas

The Canvas is a container for items.

canvas = Canvas()

Class: gaphas.item.Item

Base class (or interface) for items on a Canvas.

item = Item()
  • matrix: The item's transformation matrix
  • canvas: The canvas, which owns an item
  • constraints: list of item constraints, automatically registered when the item is added to a canvas; may be extended in subclasses

Class: gaphas.connector.Handle

Handles are used to support modifications of Items.

If the handle is connected to an item, the connected_to property should refer to the item. A disconnect handler should be provided that handles the required disconnect behaviour, for example cleaning up the constraints and connected_to.

  • pos (gaphas.connector.Position): The position of the item, default value is (0, 0).
  • strength (int): The strength of the handle to use in the constraint solver, default value is NORMAL, which is 20.
  • connectable (bool): Makes the handle connectable to other items, default value is False.
  • movable (bool): Makes the handle moveable, default value is True.
handle = Handle((10, 10), connectable=True)

Class: gaphas.connector.LinePort

The Line Port is part of an item that provides a line between two handles.

  • start (gaphas.connector.Position): The start position of the line.
  • end (gaphas.connector.Position): The end position of the line.
p1, p2 = (0.0, 0.0), (100.0, 100.0)
port = LinePort(p1, p2)

Class: gaphas.connector.PointPort

The Point Port connects the handle to an item using a port at the location of the handle.

h = Handle((10, 10))
port = PointPort(h.pos)

Class: gaphas.solver.Solver

A Solver solves constraints.

a, b, c = Variable(1.0), Variable(2.0), Variable(3.0)
solver = Solver()
c_eq = EquationConstraint(lambda a,b: a+b, a=a, b=b)

Class: gaphas.constraint.EqualsConstraint

Make 'a' and 'b' equal.

a, b = Variable(1.0), Variable(2.0)
eq = EqualsConstraint(a, b)

Class: gaphas.constraint.LessThanConstraint

Ensure one variable stays smaller than another.

a, b = Variable(3.0), Variable(2.0)
lt = LessThanConstraint(smaller=a, bigger=b)

Class: gaphas.constraint.CenterConstraint

Ensures a Variable is kept between two other variables.

a, b, center = Variable(1.0), Variable(3.0), Variable()
eq = CenterConstraint(a, b, center)

Class: gaphas.constraint.EquationConstraint

Solve a linear equation.

a, b, c = Variable(), Variable(4), Variable(5)
cons = EquationConstraint(lambda a, b, c: a + b - c, a=a, b=b, c=c)

Class: gaphas.constraint.BalanceConstraint

Keeps three variables in line, maintaining a specific ratio.

a, b, c = Variable(2.0), Variable(3.0), Variable(2.3, WEAK)
bc = BalanceConstraint(band=(a,b), v=c)
c.value = 2.4

Class: gaphas.constraint.LineConstraint

Solves the equation where a line is connected to a line or side at a specific point.

line = (Variable(0), Variable(0)), (Variable(30), Variable(20))
point = (Variable(15), Variable(4))
lc = LineConstraint(line=line, point=point)


Class: gaphas.view.View

View class for gaphas.canvas.Canvas objects.

canvas = Canvas()
view = View(canvas=canvas)

Class: gaphas.view.GtkView

GTK+ widget for rendering a gaphas.canvas.Canvas to a screen.

canvas = Canvas()
win = Gtk.Window()
view = GtkView(canvas=canvas)

Class: gaphas.painter.PainterChain

Chain up a set of painters.

view.painter = (

Class: gaphas.painter.DrawContext

Special context for drawing the item. It contains a cairo context and properties like selected and focused.

  • **kwargs: Optional cairo properties for a context.
    selected=(item in view.selected_items),
    focused=(item is view.focused_item),
    hovered=(item is view.hovered_item),
    dropzone=(item is view.dropzone_item),

Class: gaphas.painter.ItemPainter

Painter to draw an item.

svgview = View(view.canvas)
svgview.painter = ItemPainter()

Class: gaphas.painter.CairoBoundingBoxContext

It is used intercept stroke(), fill(), and others context operations so that the bounding box of the item involved can be calculated.

  • cairo (cairo.Context): The cairo context to intercept.
cairo = CairoBoundingBoxContext(cairo)

Class: gaphas.painter.BoundingBoxPainter

A type of ItemPainter which is used to calculate the bounding boxes (in canvas coordinates) for the items.

view.bounding_box_painter = BoundingBoxPainter()

Class: gaphas.painter.HandlePainter

Draw handles of items that are marked as selected in the view.

Class: gaphas.painter.ToolPainter

Allows the Tool defined on a view to conduct drawing.

Class: gaphas.painter.FocusedItemPainter

Used to draw on top of all the other layers for the focused item.


