Client vs. Server

Earth Engine client libraries for Python and JavaScript translate complex geospatial analyses to Earth Engine requests. Code that you write for a client library may contain a mixture of references to client-side objects and variables that represent server-side objects.

It is important to distinguish Earth Engine objects from other Python or JavaScript objects or primitives that might be in your code. You can manipulate objects on the server by manipulating client-side "proxy" objects in your script. You can recognize a proxy object as anything starting with ee. These Earth Engine proxy objects don't contain any actual data and are just handles for objects on the server. To start, consider a client-side string object (which is NOT a proxy object):

var clientString = 'I am a String';
print(typeof clientString);  // string

import ee
import geemap.core as geemap

client_string = 'I am a String'
print(type(client_string))  # str

Observe from the output that the client (the web browser or notebook) has interpreted this code and run it, determining that the variable is type string. Now suppose you want Earth Engine to be able to do something with this string. To do that, you need to wrap up the string in a nice container and send it to Google. That container is the proxy object. Here's an example:

var serverString = ee.String('I am not a String!');
print(typeof serverString);  // object
print('Is this an EE object?',
    serverString instanceof ee.ComputedObject);  // true

import ee
import geemap.core as geemap

server_string = ee.String('I am not a String!')
print(type(server_string))  # ee.ee_string.String
print(
    'Is this an EE object?', isinstance(server_string, ee.ee_string.String)
)  # True

Observe from the output that the ee.String is an object, NOT a string. More specifically, it's an ee.computedObject, which means it's a proxy object for something on the server. Think of ee.Thing as the way to put a thing in a container for sending to Google. Your client doesn't know what's in the container, but you can find out what's in it by printing it:

print(serverString);  // I am not a String

import ee
import geemap.core as geemap

print(server_string.getInfo())  # I am not a String

To see what the container itself looks like, print the string representation of the object:

print(serverString.toString());  // ee.String("I am not a String!")

import ee
import geemap.core as geemap

print(server_string)  # ee.String({"constantValue": "I am not a String!"})

If, for some reason, you need to use Python or JavaScript running in the client to manipulate whatever is in the container, then use getInfo() to get the contents of the container and assign it to a variable:

var someString = serverString.getInfo();
var strings = someString + '  Am I?';
print(strings);  // I am not a String!  Am I?

import ee
import geemap.core as geemap

some_string = server_string.getInfo()
strings = some_string + '  Am I?'
print(strings)  # I am not a String!  Am I?

Looping

Because the client doesn't know what's in server-side ee.Thing objects, client-side operations such as conditionals and for-loops does not work with them. For that reason, and to avoid synchronous calls to getInfo(), use server functions to the extent possible. For example, consider the following two ways of creating a list:

Not recommended — client-side for-loop

var clientList = [];
for(var i = 0; i < 8; i++) {
  clientList.push(i + 1);
}
print(clientList);

import ee
import geemap.core as geemap

client_list = []
for i in range(8):
  client_list.append(i + 1)
print(client_list)

Recommended — server-side mapping

var serverList = ee.List.sequence(0, 7);
serverList = serverList.map(function(n) {
  return ee.Number(n).add(1);
});
print(serverList);

import ee
import geemap.core as geemap

server_list = ee.List.sequence(0, 7)
server_list = server_list.map(lambda n: ee.Number(n).add(1))
print(server_list.getInfo())

The server-side mapping example is a little silly because you could make the same list simply with ee.List.sequence(1, 8), but it illustrates some important concepts. The first concept is map() which simply applies the same function to everything in the list. Because this function is executed on the server, client-side functions such as getInfo() and print() won't work in a mapped function. For that reason, the i + 1 code has to be replaced with the equivalent server-side code: ee.Number(n).add(1). Importantly, n is an object that only exists on the server. Because the function doesn't know the type of its argument, it needs to be cast to an ee.Number.

It's also worth noting that occasionally client-side functionality is convenient. For example, the previous for loop could be used to build a list and wrap it with a server-side object:

var toServerList = ee.List(clientList);

import ee
import geemap.core as geemap

to_server_list = ee.List(client_list)

Realize that client-side processing is done in your notebook or browser, the host machine's CPU, so it can be less efficient than using Earth Engine to do the work on the server. Also, to avoid potentially surprising results, it is good practice to avoid mixing client and server functionality in your scripts. The Conditionals section provides an example of possibly unintended consequences.

Conditionals

Server-side objects don't necessarily work with client-side functions and vice versa. For example, consider the case of a server-side Boolean variable:

var myList = ee.List([1, 2, 3]);
var serverBoolean = myList.contains(5);
print(serverBoolean);  // false

import ee
import geemap.core as geemap

my_list = ee.List([1, 2, 3])
server_boolean = my_list.contains(5)
print(server_boolean.getInfo())  # False

As shown in the following example, the variable does not behave in a client-side conditional because it is a server-side object. To correctly check a server-side boolean, use a server-side function:

Not recommended — client-side conditional

var clientConditional;
if (serverBoolean) {
  clientConditional = true;
} else {
  clientConditional = false;
}
print('Should be false:', clientConditional);  // True!

import ee
import geemap.core as geemap

if server_boolean:
  client_conditional = True
else:
  client_conditional = False
print('Should be False:', client_conditional)  # True!

Recommended — server-side conditional

var serverConditional = ee.Algorithms.If(serverBoolean, 'True!', 'False!');
print('Should be false:', serverConditional);  // False!

import ee
import geemap.core as geemap

server_conditional = ee.Algorithms.If(server_boolean, 'True!', 'False!')
print('Should be False:', server_conditional.getInfo())  # False!

Client and Server Functions

The previous sections describe several reasons why it is inefficient or illogical to mix client and server objects and functions. Which objects and functions are client-side and which are server-side? In general, anything initialized as ee.Thing is a server object and any method on that object, ee.Thing.method(), is a server function. Objects and functions that appear in the Python or JavaScript reference are client-side. As noted previously, you can use client-side functionality to create an object, then wrap it by supplying the client-side object to an Earth Engine constructor, for example ee.String().