Extensibility Guide

All of Boto3's resource and client classes are generated at runtime. This means that you cannot directly inherit and then extend the functionality of these classes because they do not exist until the program actually starts running.

However it is still possible to extend the functionality of classes through Boto3's event system.

An Introduction to the Event System

Boto3's event system allows users to register a function to a specific event. Then once the running program reaches a line that emits that specific event, Boto3 will call every function registered to the event in the order in which they were registered. When Boto3 calls each of these registered functions, it will call each of them with a specific set of keyword arguments that are associated with that event. Then once the registered function is called, the function may modify the keyword arguments passed to that function or return a value. Here is an example of how the event system works:

import boto3

s3 = boto3.client('s3')

# Access the event system on the S3 client
event_system = s3.meta.events

# Create a function
def add_my_bucket(params, **kwargs):
    # Add the name of the bucket you want to default to.
    if 'Bucket' not in params:
        params['Bucket'] = 'mybucket'

# Register the function to an event
event_system.register('provide-client-params.s3.ListObjects', add_my_bucket)

response = s3.list_objects()

In this example, the handler add_my_bucket is registered such that the handler will inject the value 'mybucket for the Bucket parameter whenever the the list_objects client call is made without the Bucket parameter. Note that if the same list_objects call is made without the Bucket parameter and the registered handler, it will result in a validation error.

Here are the takeaways from this example:

  • All clients have their own event system that you can use to fire events and register functions. You can access the event system through the meta.events attribute on the client.
  • All functions registered to the event system must have **kwargs in the function signature. This is because emitting an event can have any number of keyword arguments emitted along side it, and so if your function is called without **kwargs, its signature will have to match every keyword argument emitted by the event. This also allows for more keyword arguments to be added to the emitted event in the future without breaking existing handlers.
  • To register a function to an event, call the register method on the event system with the name of the event you want to register the function to and the function handle. Note that if you register the event after the event is emitted, the function will not be called unless the event is emitted again. In the example, the add_my_bucket handler was registered to the 'provide-client-params.s3.ListObjects' event, which is an event that can be used to inject and modify parameters passed in by the client method. To read more about the event refer to provide-client-params

A Hierarchical Structure

The event system also provides a hierarchy for registering events such that you can register a function to a set of events depending on the event name heirarchy.

An event name can have its own heirachy by specifying . in its name. For example, take the event name 'general.specific.more_specific'. When this event is emitted, the registered functions will be called in the order from most specific to least specific registration. So in this example, the functions will be called in the following order:

  1. Functions registered to 'general.specific.more_specific'
  2. Functions registered to 'general.specific'
  3. Functions registered to 'general'

Here is a deeper example of how the event system works with respect to its hierarchial structure:

import boto3

s3 = boto3.client('s3')

# Access the event system on the S3 client
event_system = s3.meta.events

def add_my_general_bucket(params, **kwargs):
    if 'Bucket' not in params:
        params['Bucket'] = 'mybucket'

def add_my_specific_bucket(params, **kwargs):
    if 'Bucket' not in params:
        params['Bucket'] = 'myspecificbucket'

event_system.register('provide-client-params.s3', add_my_general_bucket)
event_system.register('provide-client-params.s3.ListObjects', add_my_specific_bucket)

list_obj_response = s3.list_objects()
put_obj_response = s3.put_object(Key='mykey', Body=b'my body')

In this example, the list_objects method call will use the 'myspecificbucket' for the bucket instead of 'mybucket' because the add_my_specific_bucket method was registered to the 'provide-client-params.s3.ListObjects' event which is more specific than the 'provide-client-params.s3' event. Thus, the add_my_specific_bucket function is called before the add_my_general_bucket function is called when the event is emitted.

However for the put_object call, the bucket used is 'mybucket'. This is because the event emitted for the put_object client call is 'provide-client-params.s3.PutObject' and the add_my_general_bucket method is called via its registration to 'provide-client-params.s3'. The 'provide-client-params.s3.ListObjects' event is never emitted so the registered add_my_specific_bucket function is never called.

Wildcard Matching

Another aspect of Boto3's event system is that it has the capability to do wildcard matching using the '*' notation. Here is an example of using wildcards in the event system:

import boto3

s3 = boto3.client('s3')

# Access the event system on the S3 client
event_system = s3.meta.events

def add_my_wildcard_bucket(params, **kwargs):
    if 'Bucket' not in params:
        params['Bucket'] = 'mybucket'

event_system.register('provide-client-params.s3.*', add_my_wildcard_bucket)
response = s3.list_objects()

The '*' allows you to register to a group of events without having to know the actual name of the event. This is useful when you have to apply the same handler in multiple places. Also note that if the wildcard is used, it must be isolated. It does not handle globbing with additional characters. So in the previous example, if the my_wildcard_function was registered to 'provide-client-params.s3.*objects', the handler would not be called because it will consider 'provide-client-params.s3.*objects' to be a specific event.

The wildcard also respects the hierarchical structure of the event system. If another handler was registered to the 'provide-client-params.s3' event, the add_my_wildcard_bucket would be called first because it is registered to 'provide-client-params.s3.*' which is more specific than the event 'provide-client.s3'.

