lineEdit.setValidator(QtGui.QIntValidator(0, 20))

QtGui.QIntValidator(минимальное_число, максимальное_число)

lineEdit.setValidator(QtGui.QRegExpValidator(QtCore.QRegExp(r'\d*'), self))

lineEdit.setValidator(QtGui.QRegExpValidator(QtCore.QRegExp(r'\d{0,15}'), self))

# -*- coding: utf-8 -*-
import sys
from PyQt4 import QtGui, QtCore
class Example(QtGui.QWidget):
    def __init__(self):
        super(Example, self).__init__()
        self.initUI()
    def initUI(self):
        lineEdit = QtGui.QLineEdit(self)
        lineEdit.setValidator(QtGui.QRegExpValidator(QtCore.QRegExp(r'\d{0,15}'), self))
        lineEdit.move(60, 100)
        self.setGeometry(300, 300, 280, 170)
        self.setWindowTitle('Test')
        self.show()
def main():
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())
if __name__ == '__main__':
    main()