Reducing an ImageCollection in Google Earth Engine Python API generates empty image

Question

I am trying to calculate the minimum cumulative water deficit for a region.

terraclim = (ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE')
              .filterBounds(roi)
              .filterDate('1985-01-01', '2019-12-31'))
prec = terraclim.select('pr')

# water deficit here is precipitation - 100mm/month, mean ET value for Amazonia
water_deficit = prec.map(lambda image: image.subtract(100))
monthly_wd_list = water_deficit.toList(water_deficit.size())

resultImage = ee.Image(0)
cwd_list = []

# here we calculate the monthly cumulative water deficit values
for i in range(monthly_wd_list.size().getInfo()):
    # get this month's water deficit value
    currentImage = ee.Image(monthly_wd_list.get(i))
    # add value to last month's water deficit value
    resultImage = resultImage.add(currentImage)
    # if value is positive, it rained enough to compensate for previous drought (zero deficit)
    resultImage = resultImage.where(resultImage.gt(0), 0)
    # otherwise, result is negative, and deficit carries on to coming month
    cwd_list.append(resultImage.set('month', i))

# Determining the lowest cumulative water deficit value of the year
annual_mcwd_list = []
year_count = 1984
for i in range(0, (2020-1985)*12, 12):
    year_count += 1 
    yr = cwd_list[i:i+12]
    cwd_a = ee.ImageCollection.fromImages(yr)
    mcwd_a = cwd_a.reduce(ee.Reducer.min())
    annual_mcwd_list.append(mcwd_a.set('year', year_count))

The images in annual_mcwd_list are generated, but are all empty.

I have tried testing

i = 1
yr = [cwd_list[1], cwd_list[2]]
cwd_a = ee.ImageCollection.fromImages(yr)
mcwd_a = cwd_a.reduce(ee.Reducer.min())

and cwd_a.first() is a real image that maps perfectly, but the reduction mcwd_a appears empty when added to the map. Same outcome when I use .min() instead of .reduce()

Answer 1

Try this approach using server-side functions (with code reformatting):

terraclim = (ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE')
              .filterBounds(roi)
              .filterDate('1985-01-01', '2019-12-31'))

def get_wd(img):
  result = img.select('pr').subtract(100)
  result = result.where(result.gt(0), 0).rename('wd')
  result = result.copyProperties(img, ['system:time_start'])
  return result

wd_col = terraclim.map(get_wd)

years = ee.List.sequence(1985, 2019)

def get_min_yearly(y):
  return wd_col.filter(ee.Filter.calendarRange(y, y, 'year')).select('wd').min().set('year', y)

min_yearly = ee.ImageCollection.fromImages(years.map(get_min_yearly).flatten())