Python PyQt simple calculator

Python PyQt Connecting Signals to Slots: Exercise-9 with Solution

Write a Python program that creates a simple calculator application and displays buttons for numbers and math operations. Use the PyQt module.

From doc.qt.io:

QApplication Class: The QApplication class manages the GUI application's control flow and main settings.

QWidget Class: The QWidget class is the base class of all user interface objects.

QPushButton Class: The QPushButton widget provides a command button.

QVBoxLayout Class: The QVBoxLayout class lines up widgets vertically.

QGridLayout Class: The QGridLayout class lays out widgets in a grid.

QLabel Class: The QLabel widget provides a text or image display.

Sample Solution:

Python Code:

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QVBoxLayout, QGridLayout, QLabel

class CalculatorApp(QWidget):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("Simple Calculator")
        self.setGeometry(100, 100, 300, 400)  # (x, y, width, height)

        layout = QVBoxLayout()

        # Create a label to display the calculator input and result
        self.display_label = QLabel("0", self)
        layout.addWidget(self.display_label)

        # Create a layout for the buttons
        button_layout = QGridLayout()

        # Create buttons for numbers 0-9
        for i in range(10):
            button = QPushButton(str(i), self)
            button.clicked.connect(lambda checked, num=i: self.on_digit_clicked(num))
            if i == 0:
                button_layout.addWidget(button, 3, 1)
            else:
                button_layout.addWidget(button, (9 - i) // 3, (i - 1) % 3)

        # Create buttons for operators (+, -, *, /)
        operators = ["+", "-", "*", "/"]
        for operator in operators:
            button = QPushButton(operator, self)
            button.clicked.connect(lambda checked, op=operator: self.on_operator_clicked(op))
            button_layout.addWidget(button, operators.index(operator), 3)

        # Create a button for equals (=)
        equals_button = QPushButton("=", self)
        equals_button.clicked.connect(self.calculate_result)
        button_layout.addWidget(equals_button, 3, 2)

        # Create a clear button (C)
        clear_button = QPushButton("C", self)
        clear_button.clicked.connect(self.clear_input)
        button_layout.addWidget(clear_button, 3, 0)

        layout.addLayout(button_layout)

        self.setLayout(layout)

        self.current_input = ""

    def on_digit_clicked(self, digit):
        self.current_input += str(digit)
        self.display_label.setText(self.current_input)

    def on_operator_clicked(self, operator):
        if self.current_input and self.current_input[-1] not in ["+", "-", "*", "/"]:
            self.current_input += operator
            self.display_label.setText(self.current_input)

    def calculate_result(self):
        try:
            result = eval(self.current_input)
            self.display_label.setText(str(result))
            self.current_input = str(result)
        except Exception:
            self.display_label.setText("Error")
            self.current_input = ""

    def clear_input(self):
        self.current_input = ""
        self.display_label.setText("0")

def main():
    app = QApplication(sys.argv)
    window = CalculatorApp()
    window.show()
    sys.exit(app.exec_())

if __name__ == "__main__":
    main()

Explanation:

In the exercise above -

• Import the necessary modules from PyQt5.
• Create a custom "QWidget" class named "CalculatorApp" that builds a simple calculator application.
• Design a layout with buttons for numbers (0-9), operators (+, -, *, /), an equals (=) button, and a clear (C) button.
• When we click the number buttons, operators, equals, or clear buttons, it updates the input and displays the result.
• We use a "QLabel" (self.display_label) to display input and results.
• The "on_digit_clicked()", "on_operator_clicked()", "calculate_result()", and "clear_input()" methods handle button clicks and perform calculations.
• In the main function, we create the PyQt application, instantiate the CalculatorApp class, and show the main window.

Output:

Flowchart:

Python Code Editor:

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