Interacting with the canvas is done through tools. Tools tell what has to be done (like moving). To make an element move aspects are defined. Aspects tell how the behaviour has to be performed.


Used to chain tools together. For example, chain a HoverTool, HandleTool, and SelectionTool in order to combine their functionality in to a new tool.

  • view (gaphas.view.View): The view to use for the tool chain.


Makes the item under the mouse cursor the hovered item.

  • view (gaphas.view.View): The view to use for the tool, default is None.


Does selection and dragging of items.

  • view (gaphas.view.View): The view to use for the tool, default is None.


Tool to move handles around.

  • view (gaphas.view.View): The view to use for the tool, default is None.


Allows the user to drag a "rubber band" for selecting items in an area.

  • view (gaphas.view.View): The view to use for the tool, default is None.


Captures drag events with the middle mouse button and uses them to translate the canvas within the view.

  • view (gaphas.view.View): The view to use for the tool, default is None.


Tool for zooming using two different user inputs:

  1. Ctrl + middle-mouse dragging in the up and down direction
  2. Ctrl + mouse-wheel
  • view (gaphas.view.View): The view to use for the tool, default is None.


Tool for placing items on the canvas.

  • view (gaphas.view.View): The view to use for the tool.
  • factory (factory object): A canvas item factory for creating new items.
  • handle_tool ( The handle tool to use.
  • handle_index (int): The index of the handle to be used by the handle tool.
def on_clicked(button):
    view.tool.grab(PlacementTool(view, factory(view, MyLine), HandleTool(), 1))

Class: gaphas.aspects.ItemFinder

Find an item on the canvas.

  • view (gaphas.view.View): The view to use in order to search for an item.

Class: gaphas.aspects.ItemSelection

Manages selection and unselection of items.

  • item (gaphas.item.Item): The item to set as focused or unfocused.
  • view (gaphas.view.View): The view to focus or unfocus on.

Class: gaphas.aspects.ItemInMotion

Manages motion of an item.

  • item (gaphas.item.Item): The item to move.
  • view (gaphas.view.View): The view to to use for move coordinates.

Class: gaphas.aspects.ItemHandleFinder

Finds handles.

  • item (gaphas.item.Item): The item.
  • view (gaphas.view.View): The view to get the handle at the position from.
canvas = Canvas()
line = Line()
view = View(canvas)
finder = HandleFinder(line, view)

Class: gaphas.aspects.ElementHandleSelection

Selects the handle of a gaphas.item.Element.

  • item (gaphas.item.Item): The Element item that the handle belongs to.
  • handle (gaphas.connector.Handle): The handle to select or unselect.
  • view (gaphas.view.View): The view to use to apply the cursor to.

Class: gaphas.aspects.ItemHandleInMotion

Move a handle.

  • item (gaphas.item.Item): The item that the handle belongs to.
  • handle (gaphas.connector.Handle): The handle to move.
  • view (gaphas.view.View): The view to use for the coordinate system.

Class: gaphas.aspects.ItemConnector

Connect or disconnect an item's handle to another item or port.

  • item (gaphas.item.Item): The item that the handle belongs to.
  • handle (gaphas.connector.Handle): The handle to connect.

Class: gaphas.aspects.ItemConnectionSink

Makes an item a sink, which is another item that an item's handle is connected to like a connected item or port.

  • item (gaphas.item.Item): The item to look for ports on.
  • port (gaphas.connector.Port): The port to use as the sink.

Class: gaphas.aspects.ItemPaintFocused

Paints on top of all items, just for the focused item and only when it's hovered (see gaphas.painter.FocusedItemPainter).

  • item (gaphas.item.Item): The focused item.
  • view (gaphas.view.View): The view to paint with.

Extended Behaviour

By importing the following modules, extra behaviour is added to the default view behaviour.

Class: gaphas.segment.LineSegment

Split and merge line segments.

  • item (gaphas.item.Item): The item of the segment.
  • view (gaphas.view.View): The view to use for the split coordinate system.

Class: gaphas.segment.SegmentHandleFinder

Extends the gaphas.aspects.ItemHandleFinder to find a handle on a line, and create a new handle if the mouse is located between two handles. The position aligns with the points drawn by the SegmentPainter.

  • item (gaphas.item.Item): The item.
  • view (gaphas.view.View): The view to get the handle at the position from.

Class: gaphas.segment.SegmentHandleSelection

Extends the gaphas.aspects.ItemHandleSelection to merge segments if the handle is released.

  • item (gaphas.item.Item): The item that the handle belongs to.
  • handle (gaphas.connector.Handle): The handle to select or unselect.
  • view (gaphas.view.View): The view to use to apply the cursor to.

Class: gaphas.segment.LineSegmentPainter

This painter draws pseudo-handles on a gaphas.item.Line by extending gaphas.aspects.ItemPaintFocused. Each line can be split by dragging those points, which will result in a new handle.

