Merge a list of ImageCollection into a single ImageCollection in Google Earth Engine Python API

Question

I am learning Earth Engine with Python and I am stuck when I want to split the entire terraclimate Image Collection with province feature collection.

admin = ee.FeatureCollection('FAO/GAUL_SIMPLIFIED_500m/2015/level2')
terraclimate = ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE')

province_names = list(
    np.unique(
        np.array(
            admin.filter(
                ee.Filter.eq('ADM0_NAME', 'Indonesia')
            ).aggregate_array('ADM1_NAME').getInfo()
        )
    )
)

province_list = ee.List(province_names)

def get_province(province):
    img_col = terraclimate.filterBounds(admin.filter(ee.Filter.eq('ADM1_NAME', province)))
    return img_col.set({'province':province})

new_img_col = province_list.map(get_province)

What I get from the code above is a list of ImageCollection. How can I convert this into a single ImageCollection? I try to unpack the list with another map but it gave me error:

'ComputedObject' object has no attribute 'map'

What I'm trying to achieve with this is to get an image collection where each image is separated by province, and afterward, I will use it to calculate mean yearly temperature for each province.

Answer 1

First, a general note: Always try to avoid getInfo() calls whenever you can. It evaluates the value server-side, and returns a client-side object. This often makes your script significantly slower than it needs to be. You can read up on it here.

You have a list where each element is an ImageCollection that you want to merge into a single ImageCollection. Below are two approaches for doing this.

First approach uses iterate(). It manually accumulates the images into a single collection. It's quite explicit in what it does while being quite verbose, due to all the casts needed.

Second approach uses a trick where the list of collections are put into a FeatureCollection, which then is flattened. More compact, but maybe a bit magical.

import ee

ee.Initialize()

admin = ee.FeatureCollection('FAO/GAUL_SIMPLIFIED_500m/2015/level2')
terraclimate = ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE')

province_names = admin \
  .filter(ee.Filter.eq('ADM0_NAME', 'Indonesia')) \
  .aggregate_array('ADM1_NAME') \
  .distinct() # Remove duplicates directly in EE. Avoids getInfo() call

def get_province(province):
    img_col = terraclimate.filterBounds(admin.filter(ee.Filter.eq('ADM1_NAME', province)))
    return img_col.set({'province':province})


# Option 1 - iterate
new_img_col = province_names.map(get_province)
collection1 = ee.ImageCollection( # Cast result of iterate() to ImageCollection
  new_img_col.iterate(
    lambda proviceCollection, acc:
      ee.ImageCollection(acc).merge( # Both arguments must be cast too
        ee.ImageCollection(proviceCollection)
      ),
    ee.ImageCollection([])
  )
)

# Option 2 - flatten
collection2 = ee.ImageCollection( # Cast result to ImageCollection
  ee.FeatureCollection( # Create an intermediate FeatureCollection containing ImageCollections
    province_names.map(get_province)
  ).flatten() # Remove the intermediate feature collection with the merged image collections
)