Python GUI Programming Cookbook

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

Here are the four lines of Python code required to create the resulting GUI:

import tkinter as tk     # 1
win = tk.Tk()            # 2
win.title("Python GUI")  # 3
win.mainloop()           # 4

Execute this code and admire the result:

In line 1, we import the built-in tkinter module and alias it as tk to simplify our Python code. In line 2, we create an instance of the Tk class by calling its constructor (the parentheses appended to Tk turn 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.

class AClass(object):
    print('Hello from AClass')

classInstance = AClass()

In line 3, we use the instance variable of the class (win) to give our window a title via the title property. In line 4, 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. Therefore, it is necessary to have typed Recipe 1 yourself into a project of your own or downloaded the code from https://www.packtpub.com/support.

We are preventing the GUI from being resized.

import tkinter as tk        # 1 imports

win = tk.Tk()               # 2 Create instance
win.title("Python GUI")     # 3 Add a title       
    
win.resizable(0, 0)         # 4 Disable resizing the GUI

win.mainloop()              # 5 Start GUI

Running the code creates this GUI:

Line 4 prevents the Python GUI from being resized.

Running this code will result in a GUI similar to the one we created in Recipe 1. However, the user can no longer resize it. Also, notice 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.

Resizable() is a method of the Tk() class and, by passing in (0, 0), we prevent the GUI from being resized. If we pass in other values, we hard-code the x and y start up size of the GUI, but that won't make it nonresizable.

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

We are extending the first recipe. We will leave the GUI resizable, so don't use the code from the second recipe (or comment the win.resizable line 4 out).

In order to add a Label widget to our GUI, we are importing the ttk module from tkinter. Please note the two import statements.

# imports                  # 1
import tkinter as tk       # 2
from tkinter import ttk    # 3

Add the following code just above win.mainloop() located at the bottom of recipes 1 and 2.

# Adding a Label           # 4
ttk.Label(win, text="A Label").grid(column=0, row=0) # 5

Running the code adds a label to our GUI:

In line 3 of the above code, we are importing a separate module from tkinter. The ttk module has some advanced widgets that make our GUI look great. In a sense, ttk is an extension within tkinter.

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

Line 5 above adds the label to the GUI, just before we call mainloop (not shown here to preserve space. See recipes 1 or 2).

We are passing our window instance into the ttk.Label constructor and setting the text property. This becomes the text our Label will display.

We are also making 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 previous recipes.

The reason why it became so small is that we added a widget to our form. Without a widget, tkinter 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. You can download the entire code from the Packt Publishing website.

We are adding 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 updating the text property of the button.

# Modify adding a Label                                      # 1
aLabel = ttk.Label(win, text="A Label")                      # 2
aLabel.grid(column=0, row=0)                                 # 3


# Button Click Event Callback Function                       # 4
def clickMe():                                               # 5
    action.configure(text="** I have been Clicked! **")
    aLabel.configure(foreground='red')

# Adding a Button                                            # 6
action = ttk.Button(win, text="Click Me!", command=clickMe)  # 7
action.grid(column=1, row=0)                                 # 8

Before clicking the button:

After clicking the button, the color of the label has been changed, and so has the text of the button. Action!

In line 2 we are now assigning the label to a variable and in line 3 we use this variable to position the label within the form. We will need this variable to change its properties in the clickMe() 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 5 is the event handler that is being invoked once the button gets clicked.

In line 7, we create the button and bind the command to the clickMe() 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 did indeed change.

Lines 3 and 8 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 other recipes in the book.

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

# Modified Button Click Function   # 1
def clickMe():                     # 2
    action.configure(text='Hello ' + name.get())
    
# Position Button in second row, second column (zero-based)
action.grid(column=1, row=1)

# Changing our Label               # 3
ttk.Label(win, text="Enter a name:").grid(column=0, row=0) # 4

# Adding a Textbox Entry widget    # 5
name = tk.StringVar()              # 6
nameEntered = ttk.Entry(win, width=12, textvariable=name) # 7
nameEntered.grid(column=0, row=1)  # 8

Now our GUI looks like this:

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

In line 2 we are getting the value of the Entry widget. We are not using 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 4 gives our label a more meaningful name, because now it describes the textbox below it. We moved the button down next to the label to visually associate the two. We are still using the grid layout manager, to be explained in more detail in Chapter 2, Layout Management.

Line 6 creates a variable name. This variable is bound to the Entry and, in our clickMe() function, we are able to retrieve the value of the Entry box 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 had hard-coded it to a width of 12 in line 7.

This recipe extends the previous recipe.

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 we named nameEntered. Now we can give it the focus.

Place the following code just above the bottom of the module that starts the main windows event loop, just like in previous recipes. 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.

nameEntered.focus()            # Place cursor into name Entry

On a Mac, you might have to set the focus to the GUI window first before being able to set the focus to the Entry widget in this window.

Adding this one line of Python code places the cursor into our text Entry box, giving the text Entry box 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 set a property on the widget. We can make the button disabled by adding this one line of Python code:

action.configure(state='disabled')    # Disable the Button Widget

After adding the above 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 recipes.

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

ttk.Label(win, text="Choose a number:").grid(column=1, row=0)  # 1
number = tk.StringVar()                         # 2
numberChosen = ttk.Combobox(win, width=12, textvariable=number) #3
numberChosen['values'] = (1, 2, 4, 42, 100)     # 4
numberChosen.grid(column=1, row=1)              # 5
numberChosen.current(0)                         # 6

This code, when added to previous recipes, creates the following GUI. Note how, in line 4 in the preceding code, we assign 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 1 adds a second label to match the newly created combo box (created in line 3). Line 2 assigns the value of the box to a variable of a special tkinter type (StringVar), as we did in a previous recipe.

