Manage IAM policies using an Amazon SDK

Create functions that wrap IAM policy actions.

import json
import logging
import operator
import pprint
import time

import boto3
from botocore.exceptions import ClientError

logger = logging.getLogger(__name__)
iam = boto3.resource("iam")

def create_policy(name, description, actions, resource_arn):
    """
    Creates a policy that contains a single statement.

    :param name: The name of the policy to create.
    :param description: The description of the policy.
    :param actions: The actions allowed by the policy. These typically take the
                    form of service:action, such as s3:PutObject.
    :param resource_arn: The Amazon Resource Name (ARN) of the resource this policy
                         applies to. This ARN can contain wildcards, such as
                         'arn:aws:s3:::my-bucket/*' to allow actions on all objects
                         in the bucket named 'my-bucket'.
    :return: The newly created policy.
    """
    policy_doc = {
        "Version": "2012-10-17",
        "Statement": [{"Effect": "Allow", "Action": actions, "Resource": resource_arn}],
    }
    try:
        policy = iam.create_policy(
            PolicyName=name,
            Description=description,
            PolicyDocument=json.dumps(policy_doc),
        )
        logger.info("Created policy %s.", policy.arn)
    except ClientError:
        logger.exception("Couldn't create policy %s.", name)
        raise
    else:
        return policy



def list_policies(scope):
    """
    Lists the policies in the current account.

    :param scope: Limits the kinds of policies that are returned. For example,
                  'Local' specifies that only locally managed policies are returned.
    :return: The list of policies.
    """
    try:
        policies = list(iam.policies.filter(Scope=scope))
        logger.info("Got %s policies in scope '%s'.", len(policies), scope)
    except ClientError:
        logger.exception("Couldn't get policies for scope '%s'.", scope)
        raise
    else:
        return policies



def create_policy_version(policy_arn, actions, resource_arn, set_as_default):
    """
    Creates a policy version. Policies can have up to five versions. The default
    version is the one that is used for all resources that reference the policy.

    :param policy_arn: The ARN of the policy.
    :param actions: The actions to allow in the policy version.
    :param resource_arn: The ARN of the resource this policy version applies to.
    :param set_as_default: When True, this policy version is set as the default
                           version for the policy. Otherwise, the default
                           is not changed.
    :return: The newly created policy version.
    """
    policy_doc = {
        "Version": "2012-10-17",
        "Statement": [{"Effect": "Allow", "Action": actions, "Resource": resource_arn}],
    }
    try:
        policy = iam.Policy(policy_arn)
        policy_version = policy.create_version(
            PolicyDocument=json.dumps(policy_doc), SetAsDefault=set_as_default
        )
        logger.info(
            "Created policy version %s for policy %s.",
            policy_version.version_id,
            policy_version.arn,
        )
    except ClientError:
        logger.exception("Couldn't create a policy version for %s.", policy_arn)
        raise
    else:
        return policy_version



def get_default_policy_statement(policy_arn):
    """
    Gets the statement of the default version of the specified policy.

    :param policy_arn: The ARN of the policy to look up.
    :return: The statement of the default policy version.
    """
    try:
        policy = iam.Policy(policy_arn)
        # To get an attribute of a policy, the SDK first calls get_policy.
        policy_doc = policy.default_version.document
        policy_statement = policy_doc.get("Statement", None)
        logger.info("Got default policy doc for %s.", policy.policy_name)
        logger.info(policy_doc)
    except ClientError:
        logger.exception("Couldn't get default policy statement for %s.", policy_arn)
        raise
    else:
        return policy_statement



def rollback_policy_version(policy_arn):
    """
    Rolls back to the previous default policy, if it exists.

    1. Gets the list of policy versions in order by date.
    2. Finds the default.
    3. Makes the previous policy the default.
    4. Deletes the old default version.

    :param policy_arn: The ARN of the policy to roll back.
    :return: The default version of the policy after the rollback.
    """
    try:
        policy_versions = sorted(
            iam.Policy(policy_arn).versions.all(),
            key=operator.attrgetter("create_date"),
        )
        logger.info("Got %s versions for %s.", len(policy_versions), policy_arn)
    except ClientError:
        logger.exception("Couldn't get versions for %s.", policy_arn)
        raise

    default_version = None
    rollback_version = None
    try:
        while default_version is None:
            ver = policy_versions.pop()
            if ver.is_default_version:
                default_version = ver
        rollback_version = policy_versions.pop()
        rollback_version.set_as_default()
        logger.info("Set %s as the default version.", rollback_version.version_id)
        default_version.delete()
        logger.info("Deleted original default version %s.", default_version.version_id)
    except IndexError:
        if default_version is None:
            logger.warning("No default version found for %s.", policy_arn)
        elif rollback_version is None:
            logger.warning(
                "Default version %s found for %s, but no previous version exists, so "
                "nothing to roll back to.",
                default_version.version_id,
                policy_arn,
            )
    except ClientError:
        logger.exception("Couldn't roll back version for %s.", policy_arn)
        raise
    else:
        return rollback_version



def delete_policy(policy_arn):
    """
    Deletes a policy.

    :param policy_arn: The ARN of the policy to delete.
    """
    try:
        iam.Policy(policy_arn).delete()
        logger.info("Deleted policy %s.", policy_arn)
    except ClientError:
        logger.exception("Couldn't delete policy %s.", policy_arn)
        raise

Use the wrapper functions to create policies, update versions, and get information about them.

def usage_demo():
    """Shows how to use the policy functions."""
    logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
    print("-" * 88)
    print("Welcome to the AWS Identity and Account Management policy demo.")
    print("-" * 88)
    print(
        "Policies let you define sets of permissions that can be attached to "
        "other IAM resources, like users and roles."
    )
    bucket_arn = f"arn:aws:s3:::made-up-bucket-name"
    policy = create_policy(
        "demo-iam-policy",
        "Policy for IAM demonstration.",
        ["s3:ListObjects"],
        bucket_arn,
    )
    print(f"Created policy {policy.policy_name}.")
    policies = list_policies("Local")
    print(f"Your account has {len(policies)} managed policies:")
    print(*[pol.policy_name for pol in policies], sep=", ")
    time.sleep(1)
    policy_version = create_policy_version(
        policy.arn, ["s3:PutObject"], bucket_arn, True
    )
    print(
        f"Added policy version {policy_version.version_id} to policy "
        f"{policy.policy_name}."
    )
    default_statement = get_default_policy_statement(policy.arn)
    print(f"The default policy statement for {policy.policy_name} is:")
    pprint.pprint(default_statement)
    rollback_version = rollback_policy_version(policy.arn)
    print(
        f"Rolled back to version {rollback_version.version_id} for "
        f"{policy.policy_name}."
    )
    default_statement = get_default_policy_statement(policy.arn)
    print(f"The default policy statement for {policy.policy_name} is now:")
    pprint.pprint(default_statement)
    delete_policy(policy.arn)
    print(f"Deleted policy {policy.policy_name}.")
    print("Thanks for watching!")