The extension pattern

Note

This page is vastly inspired from the xarray documentation that is the inspiration for the geetools implementation.

Google Earth Engine is designed as a general purpose library and hence tries to avoid including overly user specific functionality. But inevitably, the need for open-source community to contribute arises. This is where the extension pattern comes in.

Composition over Inheritance

One potential solution to this problem is to subclass every ee.ComputedObject to add user specific functionality. However, inheritance is not very robust. It’s easy to inadvertently use internal APIs when subclassing, which means that your code may break when earthengine-api upgrades. Furthermore, many builtin methods will only return native ee.ComputedObject objects.

The standard advice is to use composition over inheritance, but reimplementing an API as large as earthengine-api on your own objects can be an onerous task, even if most methods are only forwarding to earthengine-api implementations (That was the technical choice made prior in v0 of geetools).

Writing Custom Accessors

To resolve this issue for more complex cases, geetools has implemented 3 decorators:

• A class decorator: register_class_accessor

• A function decorator: register_function_accessor

They are used to add custom “accessors” on objects/functions/modules thereby “extending” the functionality of your ee object.

Here’s how we use these decorators to write a custom “geetools” accessor implementing a extra method to ee.Number object:

import ee
from geetools.accessor import register_class_accessor

@register_class_accessor(ee.Number, "geetools")
class NumberAccessor:

def __init__(self, obj: ee.Number):
    self._obj = obj

def truncate(self, nbDecimals = 2):
    """Truncate a number to a given number of decimals."""
    nbDecimals = ee.Number(nbDecimals).toInt()
    factor = ee.Number(10).pow(nbDecimals)
    return self._obj.multiply(factor).toInt().divide(factor)

In general, the only restriction on the accessor class is that the __init__ method must have a single parameter: the object it is supposed to work on.

This achieves the same result as if the Dataset class had a cached property defined that returns an instance of your class:

class Number:
    ...

    @property
    def geetools(self):
        return NumberAccessor(self)

However, using the register accessor decorators is preferable to simply adding your own ad-hoc property (i.e., ee.number.geetools = property(...)), for several reasons:

• It ensures that the name of your property does not accidentally conflict with any other attributes or methods (including other accessors).

• Instances of accessor object will be cached on the object that creates them. This means you can save state on them (e.g., to cache computed properties).

• Using an accessor provides an implicit namespace for your custom functionality that clearly identifies it as separate from built-in earthengine-api methods.

Note

Accessors are created once per object instance. New instances, like those created from mapping operations or when accessing a ee.Feature from a ee.FeatureCollection (ex. fc.first()), will have new accessors created.

The intent here is that libraries that extend earthengine-api could add such an accessor to implement subclass specific functionality rather than using actual subclasses or patching in a large number of domain specific methods. For further reading on ways to write new accessors and the philosophy behind the approach, see pydata/xarray#1080.