Calculator Lab Solution

The objective of this lab is to build a working calculator. In doing so, I hope to introduce some of the basics about tkinter, Python's de-facto standard GUI

(Graphical User Interface) package. I suggest that you read chapter 13 in the text to get a fuller picture of what can be done with tkinter.

















    A. Open gui1.py in Wing101. This small program illustrates how to create and display a Window and assign a title that is placed in the window’s title-bar (lines 6-9). In addition, an Entry widget is placed inside the window and a variable, self.display, is defined to hold the contents (lines 11-14). Widgets are things that can be placed inside a window or into containers in general. An Entry widget permits a user to enter data in it for processing. Finally, the main() function, uses a constructor to create a new GUI1 object (lines 20-21).

As illustrated below, the number 123 is entered into the Entry widget called entrybox and stored in the variable self.display after gui1.py is executed.






Create a new program, in Wing101, called simplecalculator.py. Copy the code from gui1.py and make the following adjustments, make the title-bar contain the string “Simple Calculator”, name the class Calculator, change the main() to create a new Calculator object.






    B. Open gui2.py in Wing101. This small program illustrates how to group together widgets in a container called a Frame widget. By grouping widgets together in a Frame, we simplify the task of programming a GUI by limiting the number of widgets we deal with at any one time. Most often, widgets are grouped by their related functionality to one another. But, in our case, we

group buttons only for aesthetics (for the calculator, if we don’t use Frames

all buttons would appear in the window in a single row from left to right). Our Frame, called frame, is defined in lines 11-13. Note, frame is placed inside self.window. Next, create two Button widgets that are each placed inside frame in lines 16-18 and in lines 20-23. The first button is labeled “1st” and the other is labeled “2nd”. Button widgets are used to carry out an action by calling a specific method. In addition, the method func1(), associated with the first button, is called when this Button is clicked see lines 28-29. Similarly, the method func2(), associated with the second button, is defined in lines 32-33. When gui2.py is executed, the following is displayed







Whenever either button is clicked, a appropriate message is printed in the
Debug/IO window as illustrated below:













    C. Now modify simplecalcuator.py by using what you have learned in part B above. Define six Frames named frameA, frameB, frameC, frameD, frameE, and frameF, each containing the Buttons shown to their left. For now, assign method func1() to each of these buttons. Note: there are 18 buttons in all.









    • frameA (3 buttons)

    • frameB (3 buttons)

    • frameC (3 buttons)

    • frameD (3 buttons)

    • frameE (5 buttons)

    • frameF (1 button)

    D. Since there are 18 different buttons, 18 different methods are required to make this GUI function as a working calculator. Remember, self.display is the variable that holds the contents of the calculator’s display. There are two methods that help manipulate self.display. The first is get(), which is used to access the contents of the calculator’s display (a string) and the other is set() which is used to change the calculator’s display. Here is the method associated with self.button1. Replace func1() with this one.

def func1(self):

self.display.set(self.display.get() + '1')

Each time this button is clicked another 1 is entered into self.display.
















Fifteen of the remaining methods are programmed in a similar way. Your

task is to create these functions and assign them to each of their respective Buttons. The (=) and negation (-) buttons are different.

    E. Suppose the calculator looks like the image above. When the negation (-) button is pressed the calculator now looks like










negation à







Note: a minus sign is appended in front of the contents of the variable self.display. Now write the method associated with the negation (-) button.


    F. The last method to implement is associated with the (=) button. Hint: use the eval() function. Here are a couple of examples illustrating how it is used. Note: eval() takes a string representing an arithmetic expression and evaluates its value.








Remember, self.display.get() returns a string representing the expression appearing in the calculator’s display. By applying eval() to this expression computes the value of this expression. Next, place that value back into self.display.

Finally, use a try-except clause (use Exception as the error to catch any that occur) to display an error in self.display() when an illegal operation is evaluated as illustrated below.

















After clicking (=) button we get


Disclaimer:

There are more economical ways to construct a calculator, with far fewer lines of

code than was presented here. The beauty of this lab is, a student without much knowledge about tkinter and a few simple examples about Windows, Frames, Buttons an Entrys, can get their calculator running close to perfect during the time
allotted for this lab.