Building a basic calculator with Python and PyQt

Last update on April 25 2025 12:56:19 (UTC/GMT +8 hours)

Write a Python program that builds a basic calculator application with buttons for numbers and operators using PyQt. When buttons are clicked, they display the input and perform calculations.

From doc.qt.io:

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

QMainWindow Class: The QMainWindow class provides a main application window.

QPushButton: The push button, or command button, is perhaps the most commonly used widget in any graphical user interface. Push (click) a button to command the computer to perform some action, or to answer a question. Typical buttons are OK, Apply, Cancel, Close, Yes, No and Help.

QVBoxLayout Class: The QVBoxLayout class lines up widgets vertically.

QHBoxLayout Class: The QHBoxLayout class lines up widgets horizontally.

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

QLineEdit Class: The QLineEdit widget is a one-line text editor.

Qt module: PyQt5 is a set of Python bindings for the Qt application framework. It allows us to use Qt, a popular C++ framework, to create graphical user interfaces (GUIs) in Python.

Sample Solution:

Python Code:

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton, QVBoxLayout, QHBoxLayout, QWidget, QLineEdit
from PyQt5.QtCore import Qt

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

        # Set the window properties (title and initial size)
        self.setWindowTitle("Basic Calculator")
        self.setGeometry(100, 100, 400, 400)  # (x, y, width, height)

        # Create a central widget for the main window
        central_widget = QWidget()
        self.setCentralWidget(central_widget)

        # Create a QLineEdit widget for input and result display
        self.input_display = QLineEdit()
        self.input_display.setAlignment(Qt.AlignmentFlag.AlignRight)
        self.input_display.setReadOnly(True)

        # Create a layout for buttons
        button_layout = QVBoxLayout()

        # Create number buttons
        number_buttons_layout = QVBoxLayout()
        for i in range(1, 10):
            number_button = QPushButton(str(i))
            number_button.clicked.connect(lambda checked, ch=i: self.on_number_button_click(ch))
            number_buttons_layout.addWidget(number_button)

        # Create special buttons (0, ., =)
        zero_button = QPushButton("0")
        zero_button.clicked.connect(lambda: self.on_number_button_click(0))

        dot_button = QPushButton(".")
        dot_button.clicked.connect(self.on_dot_button_click)

        equals_button = QPushButton("=")
        equals_button.clicked.connect(self.calculate_result)

        # Create operator buttons (+, -, *, /)
        operator_buttons_layout = QVBoxLayout()
        operators = ["+", "-", "*", "/"]
        for operator in operators:
            operator_button = QPushButton(operator)
            operator_button.clicked.connect(lambda checked, ch=operator: self.on_operator_button_click(ch))
            operator_buttons_layout.addWidget(operator_button)

        # Create a layout for the number buttons and special buttons
        number_special_buttons_layout = QHBoxLayout()
        number_special_buttons_layout.addLayout(number_buttons_layout)
        number_special_buttons_layout.addWidget(zero_button)
        number_special_buttons_layout.addWidget(dot_button)
        number_special_buttons_layout.addWidget(equals_button)

        # Create a layout for all buttons
        button_layout.addLayout(number_special_buttons_layout)
        button_layout.addLayout(operator_buttons_layout)

        # Create a vertical layout for the entire calculator
        main_layout = QVBoxLayout()
        main_layout.addWidget(self.input_display)
        main_layout.addLayout(button_layout)

        # Set the layout for the central widget
        central_widget.setLayout(main_layout)

        # Initialize the input expression
        self.input_expression = ""

    def on_number_button_click(self, digit):
        # Append the clicked digit to the input expression
        self.input_expression += str(digit)
        self.update_input_display()

    def on_dot_button_click(self):
        # Append a decimal point to the input expression
        if "." not in self.input_expression:
            self.input_expression += "."
            self.update_input_display()

    def on_operator_button_click(self, operator):
        # Append the clicked operator to the input expression
        if self.input_expression and self.input_expression[-1] != operator:
            self.input_expression += operator
            self.update_input_display()

    def calculate_result(self):
        try:
            # Calculate the result of the input expression
            result = eval(self.input_expression)
            self.input_expression = str(result)
            self.update_input_display()
        except Exception as e:
            # Handle calculation errors (e.g., division by zero)
            self.input_expression = "Error"
            self.update_input_display()

    def update_input_display(self):
        # Display the current input expression in the input field
        self.input_display.setText(self.input_expression)

def main():
    # Create a PyQt application
    app = QApplication(sys.argv)

    # Create an instance of the CalculatorApp class
    window = CalculatorApp()

    # Show the window
    window.show()

    # Run the application's event loop
    sys.exit(app.exec_())

if __name__ == "__main__":
    main()

Explanation:

The above code creates a basic calculator with buttons for numbers (0-9) and operators (+, -, *, /). Users can input an expression by clicking on the buttons. Clicking the "=" button will calculate the result, which will be displayed in the input field.

Output:

Flowchart:

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