I am trying to extract the dates and mean NDVI values of a processed Google Earth Engine image collection thru Python. All goes well until I try to use getInfo(), where I always encounter this error:

ee.ee_exception.EEException: String.slice, argument 'string': Invalid type. Expected: String. Actual: Float.

But when I run the same code within GEE, it doesn't generate that error. Here is a sample code:

// Initialize //
var roi = ee.Geometry.Polygon(
        [[[122.10367346625048, 7.389991097840974],
          [122.10670974995855, 7.389948538640855],
          [122.10667756321027, 7.392938312477742],
          [122.1036841951634, 7.392938312477729]]]);
var collection = ee.ImageCollection("LANDSAT/LE07/C01/T1_SR")
  .filterDate('1999-01-01', '2018-12-31')
print('Collection size (Initial): ', collection.size());
var cfilt = 0; // Threshold for cloud cover %

// Cloud Filtering //
var ctROI = ee.Number(collection.first().select('B3').reduceRegion({ // # of ROI pixels
  geometry: roi

var cmaskL457sr = function(image){
    var qa = image.select('pixel_qa');
    var cloud = qa.bitwiseAnd(1 << 5).and(qa.bitwiseAnd(1 << 7)).or(qa.bitwiseAnd(1 << 3)).or(qa.bitwiseAnd(1 << 4));
    var mask2 = image.mask().reduce(ee.Reducer.min());
    return image.updateMask(cloud.not()).updateMask(mask2);

var cloudScore = function(image){
    var ctCov = image.select('pixel_qa').reduceRegion({
      reducer: ee.Reducer.count(),
      geometry: roi
    var ctpct = ee.Number(100).subtract(ee.Number(ctCov.get('pixel_qa')).divide(ctROI).multiply(100));
    return image.set('cloud_cover_percentage', ctpct);

var cMasked = collection.map(cmaskL457sr); // Apply cloud mask
var dcFiltered = cMasked.map(cloudScore).filter(ee.Filter.lte('cloud_cover_percentage', cfilt)); // Apply cloud scoring and filter to threshold
print('Collection size (After cloud filtering): ',dcFiltered.size());

// NDVI Calc //
var addNDVI = function(image){
    var ndvi = image.normalizedDifference(['B4', 'B3']);
    var meanNDVI = ndvi.reduceRegion(ee.Reducer.mean(), roi);
    return image.addBands(ndvi).set(meanNDVI); // Attach mean NDVI values

var final = dcFiltered.map(addNDVI).select('nd'); // Final image collection 

// Fetch Data //
var dates_list = ee.List(final.aggregate_array('system:time_start')).getInfo();
var ndmeans_list = ee.List(final.aggregate_array('nd')).getInfo();

Any ideas on why the getInfo() fails? Why does the error say that it expects a string, when getInfo() is used to fetch different kinds of data. Also, it's weird to me that the exact code for GEE works, but not when I use the Python version (Note that the Python code works fine except for that last getInfo() part). Is there a special casting to be done when working with Python GEE code? I have tried various castings but none have worked so far.

Lastly, in the future if I want to generate a seriesbyRegion for a massive (50K+) FeatureCollection, what would be a better way to do that than looping my current code across all regions inside?

  • 1
    Hi, I think you should post the code that fails rather than the one with no issues Nov 9, 2018 at 12:24
  • @RodrigoE.Principe Oh, I forgot to add it. Although it's just a direct copy of the code converted to Python. Still thanks for the reply, I seem to have solved the issue.
    – crftdlve
    Nov 12, 2018 at 4:32
  • Great, you can answer your own question if you want Nov 12, 2018 at 10:37

1 Answer 1


I have solved it. The issue lies in a snippet for filtering ROIs that I have not included in the original question because I thought it was not involved in the error. Here is the EE version with no error:

var fidList = ee.List.sequence(0,roiFull.size().subtract(1)); // List of unique ID values inside the FeatureCollection
var fidStr = ee.String(fidList.get(idx)).slice(0,-2); // Fetch an element, idx, convert it to string then remove the decimal values (1.0 -> 1) and use it for filtering

And here is the Python version with error:

fidList = np.arange(roiFull.size().getInfo())  # List of unique ID values inside the FeatureCollection
fidStr = ee.String(fidList[idx])  # Fetch an element, idx, convert it to string and use it for filtering

To fix this, apparently I have to locally cast fidStr:

fidStr = ee.String(str(fidList[idx]))

It seems that EE has great tolerance for these conversions (by virtue of being ComputedObjects?) as long as the final server-side container is properly casted.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.