Python GUI Programming Cookbook - Second Edition

To follow this recipe, a working Python development environment is a prerequisite. The IDLE GUI, which ships with Python, is enough to start. IDLE was built using tkinter!

Here are the four lines of First_GUI.py required to create the resulting GUI:

Execute this code and admire the result:

In line nine, we import the built-in tkinter module and alias it as tk to simplify our Python code. In line 12, we create an instance of the Tk class by calling its constructor (the parentheses appended to Tk turns the class into an instance). We are using the alias tk, so we don't have to use the longer word tkinter. We are assigning the class instance to a variable named win (short for a window). As Python is a dynamically typed language, we did not have to declare this variable before assigning to it, and we did not have to give it a specific type. Python infers the type from the assignment of this statement. Python is a strongly typed language, so every variable always has a type. We just don't have to specify its type beforehand like in other languages. This makes Python a very powerful and productive language to program in.

In line 15, we use the instance variable (win) of the class to give our window a title via the title property. In line 20, we start the window's event loop by calling the mainloop method on the class instance, win. Up to this point in our code, we created an instance and set one property, but the GUI will not be displayed until we start the main event loop.

This recipe used a minimum amount of Python code to create our first GUI program. However, throughout this book we will use OOP when it makes sense.

This recipe extends the previous one, Creating our first Python GUI, so one requirement is to have typed the first recipe yourself into a project of your own, or download the code from https://github.com/PacktPublishing/Python-GUI-Programming-Cookbook-Second-Edition/.

We are preventing the GUI from being resized, look at:GUI_not_resizable.py

Running the code creates this GUI:

Line 18 prevents the Python GUI from being resized.

Running this code will result in a GUI similar to the one we created in the first recipe. However, the user can no longer resize it. Also, note how the maximize button in the toolbar of the window is grayed out.

Why is this important? Because once we add widgets to our form, resizing can make our GUI look not as good as we want it to be. We will add widgets to our GUI in the next recipes.

The resizable() method is of the Tk() class, and by passing in (False, False), we prevent the GUI from being resized. We can disable both the x and y dimensions of the GUI from being resized, or we can enable one or both dimensions by passing in True or any number other than zero. (True, False) would enable the x-dimension but prevent the y-dimension from being resized.

We also added comments to our code in preparation for the recipes contained in this book.

We are extending the first recipe, Creating our first Python GUI. We will leave the GUI resizable, so don't use the code from the second recipe (or comment the win.resizable line out).

In order to add a Label widget to our GUI, we will import the ttk module from tkinter. Please note the two import statements. Add the following code just above win.mainloop(), which is located at the bottom of the first and second recipes:

GUI_add_label.py

Running the code adds a label to our GUI:

In line 10 of the preceding code, we import a separate module from the tkinter package. The ttk module has some advanced widgets that make our GUI look great. In a sense, ttk is an extension within the tkinter package.

We still need to import the tkinter package itself, but we have to specify that we now want to also use ttk from the tkinter package.

Line 19 adds the label to the GUI, just before we call mainloop .

We pass our window instance into the ttk.Label constructor and set the text property. This becomes the text our Label will display.

We also make use of the grid layout manager, which we'll explore in much more depth in Chapter 2, Layout Management.

Note how our GUI suddenly got much smaller than in the previous recipes.

The reason why it became so small is that we added a widget to our form. Without a widget, the tkinter package uses a default size. Adding a widget causes optimization, which generally means using as little space as necessary to display the widget(s).

If we make the text of the label longer, the GUI will expand automatically. We will cover this automatic form size adjustment in a later recipe in Chapter 2, Layout Management.

Try resizing and maximizing this GUI with a label and watch what happens.

This recipe extends the previous one, Adding a label to the GUI form. You can download the entire code from https://github.com/PacktPublishing/Python-GUI-Programming-Cookbook-Second-Edition/.

We add a button that, when clicked, performs an action. In this recipe, we will update the label we added in the previous recipe as well as the text property of the button:

GUI_create_button_change_property.py

The following screenshot shows how our GUI looks before clicking the button:

After clicking the button, the color of the label changed and so did the text of the button, which can be seen as follows:

In line 19, we assign the label to a variable, and in line 20, we use this variable to position the label within the form. We need this variable in order to change its properties in the click_me() function. By default, this is a module-level variable, so we can access it inside the function, as long as we declare the variable above the function that calls it.

Line 23 is the event handler that is invoked once the button gets clicked.

In line 29, we create the button and bind the command to the click_me() function.

We also change the text of the label to include red as, in the printed book, this might otherwise not be obvious. When you run the code, you can see that the color does indeed change.

Lines 20 and 30 both use the grid layout manager, which will be discussed in the following chapter. This aligns both the label and the button.

We will continue to add more and more widgets to our GUI and we will make use of many built-in properties in the other recipes in the book.

