Python: How to control Lineedit input and output in PyQt

Question

The code posted below creates a simple dialog window with only widget: QLineEdit. Typing anything into this field triggers fixText() method (on textChanged) which cleans an incoming string argument. After a string was 'cleaned' the method updates the QLineEdit with the result.

The goal: regardless of what user types in to the lineedit field:

The first four letters are always uppercase. The fifth character is always underscore.

Remaining characters in a string don't have to be cleaned beyond of what cleanupString() does.

Example of what needs to be achieved: ABCD_helloWorld

A problem encountered: Unable to insert an underscore character without messing things up. The lines of the code responsible for insertion of underscore are currently commented...

import sys, os
from PyQt4 import QtCore, QtGui  
class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()       


        self.myQLineEdit = QtGui.QLineEdit("Type text here")
        self.myQLineEdit.textChanged.connect(self.fixText)

        myBoxLayout.addWidget(self.myQLineEdit)        

        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)
        self.setWindowTitle('Dialog 01')

    def fixText(self, arg):
        arg=str(arg)
        if not arg: return

        arg=self.cleanupString(arg)

        if len(arg)<3: result=arg.upper()
        else:     result = arg[0:4].upper()+arg[4:]
        # resultList=list(result)
        # resultList.insert(4, '_')
        # result=''.join(resultList)
        self.myQLineEdit.blockSignals(True)
        self.myQLineEdit.setText(result)
        self.myQLineEdit.blockSignals(False)

    def cleanupString(self, line=None):
        if line==None: return
        invalid = invalid = ['!','"','#','$','%','&','\\','(',')','*','+',',','-','.','/'
                    ,':',';','<','=','>','?','@','[',"'",']','^','`','{','|','}','~', ' ']
        for c in invalid: 
            if len(line)>0: line=line.replace(c,'_')
        return line


if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

FIXED CODE:

import sys, os
from PyQt4 import QtCore, QtGui    

class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()       


        self.myQLineEdit = QtGui.QLineEdit("Type text here")
        self.myQLineEdit.textChanged.connect(self.fixText)

        myBoxLayout.addWidget(self.myQLineEdit)        

        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)
        self.setWindowTitle('Dialog 01')

    def fixText(self, arg):
        arg=str(arg)
        if not arg: return

        arg=self.cleanupString(arg)

        if len(arg)<=3: result=arg.upper()
        else:     result = arg[0:4].upper()+"_"+arg[5:]

        self.myQLineEdit.blockSignals(True)
        self.myQLineEdit.setText(result)
        self.myQLineEdit.blockSignals(False)

    def cleanupString(self, line=None):
        if line==None: return
        invalid = invalid = ['!','"','#','$','%','&','\\','(',')','*','+',',','-','.','/'
                    ,':',';','<','=','>','?','@','[',"'",']','^','`','{','|','}','~', ' ']
        for c in invalid: 
            if len(line)>0: line=line.replace(c,'_')
        return line

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

Answer 1

you can do something like:

    if len(arg)<=3: result=arg.upper()
    else: result = arg[0:4].upper()+"_"+arg[5:]

Although, you won't be able to use backspace to delete… you will have to select the text to delete it.