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16.2 Widget FundamentalsThe Tkinter module supplies many kinds of widgets, and most of them have several things in common. All widgets are instances of classes that inherit from class Widget. Class Widget itself is abstract; that is, you never instantiate Widget itself. You only instantiate concrete subclasses corresponding to specific kinds of widgets. Class Widget's functionality is common to all the widgets you instantiate. To instantiate any kind of widget, call the widget's class. The first argument is the parent window of the widget, also known as the widget's master. If you omit this positional argument, the widget's master is the application's main window. All other arguments are in named form, option=value. You can also set or change options on an existing widget w by calling w.config(option=value). You can get an option of w by calling w.cget('option'), which returns the option's value. Each widget w is a mapping, so you can also get an option as w['option'] and set or change it with w['option']=value. 16.2.1 Common Widget OptionsMany widgets accept some common options. Some options affect a widget's colors, others affect lengths (normally in pixels), and there are various other kinds. This section details the most commonly used options. 16.2.1.1 Color optionsTkinter represents colors with strings. The string can be a color name, such as 'red' or 'orange', or it may be of the form '#RRGGBB', where each of R, G, and B is a hexadecimal digit, to represent a color by the values of red, green, and blue components on a scale of 0 to 255. Don't worry; if your screen can't display millions of different colors, as implied by this scheme; Tkinter maps any requested color to the closest color that your screen can display. The common color options are:
16.2.1.2 Length optionsTkinter normally expresses a length as an integer number of pixels; other units of measure are possible, but rarely used. The common length options are:
16.2.1.3 Options expressing numbers of charactersSome options indicate a widget's requested geometry not in pixels, but rather as a number of characters, using average width or height of the widget's fonts:
16.2.1.4 Other common optionsOther options accepted by many kinds of widgets are a mixed bag, dealing with both behavior and presentation issues.
16.2.2 Common Widget MethodsA widget w supplies many methods. Besides event-related methods, mentioned in Section 16.9 later in this chapter, commonly used widget methods are the following.
Returns the value configured in w for option.
w .config( ), without arguments, returns a dictionary where each possible option of w is mapped to a tuple that describes it. Called with one or more named arguments, config sets those options in w's configuration.
Sets focus to w, so that all keyboard events for the application are sent to w.
grab_set ensures that all of the application's events are sent to w until a corresponding call to grab_release.
Enters a Tkinter event loop. Event loops may be nested; each call to mainloop enters one further-nested level of the event loop.
Quits a Tkinter event loop. When event loops are nested; each call to quit exits one nested level of the event loop.
Handles all pending events. Never call this while handling an event!
Handles those pending events that would normally be handled only when the event loop is idle (such as layout-manager updates and widget redrawing) but does not perform any callbacks. You can safely call this method at any time.
v must be a Tkinter variable object (covered in the next section). wait_variable returns only when the value of v changes. Meanwhile, other parts of the application remain active.
w1 must be a widget. wait_visibility returns only when w1 becomes visible. Meanwhile, other parts of the application remain active.
w1 must be a widget. wait_window returns only when w1 is destroyed. Meanwhile, other parts of the application remain active.
Returns w's height in pixels.
Returns w's width in pixels. w supplies many other methods whose names start with winfo_, but the two above are the most often called, typically after calling w.update_idletasks. They let you ascertain a widget's dimensions after the user has resized a window, causing the layout manager to rearrange the widgets' geometry. 16.2.3 Tkinter Variable ObjectsThe Tkinter module supplies classes whose instances represent variables. Each class deals with a specific data type: DoubleVar for float, IntVar for int, StringVar for str. You can instantiate any of these classes without arguments to obtain an instance x, also known in Tkinter as a variable object. Then, x.set(datum) sets x's value to the given value, and x.get( ) returns x's current value. You can pass x as the textvariable or variable configuration option for a widget. Once you do this, the widget's text changes to track any change to x's value, and x's value, in turn, tracks changes to the widget (for some kinds of widgets). Further, a single Tkinter variable can control more than one widget. Tkinter variables let you control widget contents more transparently, and sometimes more conveniently, than explicitly querying and setting widget properties. The following example shows how to use a StringVar to connect an Entry widget and a Label widget automatically: import Tkinter
root = Tkinter.Tk( )
tv = Tkinter.StringVar( )
Tkinter.Label(textvariable=tv).pack( )
Tkinter.Entry(textvariable=tv).pack( )
tv.set('Welcome!')
Tkinter.Button(text="Exit", command=root.quit).pack( )
Tkinter.mainloop( )
print tv.get( )
As you edit the Entry, you'll see the Label change automatically. This example instantiates the Tkinter main window explicitly, binds it to name root, and then sets as the Button's command the bound method root.quit, which quits Tkinter's main loop but does not terminate the Python application. Thus, the example ends with a print statement, to show on standard output the final value of variable object tv. 16.2.4 Tkinter ImagesThe Tkinter class PhotoImage supports Graphical Interchange Format (GIF) and Portable PixMap (PPM) images. You instantiate class PhotoImage with a keyword argument file=path to load the image's data from the image file at the given path and get an instance x. You can set x as the image configuration option for one or more widgets. When you do this, the widget displays the image rather than text. If you need image processing functionality and support for many image formats (including JPEG, PNG, and TIFF), use PIL, the Python Imaging Library (http://www.pythonware.com/products/pil/), designed to work with Tkinter. I do not cover PIL further in this book. Tkinter also supplies class BitmapImage, whose instances are usable wherever instances of PhotoImage are. BitmapImage supports some file formats known as bitmaps. I do not cover BitmapImage further in this book. Being set as the image configuration option of a widget does not suffice to keep instances of PhotoImage and BitmapImage alive. Be sure to hold such instances in a Python container object, typically a list or dictionary, to ensure that the instances are not garbage-collected. The following example shows how to display GIF images: import os
import Tkinter
root = Tkinter.Tk( )
L = Tkinter.Listbox(selectmode=Tkinter.SINGLE)
gifsdict = { }
dirpath = 'imgs'
for gifname in os.listdir(dirpath):
if not gifname[0].isdigit( ): continue
gifpath = os.path.join(dirpath, gifname)
gif = Tkinter.PhotoImage(file=gifpath)
gifsdict[gifname] = gif
L.insert(Tkinter.END, gifname)
L.pack( )
img = Tkinter.Label( )
img.pack( )
def list_entry_clicked(*ignore):
imgname = L.get(L.curselection( )[0])
img.config(image=gifsdict[imgname])
L.bind('<ButtonRelease-1>', list_entry_clicked)
root.mainloop( )
Assuming you have in some directory ('imgs' in the example) several GIF files whose filenames start with digits, the example loads the images into memory, shows the filenames in a Listbox instance, and shows in a Label instance the GIF whose filename you click on. Note that for simplicity, the example does not give the Listbox widget a Scrollbar (we'll see how to equip a Listbox with a Scrollbar shortly). |
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