Isolation of Event Systems

The event system in Boto3 has the notion of isolation: all clients maintain their own set of registered handlers. For example if a handler is registered to one client's event system, it will not be registered to another client's event system:

import boto3

client1 = boto3.client('s3')
client2 = boto3.client('s3')

def add_my_bucket(params, **kwargs):
    if 'Bucket' not in params:
        params['Bucket'] = 'mybucket'

def add_my_other_bucket(params, **kwargs):
    if 'Bucket' not in params:
        params['Bucket'] = 'myotherbucket'

client1.meta.events.register(
    'provide-client-params.s3.ListObjects', add_my_bucket)
client2.meta.events.register(
    'provide-client-params.s3.ListObjects', add_my_other_bucket)

client1_response = client1.list_objects()
client2_response = client2.list_objects()

Thanks to the isolation of clients' event systems, client1 will inject 'mybucket' for its list_objects method call while client2 will inject 'myotherbucket' for its list_objects method call because add_my_bucket was registered to client1 while add_my_other_bucket was registered to client2.

Boto3 Specific Events

Boto3 emits a set of events that users can register to customize clients or resources and modify the behavior of method calls.

Here is the list of events that users of boto3 can register handlers to:

  • 'creating-client-class
  • 'creating-resource-class
  • 'provide-client-params'

creating-client-class

Full Event Name

'creating-client-class.service-name'

Note: service-name refers to the value used to instantiate a client i.e. boto3.client('service-name')

Description

This event is emitted upon creation of the client class for a service. The client class for a service is not created until the first instantiation of the client class. Use this event for adding methods to the client class or adding classes for the client class to inherit from.

Keyword Arguments Emitted
type class_attributes
dict
param class_attributes
A dictionary where the keys are the names of the attributes of the class and the values are the actual attributes of the class.
type base_classes
list
param base_classes
A list of classes that the client class will inherit from where the order of inheritance is the same as the order of the list.
Expected Return Value

Do not return anything.

Example

Here is an example of how to add a method to the client class:

from boto3.session import Session

def custom_method(self):
    print('This is my custom method')

def add_custom_method(class_attributes, **kwargs):
    class_attributes['my_method'] = custom_method

session = Session()
session.events.register('creating-client-class.s3', add_custom_method)

client = session.client('s3')
client.my_method()

This should output:

This is my custom method

Here is an example of how to add a new class for the client class to inherit from:

from boto3.session import Session

class MyClass(object):
    def __init__(self, *args, **kwargs):
        super(MyClass, self).__init__(*args, **kwargs)
        print('Client instantiated!')

def add_custom_class(base_classes, **kwargs):
    base_classes.insert(0, MyClass)

session = Session()
session.events.register('creating-client-class.s3', add_custom_class)

client = session.client('s3')

This should output:

Client instantiated!

creating-resource-class

Full Event Name

'creating-resource-class.service-name.resource-name'

Note: service-name refers to the value used to instantiate a service resource i.e. boto3.resource('service-name') and resource-name refers to the name of the resource class.

Description

This event is emitted upon creation of the resource class. The resource class is not created until the first instantiation of the resource class. Use this event for adding methods to the resource class or adding classes for the resource class to inherit from.

Keyword Arguments Emitted
type class_attributes
dict
param class_attributes
A dictionary where the keys are the names of the attributes of the class and the values are the actual attributes of the class.
type base_classes
list
param base_classes
A list of classes that the resource class will inherit from where the order of inheritance is the same as the order of the list.
Expected Return Value

Do not return anything.

Example

Here is an example of how to add a method to a resource class:

from boto3.session import Session

def custom_method(self):
    print('This is my custom method')

def add_custom_method(class_attributes, **kwargs):
    class_attributes['my_method'] = custom_method

session = Session()
session.events.register('creating-resource-class.s3.ServiceResource',
                        add_custom_method)

resource = session.resource('s3')
resource.my_method()

This should output:

This is my custom method

Here is an example of how to add a new class for a resource class to inherit from:

from boto3.session import Session

class MyClass(object):
    def __init__(self, *args, **kwargs):
        super(MyClass, self).__init__(*args, **kwargs)
        print('Resource instantiated!')

def add_custom_class(base_classes, **kwargs):
    base_classes.insert(0, MyClass)

session = Session()
session.events.register('creating-resource-class.s3.ServiceResource',
                        add_custom_class)

resource = session.resource('s3')

This should output:

Resource instantiated!

provide-client-params

Full Event Name

'provide-client.service-name.operation-name'

Note: service-name refers to the value used to instantiate a client i.e. boto3.client('service-name'). operation-name refers to the underlying API operation of the corresponding client method. To access the operation API name, retrieve the value from the client.meta.method_to_api_mapping dictionary using the name of the desired client method as the key.

Description

This event is emitted before validation of the parameters passed to client method. Use this event to inject or modify parameters prior to the parameters being validated and built into a request that is sent over the wire.

Keyword Arguments Emitted
type params
dict
param params
A dictionary where the keys are the names of the parameters passed through the client method and the values are the values of those parameters.
type model
botocore.model.OperationModel
param model
A model representing the underlying API operation of the client method.
Expected Return Value

Do not return anything or return a new dictionary of parameters to use when making the request.

Example

Here is an example of how to inject a parameter using the event:

import boto3

s3 = boto3.client('s3')

# Access the event system on the S3 client
event_system = s3.meta.events

# Create a function
def add_my_bucket(params, **kwargs):
    # Add the name of the bucket you want to default to.
    if 'Bucket' not in params:
        params['Bucket'] = 'mybucket'

# Register the function to an event
event_system.register('provide-client-params.s3.ListObjects', add_my_bucket)

response = s3.list_objects()