Extension-Specific User Interface

Chimera implements its graphical user interface (GUI) using a Python interface (Tkinter module) to the Tcl/Tk toolkit. Since Chimera extensions are also written in Python, they can extend the user interface using the same mechanism. Any extension that requires user input will need to present a GUI. This example assumes that the reader is familiar with Tkinter and does not describe the Tkinter-specific code in detail.

The code below demonstrates how to change the display mode of protein backbone to a user-selected representation. The graphical window renders atoms and bonds according to their drawMode attribute. Thus, all that the example code in the main package, __init__.py, does is to change the attribute values of backbone atoms and bonds. The example code in the graphical user interface submodule, gui.py, adds a button to the Chimera toolbar. When the user clicks the toolbar button, the window below is displayed.

The user can select the desired display representation for atoms and bonds using the option menus, and then set the backbone atom and bond representations by clicking the Apply button.

Example ExtensionUI/__init__.py

Import the standard modules used in this example.
import re


Import the Chimera modules used in this example.
import chimera


Define a regular expression for matching the names of protein backbone atoms (we do not include the carbonyl oxygens because they tend to clutter up the graphics display without adding much information).
MAINCHAIN = re.compile("^(N|CA|C)$", re.I)


Define mainchain function for setting the display representation of protein backbone atoms and bonds. See Molecular Editing for a more detailed example on changing molecular attributes.
def mainchain(atomMode, bondMode):


for m in chimera.openModels.list(modelTypes=[chimera.Molecule]):


for a in m.atoms:

if MAINCHAIN.match(a.name):

a.drawMode = atomMode

for b in m.bonds:

ends = b.atoms
if MAINCHAIN.match(ends[0].name) \
and MAINCHAIN.match(ends[1].name):


b.drawMode = bondMode


Example ExtensionUI/gui.py

Import the standard modules used by this example.
import os
import Tkinter


Import the Chimera modules and classes used by this example.
import chimera
from chimera.baseDialog import ModelessDialog


Import the package for which the graphical user interface is designed. In this case, the package is named ExtensionUI.
import ExtensionUI


Define two module variables: atomMode and bondMode are Tk variables that keep track of the last selected display representations. These variables are initialized to be None, and are set to usable values when the GUI is created.
atomMode = None
bondMode = None


Define two dictionaries that map string names into Chimera enumerated constant values. The two variables atomMode and bondMode keep track of the representations as strings because they are displayed directly in the user interface. However, the mainchain function in the main package expects Chimera constants as its arguments. The dictionaries atomModeMap and bondModeMap provides the translation from string to enumerated constants.
atomModeMap = {


'Dot': chimera.Atom.Dot,
'Sphere': chimera.Atom.Sphere,
'EndCap': chimera.Atom.EndCap,
'Ball': chimera.Atom.Ball


Chimera offers two base classes to somewhat simplify the task of creating user interfaces: ModalDialog and ModelessDialog. The former is designed for situations when information or response is required of the user immediately; the dialog stays in front of other Chimera windows until dismissed and prevents input from going to other Chimera windows. The latter dialog type is designed for "ongoing" interfaces; it allows input focus to go to other windows, and other windows can obscure it.

Here we declare a class that derives from ModelessDialog and customize it for the specific needs of this extension.
class MainchainDialog(ModelessDialog):


Chimera dialogs can either be named or nameless. Named dialogs are displayed using the display(name) function of the chimera.dialogs module. The name that should be used as an argument to the display function is given by the class variable name. Using a named dialog is appropriate when it might be desirable to invoke the dialog from other extensions or from Chimera itself.

A nameless dialog is intended for use only in the extension that defines the dialog. A nameless dialog is typically created and displayed by calling its constructor. Once created, a nameless dialog can be redisplayed (if it was withdrawn by clicking its Cancel button for example) by calling its enter() method.

For demonstration purposes, we use a named dialog here. A nameless dialog is used in the Color and Color Wells example.
name = "extension ui"


The buttons displayed at the bottom of the dialog are given in the class variable buttons. For modeless dialogs, a help button will automatically be added to the button list (the help button will be grayed out if no help information is provided). For buttons other than Help, clicking on them will invoke a class method of the same name.