This recipe builds upon the Creating buttons and changing their text property recipe.

Check out the following code:

GUI_textbox_widget.py

Now, our GUI looks like this:

After entering some text and clicking the button, there is the following change in the GUI:

In line 24, we get the value of the Entry widget. We have not used OOP yet, so how come we can access the value of a variable that was not even declared yet?

Without using OOP classes, in Python procedural coding, we have to physically place a name above a statement that tries to use that name. So how come this works (it does)?

The answer is that the button click event is a callback function, and by the time the button is clicked by a user, the variables referenced in this function are known and do exist.

Life is good.

Line 27 gives our label a more meaningful name; for now, it describes the text box below it. We moved the button down next to the label to visually associate the two. We are still using the grid layout manager, which will be explained in more detail in Chapter 2, Layout Management.

Line 30 creates a variable, name. This variable is bound to the Entry widget and, in our click_me() function, we are able to retrieve the value of the Entry widget by calling get() on this variable. This works like a charm.

Now we see that while the button displays the entire text we entered (and more), the textbox Entry widget did not expand. The reason for this is that we hardcoded it to a width of 12 in line 31.

This recipe extends the previous recipe, Text box widgets.

Python is truly great. All we have to do to set the focus to a specific control when the GUI appears is call the focus() method on an instance of a tkinter widget we previously created. In our current GUI example, we assigned the ttk.Entry class instance to a variable named, name_entered. Now, we can give it the focus.

Place the following code just above the code which is located at the bottom of the module and which starts the main windows event loop, like we did in the previous recipes:

GUI_set_focus.py

If you get some errors, make sure you are placing calls to variables below the code where they are declared. We are not using OOP as of yet, so this is still necessary. Later, it will no longer be necessary to do this.

Adding this one line (38) of Python code places the cursor in our text Entry widget, giving the text Entry widget the focus. As soon as the GUI appears, we can type into this text box without having to click it first.

We can also disable widgets. To do that, we will set a property on the widget. We can make the button disabled by adding this one line (37 below) of Python code to create the button:

After adding the preceding line of Python code, clicking the button no longer creates any action:

This code is self-explanatory. We set the focus to one control and disable another widget. Good naming in programming languages helps to eliminate lengthy explanations. Later in this book, there will be some advanced tips on how to do this while programming at work or practicing our programming skills at home.

Yes. This is only the first chapter. There is much more to come.

This recipe extends the previous recipe, Setting the focus to a widget and disabling widgets.

We insert another column between the Entry widget and the Button widget using the grid layout manager. Here is the Python code:

GUI_combobox_widget.py

This code, when added to the previous recipes, creates the following GUI. Note how, in line 43 in the preceding code, we assigned a tuple with default values to the combo box. These values then appear in the drop-down box. We can also change them if we like (by typing in different values when the application is running):

Line 40 adds a second label to match the newly created combo box (created in line 42). Line 41 assigns the value of the box to a variable of a special tkinter type StringVar, as we did in a previous recipe.

Line 44 aligns the two new controls (label and combobox) within our previous GUI layout, and line 45 assigns a default value to be displayed when the GUI first becomes visible. This is the first value of the number_chosen['values'] tuple, the string "1". We did not place quotes around our tuple of integers in line 43, but they got casted into strings because, in line 41, we declared the values to be of the tk.StringVar type.

The preceding screenshot shows the selection made by the user as 42. This value gets assigned to the number variable.

If we want to restrict the user to only be able to select the values we have programmed into the Combobox, we can do that by passing the state property into the constructor. Modify line 42 as follows:

GUI_combobox_widget_readonly_plus_display_number.py

Now, users can no longer type values into the Combobox. We can display the value chosen by the user by adding the following line of code to our Button Click Event Callback function:

After choosing a number, entering a name, and then clicking the button, we get the following GUI result, which now also displays the number selected:

This recipe extends the previous recipe, Combo box widgets.

We are creating three check button widgets that differ in their states. The first is disabled and has a check mark in it. The user cannot remove this check mark as the widget is disabled.

The second check button is enabled, and by default, has no check mark in it, but the user can click it to add a check mark.

The third check button is both enabled and checked by default. The users can uncheck and recheck the widget as often as they like. Look at the following code:

GUI_checkbutton_widget.py

Running the new code results in the following GUI:

In lines 47, 52, and 57 we create three variables of the IntVar type. In the line following each of these variables, we create a Checkbutton, passing in these variables. They will hold the state of the Checkbutton (unchecked or checked). By default, that is either 0 (unchecked) or 1 (checked), so the type of the variable is a tkinter integer.

We place these Checkbutton widgets in our main window, so the first argument passed into the constructor is the parent of the widget, in our case, win. We give each Checkbutton widget a different label via its text property.

