EEException: Image.bandNames: Parameter 'image' is required

Question

I am trying to compute the NDVI index using the MODIS Landset in Python and to visualize it in a time series chart, but I am facing an error and I cannot understand where is the problem. This is my code:

campo_1= ee.Geometry.Polygon(
 [[10.6552805, 45.0993966], 
 [10.6584562, 45.0978214], 
 [10.6608595, 45.1003054], 
 [10.6576837, 45.1019109],
 [10.6552805, 45.0993966]]])`

campo1_points= ee.FeatureCollection.randomPoints(campo_1, 8).geometry()

def create_reduce_region_function(geometry,
                        reducer=ee.Reducer.mean(),
                        scale=1000,
                        crs='EPSG:4326',
                        bestEffort=True,
                        maxPixels=1e13,
                        tileScale=4):
        
    def reduce_region_function(img):
        stat = img.reduceRegion(
                reducer=reducer,
                geometry=geometry,
                scale=scale,
                crs=crs,
                bestEffort=bestEffort,
                maxPixels=maxPixels,
                tileScale=tileScale)
        
        return ee.FeatureCollection(geometry, stat).set({'millis': img.date().millis()})
    return reduce_region_function

def fc_to_dict(fc):
    prop_names = fc.first().propertyNames()
    prop_lists = fc.reduceColumns(
      reducer=ee.Reducer.toList().repeat(prop_names.size()),
      selectors=prop_names).get('list')

    return ee.Dictionary.fromLists(prop_names, prop_lists)

date_range= ee.DateRange('2023-07-24','2023-07-31')
ndvi = ee.ImageCollection('MODIS/006/MOD13A2').filterDate(date_range).select('NDVI')
reduce_ndvi = create_reduce_region_function(
    geometry=campo1_points, reducer=ee.Reducer.mean(), scale=1000, crs='EPSG:3310')
ndvi_stat_fc = ee.FeatureCollection(ndvi.map(reduce_ndvi)).filter(
    ee.Filter.notNull(ndvi.first().bandNames()))

fc_to_dict(ndvi_stat_fc).getInfo()

When trying to get the features of the variable I got the following error:

EEException: Image.bandNames: Parameter 'image' is required.

Can someone explain to me why and how to fix this error?

Answer 1

The reduce_region_function() needs a parameter img. The function is declared within the create_reduce_region_function() function and the img parameter is not mentioned there. Consequently, calling the function throws an error for the reduce_region_function().

What you can do is add the parameter img to the main create_reduce_region_function() and reformat the map function as follows.

def create_reduce_region_function(img, geometry,
                        reducer=ee.Reducer.mean(),
                        scale=1000,
                        crs='EPSG:4326',
                        bestEffort=True,
                        maxPixels=1e13,
                        tileScale=4):
                      # Rest similar to your code
ndvi_stat_fc=ee.FeatureCollection(ndvi.map(lambda i:create_reduce_region_function(img=i,geometry=campo1_points))).filter(
    ee.Filter.notNull(ndvi.first().bandNames()))

NB: You can change other parameters according to your requirements.