Simple calculator made with pyqt5 (for basic qt training)

mycalc.py

import os
import sys
from PyQt5 import Qt, uic


class MyCalc(Qt.QWidget):
    """A specialized QLineEdit (implemented using signal connections declared
    in the loaded .ui file)
    """

    def __init__(self, parent=None):

        # call the parent class init
        Qt.QWidget.__init__(self, parent=parent)

        # load the .ui file
        uic.loadUi(os.path.join(os.path.dirname(__file__), "mycalc.ui"), self)

        # initialize the _reset_needed flag to True to remove the initial 0
        self._reset_needed = True

        # connect the buttons to the slots
        for b in (
            self.b0,
            self.b1,
            self.b2,
            self.b3,
            self.b4,
            self.b5,
            self.b6,
            self.b7,
            self.b8,
            self.b9,
            self.bplus,
            self.bminus,
            self.bmult,
            self.bdiv,
            self.bdot,
        ):
            b.clicked.connect(self.onKeyClicked)

        self.beq.clicked.connect(self.onEqClicked)
        self.bclear.clicked.connect(self.onClearClicked)

    def onKeyClicked(self):
        # do not accept entering more than 15 digits
        if len(self.label.text()) > 15:
            return
        # before appending new text, reset the existing one if required
        if self._reset_needed:
            self.label.setText("")
            self._reset_needed = False
        # append the corresponding button text to the text currently displayed
        text = self.label.text() + self.sender().text()
        self.label.setText(text)

    def onEqClicked(self):
        try:
            # use python eval to evaluate the expression being displayed
            text = "{:g}".format(eval(self.label.text()))
            if len(text) > 15:
                raise OverflowError
        except:
            text = "Error"
            self._reset_needed = True

        self.label.setText(text)

    def onClearClicked(self):
        self.label.setText("0")
        self._reset_needed = True




if __name__ == "__main__":

    # Initialize a Qt application (Qt will crash if you do not do this first)
    app = Qt.QApplication(sys.argv)

    # instantiate the widget
    w = MyCalc()
    w.setWindowTitle("MyCalc")

    # show it (if you do not show the widget, it won't be visible)
    w.show()

    # Initialize the Qt event loop (and exit when we close the app)
    sys.exit(app.exec_())

mycalc.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>358</width>
    <height>196</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <layout class="QVBoxLayout" name="verticalLayout">
   <item>
    <widget class="QLabel" name="label">
     <property name="sizePolicy">
      <sizepolicy hsizetype="Preferred" vsizetype="Fixed">
       <horstretch>0</horstretch>
       <verstretch>0</verstretch>
      </sizepolicy>
     </property>
     <property name="maximumSize">
      <size>
       <width>16777215</width>
       <height>300</height>
      </size>
     </property>
     <property name="font">
      <font>
       <pointsize>18</pointsize>
      </font>
     </property>
     <property name="frameShape">
      <enum>QFrame::Panel</enum>
     </property>
     <property name="frameShadow">
      <enum>QFrame::Sunken</enum>
     </property>
     <property name="midLineWidth">
      <number>1</number>
     </property>
     <property name="text">
      <string>0</string>
     </property>
     <property name="scaledContents">
      <bool>false</bool>
     </property>
     <property name="alignment">
      <set>Qt::AlignRight|Qt::AlignTrailing|Qt::AlignVCenter</set>
     </property>
    </widget>
   </item>
   <item>
    <layout class="QGridLayout" name="gridLayout">
     <item row="0" column="0">
      <widget class="QPushButton" name="b1">
       <property name="text">
        <string>1</string>
       </property>
      </widget>
     </item>
     <item row="0" column="1">
      <widget class="QPushButton" name="b2">
       <property name="text">
        <string>2</string>
       </property>
      </widget>
     </item>
     <item row="0" column="2">
      <widget class="QPushButton" name="b3">
       <property name="text">
        <string>3</string>
       </property>
      </widget>
     </item>
     <item row="1" column="0">
      <widget class="QPushButton" name="b4">
       <property name="text">
        <string>4</string>
       </property>
      </widget>
     </item>
     <item row="1" column="1">
      <widget class="QPushButton" name="b5">
       <property name="text">
        <string>5</string>
       </property>
      </widget>
     </item>
     <item row="1" column="2">
      <widget class="QPushButton" name="b6">
       <property name="text">
        <string>6</string>
       </property>
      </widget>
     </item>
     <item row="2" column="0">
      <widget class="QPushButton" name="b7">
       <property name="text">
        <string>7</string>
       </property>
      </widget>
     </item>
     <item row="2" column="1">
      <widget class="QPushButton" name="b8">
       <property name="text">
        <string>8</string>
       </property>
      </widget>
     </item>
     <item row="2" column="2">
      <widget class="QPushButton" name="b9">
       <property name="text">
        <string>9</string>
       </property>
      </widget>
     </item>
     <item row="3" column="1">
      <widget class="QPushButton" name="b0">
       <property name="text">
        <string>0</string>
       </property>
      </widget>
     </item>
     <item row="3" column="0">
      <widget class="QPushButton" name="bdot">
       <property name="text">
        <string>.</string>
       </property>
      </widget>
     </item>
     <item row="3" column="3">
      <widget class="QPushButton" name="bdiv">
       <property name="text">
        <string>/</string>
       </property>
      </widget>
     </item>
     <item row="0" column="3">
      <widget class="QPushButton" name="bplus">
       <property name="text">
        <string>+</string>
       </property>
      </widget>
     </item>
     <item row="2" column="3">
      <widget class="QPushButton" name="bmult">
       <property name="text">
        <string>*</string>
       </property>
      </widget>
     </item>
     <item row="1" column="3">
      <widget class="QPushButton" name="bminus">
       <property name="text">
        <string>-</string>
       </property>
      </widget>
     </item>
     <item row="3" column="2">
      <widget class="QPushButton" name="bclear">
       <property name="text">
        <string>C</string>
       </property>
      </widget>
     </item>
     <item row="4" column="2" colspan="2">
      <widget class="QPushButton" name="beq">
       <property name="text">
        <string>=</string>
       </property>
      </widget>
     </item>
    </layout>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>