Setting the sticky property of the grid to tk.W means that the widget will be aligned to the west of the grid. This is very similar to Java syntax and it means that it will be aligned to the left. When we resize our GUI, the widget will remain on the left side and not be moved towards the center of the GUI.

Lines 49 and 59 place a checkmark into the Checkbutton widget by calling the select() method on these two Checkbutton class instances.

We continue to arrange our widgets using the grid layout manager, which will be explained in more detail in Chapter 2, Layout Management.

This recipe extends the previous recipe, Creating a check button with different initial states.

We add the following code to the previous recipe:

GUI_radiobutton_widget.py

Running this code and selecting the Radiobutton named Gold creates the following window:

In lines 75-77, we create some module-level global variables which we will use in the creation of each radio button as well as in the callback function that creates the action of changing the background color of the main form (using the instance variable win).

We are using global variables to make it easier to change the code. By assigning the name of the color to a variable and using this variable in several places, we can easily experiment with different colors. Instead of doing a global search-and-replace of a hardcoded string (which is prone to errors), we just need to change one line of code and everything else will work. This is known as the DRY principle, which stands for Don't Repeat Yourself. This is an OOP concept which we will use in the later recipes of the book.

Line 80 is the callback function that changes the background of our main form (win) depending upon the user's selection.

In line 87 we create a tk.IntVar variable. What is important about this is that we create only one variable to be used by all three radio buttons. As can be seen from the screenshot, no matter which Radiobutton we select, all the others will automatically be unselected for us.

Lines 89 to 96 create the three radio buttons, assigning them to the main form, passing in the variable to be used in the callback function that creates the action of changing the background of our main window.

Here is a small sample of the available symbolic color names that you can look up at the official tcl manual page at http://www.tcl.tk/man/tcl8.5/TkCmd/colors.htm.

Some of the names create the same color, so alice blue creates the same color as AliceBlue. In this recipe, we used the symbolic names Blue, Gold, and Red.

This recipe extends the previous recipe, Using radio button widgets. You can download the code for each chapter of this book from https://github.com/PacktPublishing/Python-GUI-Programming-Cookbook-Second-Edition/.

By adding the following lines of code, we create a ScrolledText widget:

GUI_scrolledtext_widget.py

We can actually type into our widget, and if we type enough words, the lines will automatically wrap around:

Once we type in more words than the height the widget can display, the vertical scrollbar becomes enabled. This all works out-of-the-box without us needing to write any more code to achieve this:

In line 11, we import the module that contains the ScrolledText widget class. Add this to the top of the module, just below the other two import statements.

Lines 100 and 101 define the width and height of the ScrolledText widget we are about to create. These are hardcoded values we are passing into the ScrolledText widget constructor in line 102.

These values are magic numbers found by experimentation to work well. You might experiment by changing scol_w from 30 to 50 and observe the effect!

In line 102, we are also  setting a property on the widget by passing in wrap=tk.WORD.

By setting the wrap property to tk.WORD we are telling the ScrolledText widget to break lines by words so that we do not wrap around within a word. The default option is tk.CHAR, which wraps any character regardless of whether we are in the middle of a word.

The second screenshot shows that the vertical scrollbar moved down because we are reading a longer text that does not entirely fit into the x, y dimensions of the SrolledText control we created.

Setting the columnspan property of the grid widget to 3 for the SrolledText widget makes this widget span all the three columns. If we do not set this property, our SrolledText widget would only reside in column one, which is not what we want.

We are refactoring some parts of the previous recipe's code, Using scrolled text widgets, so you need that code to apply this recipe to.

Here's how we refactor our code:

GUI_adding_widgets_in_loop.py

Running this code will create the same window as before, but our code is much cleaner and easier to maintain. This will help us when we expand our GUI in the coming recipes.

In line 77, we have turned our global variables into a list.

In line 89, we set a default value to the tk.IntVar variable that we named radVar. This is important because, while in the previous recipe we had set the value for Radiobutton widgets starting at 1, in our new loop it is much more convenient to use Python's zero-based indexing. If we did not set the default value to a value outside the range of our Radiobutton widgets, one of the radio buttons would be selected when the GUI appears. While this in itself might not be so bad, it would not trigger the callback and we would end up with a radio button selected that does not do its job (that is, change the color of the main win form).

In line 95, we replace the three previously hardcoded creations of the Radiobutton widgets with a loop that does the same. It is just more concise (fewer lines of code) and much more maintainable. For example, if we want to create 100 instead of just three Radiobutton widgets, all we have to change is the number inside Python's range operator. We would not have to type or copy and paste 97 sections of duplicate code, just one number.

Line 82 shows the modified callback function.

This recipe concludes the first chapter of this book. All the following recipes in all of the next chapters will build upon the GUI we have constructed so far, greatly enhancing it.