Both dialog base classes provide appropriate methods for Close, so we won't provide a Close method in this subclass. The ModelessDialog base class also provides a stub method for Apply, but we will override it with our own Apply method later in the class definition.
buttons = ("Apply", "Close")


A help file or URL can be specified with the help class variable. A URL would be specified as a string (typically starting with "http://..."). A file would be specified as a 2-tuple of file name followed by a package. The file would then be looked for in the helpdir subdirectory of the package. A dialog of Chimera itself (rather than of an imported package) might only give a filename as the class help variable. That file would be looked for in /usr/local/chimera/share/chimera/helpdir.
help = ("ExtensionUI.html", ExtensionUI)


The title displayed in the dialog window's title bar is set via the class variable title.
title = "Set Backbone Representation"


Both ModelessDialog and ModalDialog, in their __init__ functions, set up the standard parts of the dialog interface (top-level window, bottom-row buttons, etc.) and then call a function named fillInUI so that the subclass can fill in the parts of the interface specific to the dialog. As an argument to the function, a Tkinter Frame is provided that should be the parent to the subclass-provided interface elements.
def fillInUI(self, parent):


Declare that, in fillInUI, the names atomMode and bondMode refer to the global variables defined above.
global atomMode, bondMode


Create and initialize atomMode and bondMode, the two global Tk string variables that keep track of the currently selected display representation.
atomMode = Tkinter.StringVar(parent)
atomMode.set('Dot')
bondMode = Tkinter.StringVar(parent)
bondMode.set('Wire')


Create the label and option menu for selecting atom display representation. First create the label Atom Representation and place it on the left-hand side of the top row in the GUI window.
atomLabel = Tkinter.Label(parent, text='Atom Representation')
atomLabel.grid(column=0, row=0)


Create the menu button and the option menu that it brings up.
atomButton = Tkinter.Menubutton(parent, indicatoron=1,


textvariable=atomMode, width=6,
relief=Tkinter.RAISED, borderwidth=2)


atomButton.grid(column=1, row=0)
atomMenu = Tkinter.Menu(atomButton, tearoff=0)


Add radio buttons for all possible choices to the menu. The list of choices is obtained from the keys of the string-to-enumeration dictionary, and the radio button for each choice is programmed to update the atomMode variable when selected.
for mode in atomModeMap.keys():


atomMenu.add_radiobutton(label=mode, variable=atomMode, value=mode)


Assigns the option menu to the menu button.
atomButton['menu'] = atomMenu


The lines below do the same thing for bond representation as the lines above do for atom representation.
bondLabel = Tkinter.Label(parent, text='Bond Representation')
bondLabel.grid(column=0, row=1)
bondButton = Tkinter.Menubutton(parent, indicatoron=1,


textvariable=bondMode, width=6,
relief=Tkinter.RAISED, borderwidth=2)


bondButton.grid(column=1, row=1)
bondMenu = Tkinter.Menu(bondButton, tearoff=0)
for mode in bondModeMap.keys():

bondMenu.add_radiobutton(label=mode, variable=bondMode, value=mode)

bondButton['menu'] = bondMenu

Define the method that is invoked when the Apply button is clicked. The function simply converts the currently selected representations from strings to enumerated constants, using the atomModeMap and bondModeMap dictionaries, and invokes the main package function mainchain.
def Apply(self):


ExtensionUI.mainchain(atomModeMap[atomMode.get()],

bondModeMap[bondMode.get()])

Now we register the above dialog with Chimera, so that it may be invoked via the display(name) method of the chimera.dialogs module. Here the class itself is registered, but since it is a named dialog deriving from ModalDialog/ModelessDialog, the instance will automatically reregister itself when first created. This allows the dialogs.find() function to return the instance instead of the class.
chimera.dialogs.register(MainchainDialog.name, MainchainDialog)


Create the Chimera toolbar button that displays the dialog when pressed. Note that since the package is not normally searched for icons, we have to prepend the path of this package to the icon's file name.
dir, file = os.path.split(__file__)
icon = os.path.join(dir, 'ExtensionUI.tiff')
chimera.tkgui.app.toolbar.add(icon, lambda d=chimera.dialogs.display, n=MainchainDialog.name: d(n), 'Set Main Chain Representation', None)


Code Notes

The example above requires the user to first select the desired representation, then apply the selection to the protein backbone. An alternative interface style is to apply user selections immediately. The appropriate choice of style depends on the extension application. The reason for choosing the "Apply" style for this example is that the user is expected to change both atom and bond representations, and there is no need to edit and display intermediate representations.

The ratio of 13 lines of functionality code to 34 lines of user interface code is fairly typical. Doing things is easy; figuring out what a user wants to do, that's hard.

For extensions based on the ModalDialog class, a different approach is typically used. The dialog is not registered (no call to chimera.dialogs.register). The function associated with the toolbar icon (the second argument to chimera.tkgui.app.toolbar.add) creates the modal dialog, calls the dialog's run() method, and uses that method's return value as appropriate (None is returned by a user-initiated Cancel of the dialog). When writing dialog methods, the return value is kept in the self.returnValue attribute of the dialog. The dialog is destroyed when the toolbar function runs out of scope.

Running the Example

The example code files and toolbar icon must be saved in a directory named ExtensionUI. To run the example, start chimera in the directory above the package directory and type the following command into the IDLE command line:

import ExtensionUI.gui

This should display a button on the Chimera toolbar. Clicking the button should bring up a window similar to the one shown above.