Using PyQt to build a interest rate calculator GUI.

pyQtExample.py

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Form(QDialog):
	def __init__(self, parent = None):
		super(Form, self).__init__(parent)
		#Labels
		prin_label = QLabel("Principal:")
		rate_label = QLabel("Rate:")
		year_label = QLabel("Year(s):")
		amount_label = QLabel("Interest total:")
		self.result_label = QLabel()
		value_label = QLabel("Monthly contribution:")
		amount_label = QLabel("Annuity total:")
		self.result_Label = QLabel()
		total_label = QLabel("TOTAL:")
		self.total_result = QLabel()

		#Widgets
		self.valueSpinBox = QDoubleSpinBox()
		self.valueSpinBox.setRange(0.01, 1000000.00)
		self.valueSpinBox.setValue(100)
		self.valueSpinBox.setPrefix("$")

		self.PrincipalSpinBox = QDoubleSpinBox()
		self.PrincipalSpinBox.setRange(0.01, 1000000.00)
		self.PrincipalSpinBox.setValue(100)
		self.PrincipalSpinBox.setPrefix("$")

		self.RateSpinBox = QDoubleSpinBox()
		self.RateSpinBox.setRange(0.01, 100.00)
		self.RateSpinBox.setValue(1)
		self.RateSpinBox.setSuffix("%")

		self.YearComboBox = QComboBox()
		self.YearComboBox.addItem("1 year ")
		for n in range (2, 11):
		  year = "%d years" % n	
		  self.YearComboBox.addItem(year)

		#Layout
		grid = QGridLayout()
		grid.addWidget(prin_label, 0, 0)
		grid.addWidget(self.PrincipalSpinBox, 0, 1)
		grid.addWidget(valueLabel, 1, 0)
		grid.addWidget(self.valueSpinBox, 1, 1)
		grid.addWidget(rate_label, 2, 0)
		grid.addWidget(self.RateSpinBox, 2, 1)
		grid.addWidget(year_label, 3, 0)
		grid.addWidget(self.YearComboBox, 3, 1)
		grid.addWidget(amount_label, 4, 0)
		grid.addWidget(self.resultLabel, 4, 1)
		grid.addWidget(amountLabel, 5, 0)
		grid.addWidget(self.result_Label, 5, 1)
		grid.addWidget(totalLabel, 6, 0)
		grid.addWidget(self.totalresult, 6, 1)
		self.setLayout(grid)

		#Form behavior
		self.valueSpinBox.valueChanged.connect(self.update_ui)
		self.PrincipalSpinBox.valueChanged.connect(self.update_ui)
		self.RateSpinBox.valueChanged.connect(self.update_ui)
		self.YearComboBox.currentIndexChanged.connect(self.update_ui)

	@pyqtSlot()
	def update_ui(self):
		principal = self.PrincipalSpinBox.value()
		rate = self.RateSpinBox.value()
		years = float(self.YearComboBox.currentIndex()) + 1
		amount_int = principal *((1 + (rate/100.00))**years)
		self.resultLabel.setText("${0:0.2f}".format(amount_int))

		value = self.valueSpinBox.value()
		i = (self.RateSpinBox.value()/100.00) / 12
		n = years * 12
		amount = ((1 + i)**n-1)*(value/i)
		self.result_Label.setText("${0:0.2f}".format(amount))

		total = amount + amount_int
		self.totalresult.setText("${0:0.2f}".format(total))
    
app = QApplication(sys.argv)
form = Form()
form.show()
app.exec_()