Sum of multiple rasters in QGIS Python

Question

I would like to make the sum of 12 different rasters to get one raster as output. I wrote the following code but it doesn't work (or better, it works, but at the end it doesn't give me any layer, but the writing "Invalid layer, Gdal provider cannot open Gdal dataset"), any suggestions to how modifying it?

import qgis
import gdal 
from qgis.analysis import QgsRasterCalculator, QgsRasterCalculatorEntry
from qgis.core import QgsRasterLayer

suffix_input=['I_jan', 'I_feb', 'I_mar', 'I_apr', 'I_may', 'I_jun', 'I_jul', 'I_aug', 'I_sep', 'I_oct', 'I_nov' ,'I_dec']
inputpath = '/Users/macbook/Desktop/TESI/PROGETTO/I/'

list=[0,1,2,3,4,5,6,7,8,9,10,11]
sum=0
for i in list:
  inputrasterfile = QgsRasterLayer(inputpath + suffix_input[i] + ".tif")

  entries = []
  ras = QgsRasterCalculatorEntry()
  ras.ref = 'ras@1' 
  ras.raster = inputrasterfile
  ras.bandNumber = 1
  entries.append( ras )

  sum = 'sum + ras@1'

  calc = QgsRasterCalculator( sum, '/Users/macbook/Desktop/TESI/CAPITANATA PROGETTO/Itot.tif', 'GTiff', inputrasterfile.extent(), inputrasterfile.width(), inputrasterfile.height(), entries )
  print(calc.processCalculation())
  iface.addRasterLayer('/Users/macbook/Desktop/TESI/PROGETTO/Itot.tif')

Answer 1

1. If you want to iterate a list while having access to iteration number then use

for i, item in enumerate(list):

If you don't need the counter, then just do it like for item in list:

2. I would propose that you don't sum the rasters in a loop but instead calculate the sum with a single calculation after the loop like this:

    expr = ''
    rasters = []
    for i, suffix in enumerate(suffix_input):
        inputrasterfile = QgsRasterLayer(inputpath + suffix + ".tif", suffix)
        rasters.append(inputrasterfile)
   
        expr = expr + '"{}@1"+'.format(suffix)
   
    expr = expr.rstrip('+')
   
    alg_params = {
        'CELLSIZE': 0,
        'CRS': None,
        'EXPRESSION': expr,
        'EXTENT': rasters[0].extent(),
        'LAYERS': rasters,
        'OUTPUT': '/Users/macbook/Desktop/TESI/PROGETTO/Itot.tif')
    }
   
    result = processing.run('qgis:rastercalculator', alg_params)
    iface.addRasterLayer('/Users/macbook/Desktop/TESI/PROGETTO/Itot.tif')