The update_function_configuration currently returns an empty dict no matter what but certain lambda libraries (ie, Zappa) require a realistic return value... which is simply the current config after being updated.

https://github.com/localstack/localstack/blob/9cba1010dfcbf2b7672df4d7da46b7e8ab287f64/localstack/services/awslambda/lambda_api.py#L872

https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/lambda.html#Lambda.Client.update_function_configuration

There might also be some other magic enriching that result but my quick read-through looks like it's an empty dict.



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result = []
    for m in event_source_mappings:
        if not func_name or (m['FunctionArn'] in [func_name, func_arn(func_name)]):
            if not source_arn or (m['EventSourceArn'].startswith(source_arn)):
                result.append(m)
    return result
def get_function_version(arn, version):
    func = arn_to_lambda.get(arn)
    return format_func_details(func, version=version, always_add_version=True)
def publish_new_function_version(arn):
    func_details = arn_to_lambda.get(arn)
    versions = func_details.versions
    last_version = func_details.max_version()
    versions[str(last_version + 1)] = {
        'CodeSize': versions.get('$LATEST').get('CodeSize'),
        'CodeSha256': versions.get('$LATEST').get('CodeSha256'),
        'Function': versions.get('$LATEST').get('Function'),
        'RevisionId': str(uuid.uuid4())
    }
    return get_function_version(arn, str(last_version + 1))
def do_list_versions(arn):
    return sorted([get_function_version(arn, version) for version in
                   arn_to_lambda.get(arn).versions.keys()], key=lambda k: str(k.get('Version')))
def do_update_alias(arn, alias, version, description=None):
    new_alias = {
        'AliasArn': arn + ':' + alias,
        'FunctionVersion': version,
        'Name': alias,
        'Description': description or '',
        'RevisionId': str(uuid.uuid4())