py_calculator.py

import sys

from PyQt5.QtWidgets import QApplication
from PyQt5.QtWidgets import QMainWindow
from PyQt5.QtWidgets import QWidget
from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QGridLayout
from PyQt5.QtWidgets import QLineEdit
from PyQt5.QtWidgets import QPushButton
from PyQt5.QtWidgets import QVBoxLayout
from functools import partial

__version__ = '0.1'
__author__ = 'abhishek kumar'

ERROR_MSG = 'ERROR'

class PyCalcUI(QMainWindow):
    def __init__(self):
        super().__init__()
        
        self.setWindowTitle('Python Calculator')
        self.setFixedSize(350,450)

        self._centralWidget = QWidget(self)
        self.setCentralWidget(self._centralWidget)

        self.generalLayout = QVBoxLayout()
        self._centralWidget.setLayout(self.generalLayout)

        self._createDisplay()
        self._createButtons()

    def _createDisplay(self):

        self.display = QLineEdit()

        self.display.setFixedHeight(80)
        self.display.setAlignment(Qt.AlignRight)
        self.display.setReadOnly(True)

        self.generalLayout.addWidget(self.display)

    def _createButtons(self):
        self.buttons = {}
        buttonsLayout = QGridLayout()
        buttons = {
                    '7':(0,0),
                    '8':(0,1),
                    '9':(0,2),
                    '/':(0,3),
                    'C':(0,4),
                    '4':(1,0),
                    '5':(1,1),
                    '6':(1,2),
                    '*':(1,3),
                    '(':(1,4),
                    '1':(2,0),
                    '2':(2,1),
                    '3':(2,2),
                    '-':(2,3),
                    ')':(2,4),
                    '0':(3,0),
                    '00':(3,1),
                    '.':(3,2),
                    '+':(3,3),
                    '=':(3,4),
                }
        
        for btnText,pos in buttons.items():
            self.buttons[btnText] = QPushButton(btnText)
            self.buttons[btnText].setFixedSize(60,60)
            buttonsLayout.addWidget(self.buttons[btnText],pos[0],pos[1])
        self.generalLayout.addLayout(buttonsLayout)

    def setDisplayText(self,text):

        self.display.setText(text)
        self.display.setFocus()
    
    def displayText(self):
        return self.display.text()

    def clearDisplay(self):
        self.setDisplayText('')
        
class PyCalcCtrl:
    def __init__(self,model,view):

        self._view = view
        self._evaluate = model

        self._connectSignals()

    def _calculateResult(self):

        result = self._evaluate(expression = self._view.displayText())
        self._view.setDisplayText(result)

    def _buildExpression(self,sub_exp):

        if self._view.displayText() == ERROR_MSG:
            self._view.clearDisplay()

        expression = self._view.displayText()+sub_exp
        self._view.setDisplayText(expression)

    def _connectSignals(self):

        for btnText,btn in self._view.buttons.items():
            if btnText not in {'=','C'}:
                btn.clicked.connect(partial(self._buildExpression,btnText))
        
        self._view.buttons['='].clicked.connect(self._calculateResult)
        self._view.display.returnPressed.connect(self._calculateResult)
        self._view.buttons['C'].clicked.connect(self._view.clearDisplay)
        
def evaluateExpression(expression):
    
    try:
        result = str(eval(expression,{},{}))
    except Exception:
        result = ERROR_MSG

    return result

def main():

    pycalc = QApplication(sys.argv)
    
    view = PyCalcUI()
    view.show()

    model = evaluateExpression
    PyCalcCtrl(model=model,view=view)
   
    sys.exit(pycalc.exec_())

if __name__ =='__main__':
    main()