Line 5 aligns the two new controls (label and combo box) within our previous GUI layout, and line 6 assigns a default value to be displayed when the GUI first becomes visible. This is the first value of the numberChosen['values'] tuple, the string "1". We did not place quotes around our tuple of integers in line 4, but they got casted into strings because, in line 2, we declared the values to be of type tk.StringVar.

The screenshot shows the selection made by the user (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 3 in the previous code to:

numberChosen = ttk.Combobox(win, width=12, textvariable=number, state='readonly')

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:

# Modified Button Click Callback Function
def clickMe():
    action.configure(text='Hello ' + name.get()+ ' ' + numberChosen.get())

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 recipes.

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

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

The third Checkbutton is both enabled and checked by default. The users can uncheck and recheck the widget as often as they like.

# Creating three checkbuttons    # 1
chVarDis = tk.IntVar()           # 2
check1 = tk.Checkbutton(win, text="Disabled", variable=chVarDis, state='disabled')                     # 3
check1.select()                  # 4
check1.grid(column=0, row=4, sticky=tk.W) # 5

chVarUn = tk.IntVar()            # 6
check2 = tk.Checkbutton(win, text="UnChecked", variable=chVarUn)
check2.deselect()                # 8
check2.grid(column=1, row=4, sticky=tk.W) # 9                  

chVarEn = tk.IntVar()            # 10
check3 = tk.Checkbutton(win, text="Enabled", variable=chVarEn)
check3.select()                  # 12
check3.grid(column=2, row=4, sticky=tk.W) # 13

Running the new code results in the following GUI:

In lines 2, 6, and 10, we create three variables of type IntVar. In the following line, for 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 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 4 and 12 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 recipes.

We are adding the following code to the previous recipe:

# Radiobutton Globals   # 1
COLOR1 = "Blue"         # 2
COLOR2 = "Gold"         # 3
COLOR3 = "Red"          # 4

# Radiobutton Callback  # 5
def radCall():          # 6
   radSel=radVar.get()
   if   radSel == 1: win.configure(background=COLOR1)
   elif radSel == 2: win.configure(background=COLOR2)
   elif radSel == 3: win.configure(background=COLOR3)

# create three Radiobuttons   # 7
radVar = tk.IntVar()          # 8
rad1 = tk.Radiobutton(win, text=COLOR1, variable=radVar, value=1,               command=radCall)              # 9
rad1.grid(column=0, row=5, sticky=tk.W)  # 10

rad2 = tk.Radiobutton(win, text=COLOR2, variable=radVar, value=2, command=radCall)                             # 11
rad2.grid(column=1, row=5, sticky=tk.W)  # 12

rad3 = tk.Radiobutton(win, text=COLOR3, variable=radVar, value=3, command=radCall)                             # 13
rad3.grid(column=2, row=5, sticky=tk.W)  # 14

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

In lines 2-4 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 hard-coded 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 that we will use in later recipes of the book.

Line 6 is the callback function that changes the background of our main form (win) depending upon the user's selection.

In line 8 we are creating a tk.IntVar variable. What is important about this is that we are creating only one variable to be used by all three radio buttons. As can be seen from the above screenshot, no matter which Radiobutton we select, all the others will automatically be unselected for us.

Lines 9 to 14 create the three radio buttons, assign them to the main form, and pass 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:

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 recipes. You can download the code for each chapter of this book from the Packt Publishing website.

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

# Add this import to the top of the Python Module    # 1
from tkinter import scrolledtext      # 2

# Using a scrolled Text control       # 3
scrolW  = 30                          # 4
scrolH  =  3                          # 5
scr = scrolledtext.ScrolledText(win, width=scrolW, height=scrolH, wrap=tk.WORD)                         # 6
scr.grid(column=0, columnspan=3)      # 7

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 of 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 2 we are importing the module that contains the ScrolledText widget class. Add that to the top of the module, just below the other two import statements.

Lines 4 and 5 define the width and height of the ScrolledText widget we are about to create. These are hard-coded values we are passing into the ScrolledText widget constructor in line 6.

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

In line 6 we are 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 three columns. If we did 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, so you need that code to apply to this recipe to.

# First, we change our Radiobutton global variables into a list.
colors = ["Blue", "Gold", "Red"]              # 1

# create three Radiobuttons using one variable
radVar = tk.IntVar()

Next we are selecting a non-existing index value for radVar.
radVar.set(99)                                # 2

Now we are creating all three Radiobutton widgets within one loop.

for col in range(3):                          # 3
    curRad = 'rad' + str(col)  
    curRad = tk.Radiobutton(win, text=colors[col], variable=radVar,     value=col, command=radCall)
    curRad.grid(column=col, row=5, sticky=tk.W)


We have also changed the callback function to be zero-based, using the list instead of module-level global variables. 

# Radiobutton callback function                # 4
def radCall():
   radSel=radVar.get()
   if   radSel == 0: win.configure(background=colors[0])
   elif radSel == 1: win.configure(background=colors[1])
   elif radSel == 2: win.configure(background=colors[2])

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 following recipes.

In line 1, we have turned our global variables into a list.

In line 2, we are setting a default value to the tk.IntVar variable 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 3 we are replacing the three previously hard-coded creations of the Radiobutton widgets with a loop, which 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 3 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 4 shows the modified callback, which physically lives above the previous lines. We placed it below to give emphasis to the more important parts of this recipe.

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.