How to convert string to float with Python .get()?

Question

So this is pretty simple I', sure I just can't seem to figure it out.

Using Tk module to create a GUI, I have an entry field that requires a float input to do math with.

At the moment I get this error

floatval1 = float(val1)

could not convert string to float

The code section looks like this:

val1 = entry1.get()
val2 = entry2.get()
floatval1 = float(val1)
floatval2 = float(val2)
formula12 = (((((((floatval1/100)*(floatval2)/100))*86.67)+bevel+44)*2.4)*1.1)
bevel = (((floatval1/100)*2)+((floatval2/100)*2))*3.5
glprice12 = formula12 + bevel

Obviously I've tried to convert the entry fields, entries to a float so it can be used in this formula.

I cant see a reason why it would not allow this, any help would be greatly appreciated.

Kind Regards,

Answer 1

Are you certain that the strings contained by your input fields are valid numbers?

That is, input fields normally start out as empty. If you try to parse an empty string ("") as a number then you will get an error. What number does "" represent?

It's possible that the string in the field is not a valid number so you should right code for that possibility.

eg.

import tkMessageBox

...

# in your function
val1 = entry1.get()
val2 = entry2.get()
try:
    floatval1 = float(val1)
    floatval2 = float(val2)
except ValueError:
    message = "Either val1 ({!r}) or val2 ({!r}) is not a valid number".format(val1, 
        val2)
    print("Error!", message)
    tkMessageBox.showerror("Invalid Inputs", message)
    return # or reraise the error or something sensible
# continue rest of function as normal

Answer 2

If you have PySide installed on your machine, the following will create a dialog that does what you want.

IMO, PySide is much easier to use than Tk, and you can very quickly create little applications like this which work quite nicely.

from PySide import QtGui, QtCore
import sys

"""
This class just sets up the actual dialog widget, and puts everything in place. 
It was mostly just generated by Qt Designer & pyside-uic though, so there's 
no real need to understand it all.
"""
class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(402, 182)
        self.gridLayout = QtGui.QGridLayout(Dialog)
        self.gridLayout.setObjectName("gridLayout")
        self.label_formula12 = QtGui.QLabel(Dialog)
        self.label_formula12.setObjectName("label_formula12")
        self.gridLayout.addWidget(self.label_formula12, 3, 0, 1, 2)
        self.buttonBox = QtGui.QDialogButtonBox(Dialog)
        self.buttonBox.setOrientation(QtCore.Qt.Horizontal)
        self.buttonBox.setStandardButtons(QtGui.QDialogButtonBox.Ok)
        self.buttonBox.setObjectName("buttonBox")
        self.gridLayout.addWidget(self.buttonBox, 5, 0, 1, 2)
        self.label_v1 = QtGui.QLabel(Dialog)
        self.label_v1.setObjectName("label_v1")
        self.gridLayout.addWidget(self.label_v1, 0, 0, 1, 1)
        self.lineEdit_v1 = QtGui.QLineEdit(Dialog)
        self.lineEdit_v1.setObjectName("lineEdit_v1")
        self.gridLayout.addWidget(self.lineEdit_v1, 0, 1, 1, 1)
        self.label_v2 = QtGui.QLabel(Dialog)
        self.label_v2.setObjectName("label_v2")
        self.gridLayout.addWidget(self.label_v2, 1, 0, 1, 1)
        self.lineEdit_v2 = QtGui.QLineEdit(Dialog)
        self.lineEdit_v2.setObjectName("lineEdit_v2")
        self.gridLayout.addWidget(self.lineEdit_v2, 1, 1, 1, 1)
        self.label_bevel = QtGui.QLabel(Dialog)
        self.label_bevel.setObjectName("label_bevel")
        self.gridLayout.addWidget(self.label_bevel, 2, 0, 1, 2)
        spacerItem = QtGui.QSpacerItem(20, 40, QtGui.QSizePolicy.Minimum, QtGui.QSizePolicy.Expanding)
        self.gridLayout.addItem(spacerItem, 4, 0, 1, 2)
        self.label_v1.setBuddy(self.lineEdit_v1)
        self.label_v2.setBuddy(self.lineEdit_v2)

        self.retranslateUi(Dialog)
        QtCore.QObject.connect(self.buttonBox, QtCore.SIGNAL("accepted()"), Dialog.accept)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        Dialog.setWindowTitle("Formula 12")
        self.label_formula12.setText("Formula 12")
        self.label_v1.setText("Value &1:")
        self.label_v2.setText("Vaule &2:")
        self.label_bevel.setText("Bevel")


"""
Here we add the actual functionality to the widgets in the dialog. The
functionality it simple, there are just a few differences from Tk, namely the
use of `signals` and `slots`. The 
"""
class MainWindow(QtGui.QDialog):
    def __init__(self,parent=None):
        super(MainWindow, self).__init__(parent)
        self.ui = Ui_Dialog()
        self.ui.setupUi(self)

        self.ui.lineEdit_v1

        qDoubleValidator = QtGui.QDoubleValidator()

        self.ui.lineEdit_v1.setText("0.0")
        self.ui.lineEdit_v2.setText("0.0")

        """
        Here's the magic that forces the numbers in the `lineEdit`s to be 
        real numbers.
        """
        self.ui.lineEdit_v1.setValidator(qDoubleValidator)
        self.ui.lineEdit_v2.setValidator(qDoubleValidator)

        """
        Signals and Slots. The first set wait until editing is finished - i.e.
        when foucs leaves the box. The second set offer real time updating.        
        """
        #self.ui.lineEdit_v1.editingFinished.connect(self.updateValues)
        #self.ui.lineEdit_v2.editingFinished.connect(self.updateValues)

        self.ui.lineEdit_v1.textEdited.connect(self.updateValues)
        self.ui.lineEdit_v2.textEdited.connect(self.updateValues)

        self.updateValues()        

    def updateValues(self):
        try:
            self.value1 = float(self.ui.lineEdit_v1.text())
            self.value2 = float(self.ui.lineEdit_v2.text())

            self.bevel = self.calculateBevel()
            self.formula12 = self.calculateFormula12()

            self.ui.label_bevel.setText("Bevel = %10.5f" % self.bevel)
            self.ui.label_formula12.setText("Formula12 = %10.5f" % self.formula12)
        except ValueError:
            print "one of the values could not be cast to a float"

    def calculateBevel(self):
        return 3.5 * 2 * (self.value1 + self.value2) / 100

    def calculateFormula12(self):
        return 2.4 * 1.1 * (self.bevel + 44 + 86.67 * self.value1 * self.value2 / 10000)

def main():

    app = QtGui.QApplication(sys.argv)
    myapp = MainWindow()

    myapp.show()

    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

Answer 3

In the end the reason for the error, was because the float value was being derived from an entry field that upon its creation was blank.

A blank field in float form does not translate hence the valueerror.

It is possible to create validation rules on Tkinter forms here

However in my specific example, i moved the calculation that involved the float numbers to be inside the code block for the button itself, making it so that the float values were not being defined until after the fields had been filled.

A further try except block was added to handle incorrect values being entered.

Thankyou to everyone who helped, especially "will""Reza-S4"Dunes" and "barak manos".