ConnectHandleTool take care of performing the user interaction required for this feature.

  • item (gaphas.item.Item): The focused item.
  • view (gaphas.view.View): The view to paint with.


Provides a guide to align items for gaphas.item.Element.

  • item (gaphas.item.Item): The item to provide guides for.


Provides a guide to align items for gaphas.item.Line.

  • item (gaphas.item.Item): The item to provide guides for.


Move the item and lock the position of any element that's located at the same position.

  • item (gaphas.item.Item): The item to move.
  • view (gaphas.view.View): The view with guides to to use for move coordinates.
canvas = Canvas()
view = GtkView(canvas)
element = Element()
guider = GuidedItemInMotion(element, view)
guider.start_move((0, 0))


Move a handle and lock the position of any element that's located at the same position.

  • item (gaphas.item.Item): The item that the handle belongs to.
  • handle (gaphas.connector.Handle): The handle to move.
  • view (gaphas.view.View): The view with guides to use for the coordinate system.


Paints on top of all items with guides, just for the focused item and only when it's hovered.

  • item (gaphas.item.Item): The focused item.
  • view (gaphas.view.View): The view with guides to paint with.


Class: gaphas.tree.Tree

A Tree structure with the nodes stored in a depth-first order.

tree = Tree()
tree.add("node2", parent="node1")

Class: gaphas.matrix.Matrix

Adds @observed messages on state changes to the cairo.Matrix.

  • xx (float): xx component of the affine transformation
  • yx (float): yx component of the affine transformation
  • xy (float): xy component of the affine transformation
  • yy (float): yy component of the affine transformation
  • x0 (float): X translation component of the affine transformation
  • y0 (float): Y translation component of the affine transformation
matrix = Matrix(1, 0, 0, 1, 0, 0)

Class: gaphas.table.Table

Table is a storage class that can be used to store information, like one would in a database table, with indexes on the desired "columns." It includes indexing and is optimized for lookups.

  • columns (tuple): The columns of the table.
  • index (tuple):
from collections import namedtuple
C = namedtuple('C', "foo bar baz")
s = Table(C, (2,))

Class: gaphas.quadtree.Quadtree

A quadtree is a tree data structure in which each internal node has up to four children. Quadtrees are most often used to partition a two

  • bounds (tuple): The boundaries of the quadtree (x, y, width, height).
  • capacity (int); The number of elements in one tree bucket, default is 10.
qtree = Quadtree((0, 0, 100, 100)) 

Class: gaphas.geometry.Rectangle

Rectangle object which can be added (union), substituted (intersection), and points and rectangles can be tested to be in the rectangle.

  • x (int): X position of the rectangle.
  • y (int): Y position of the rectangle.
  • width (int): Width of the rectangle.
  • height (int): Hiehgt of the rectangle.
rect = Rectangle(1, 1, 5, 5)

Decorator: @AsyncIO

Schedule an ide handler at a given priority.

  • single (bool): Schedules the decorated function to only be called a signle time.
  • timeout (int): The time between calls of the decorated function.
  • priority (int): The GLib.PRIORITY constant to set the event priority.
@AsyncIO(single=True, timeout=60)
def c2(self):
    print('idle-c2', GLib.main_depth())

Decorator: @nonrecursive

Enforces a function or method to not be executed recursively. api/decorators

class A(object):
    def a(self, x=1):

Decorator: @recursive

Limits the recursion for a specific function.

  • limit (int): The limit for the number of recursive loops a function can be called, default is 10000.
def a(self, x=0):

❤️ Contributing

Thanks goes to these wonderful people (emoji key):

Arjan Molenaar

💻 🐛 📖 👀 💬 🔌

Dan Yeaw

💻 ⚠️ 👀 🐛 💬 🚇 📖


💻 ⚠️ 👀

Jean-Luc Stevens

💻 🐛 📖

Franz Steinmetz

💻 🐛

Adrian Boguszewski


Rico Belder

🐛 👀

Adam Boduch


This project follows the all-contributors specification. Contributions of any kind are welcome!

  1. Check for open issues or open a fresh issue to start a discussion around a feature idea or a bug. There is a first-timers-only tag for issues that should be ideal for people who are not very familiar with the codebase yet.
  2. Fork the repository on GitHub to start making your changes to the master branch (or branch off of it).
  3. Write a test which shows that the bug was fixed or that the feature works as expected.
  4. Send a pull request and bug the maintainers until it gets merged and published. 😄

See the contributing file!

©️ License

Copyright (C) Arjan Molenaar and Dan Yeaw

Licensed under the Apache License 2.0.

Summary: You can do what you like with Gaphas, as long as you include the required notices. This permissive license contains a patent license from the contributors of the code.