Python Cookbook

Giving the User Unobtrusive Feedback During Data Entry with Qt

Credit: Alex Martelli

Problem

You want to validate the contents of a line-entry widget on the fly while the user is entering data and give unobtrusive feedback about whether the current contents are valid, incomplete, or invalid.

Solution

Changing the widget’s background color to different shades is an excellent way to provide unobtrusive feedback during data entry. As usual, it’s best to package the solution as a reusable widget:

from qt import *

class ValidatingLineEdit(QLineEdit):

    colors = Qt.red, Qt.yellow, Qt.green 

    def _ _init_ _(self, validate, parent=None, name=None):
        QLineEdit._ _init_ _(self, parent, name)
        self.validate = validate
        self.connect(self, SIGNAL("textChanged(const QString &)"), self.changed)
        self.color = None
        self.changed('')

    def changed(self, newText):
        colorIndex = self.validate(unicode(newText))
        if colorIndex is None: return
        color = self.colors[colorIndex].light(196)
        if color != self.color:
            self.setPaletteBackgroundColor(color)
            self.color = color

The function passed as the validate argument must accept a Unicode string and return either None, meaning no color change, or an index into the widget’s colors attribute. By default, 0 indicates red (an incorrect entry), 1 indicates yellow (an incomplete entry), and 2 indicates green (an entry that is already acceptable).

Discussion

When the user is entering data in a line-entry field, it can be helpful to validate the field’s contents on the fly, at every change, ...