Why QgsRasterCalculator result is incorrect in pyqgis?

Question

I want to sum 4 raster layers by QgsRasterCalculator. Below code creates a raster layer as result, but pixel values in raster result is wrong. what is the error in this code?

from qgis.core import *
from PyQt4.QtCore import *
from qgis.analysis import QgsRasterCalculator, QgsRasterCalculatorEntry

entries = []
rst1 = "home/Data/lyr1.tif"
rst2 = "home/Data/lyr2.tif"
rst3 = "home/Data/lyr3.tif"
rst4 = "home/Data/lyr4.tif"
myList = [rst1, rst2, rst3, rst4]

try:
    for i in range(0, len(myList)):
         raster = myList[i]
         readRst = processing.getObject(raster)
         ras1 = QgsRasterCalculatorEntry()
         ras1.raster = readRst
         ras1.ref = "ext_lyr@" + str(i+1)
         ras1.bandNumber = 1
         entries.append(ras1)

    formula = '(' + entries[0].ref + ' + ' + entries[1].ref + ' + ' + entries[2].ref + ' + ' + entries[3].ref + ')'
    readRst = QgsRasterLayer(myList[0])
    output = "home/Data/result.tif"
    calc = QgsRasterCalculator(formula, output, 'GTiff', readRst.extent(), readRst.width(), readRst.height(), entries)
    calc.processCalculation()

except:
    self.userWarning("Error in Create map", "Can not create Create map, Exit")

Answer 1

I found solution. This line is wrong:

ras1.ref = "ext_lyr@" + str(i+1)

It must be:

ras1.ref = "ext_lyr" + str(i+1) +"@1"