I want a  complete documentation on this code import os
from typing import Optional, Union

import aws_cdk.aws_iam as iam
from aws_cdk import DockerImage, Duration, Stack
from aws_cdk import aws_ec2 as ec2
from aws_cdk import aws_lambda
from aws_cdk.aws_logs import RetentionDays
from constructs import Construct

from cdk_resources import role
from cdk_resources.construct import CDKConstruct
from cdk_resources.legacy_bridge import LEGACY_BRIDGE_SECRETS
from cdk_resources.secret_identifier import SecretIdentifier

# Set the runtime for the code bundling
RUNTIME = aws_lambda.Runtime.PYTHON_3_9
LAMBDA_BUNDLING_RUNTIME = DockerImage.from_registry("public.ecr.aws/sam/build-python3.9:latest-x86_64")
CODE_BUNDLE_PATH = os.environ.get("CODE_ZIP_PATH") or "/bundles/bundle.zip"


class PythonLambdaFunction(CDKConstruct):
    _BUNDLE_CACHE: dict[str, aws_lambda.Code] = {}

    def __init__(
        self,
        scope: Construct,
        construct_id: str,
        *,
        handler: str,
        timeout_seconds: int = 30,
        memory_size_mb: int = 256,
        log_retention: RetentionDays = RetentionDays.THREE_MONTHS,
        environment: Optional[dict[str, str]] = None,
        vpc=None,
        module_name=None,
        grants=None,
        security_groups: Optional[list[ec2.SecurityGroup]] = None,
        secrets: Optional[list[SecretIdentifier]] = None,
        reserved_concurrent_executions: Optional[Union[int, float]] = None,
        ff_tenant_provisioning=False,
    ):
        # Initialize the construct base class
        super().__init__(scope, construct_id)

        # TODO: remove once we've resolved the vpc to have an private subnet
        allow_public_subnet = ff_tenant_provisioning
        # Store the specified handler as a property so it is available
        # for inspection
        self.handler = handler

        # Add the elastic secrets into the lambda
        environment = environment or {}

        # Replace grants with an empty list if None
        grants = grants or []

        # Default secrets to an empty list
        secrets = secrets or []

        # Add in environment variables from the secrets parameter
        secrets_environment = self._environment_variables_from_secrets(secrets)
        if secrets_environment:
            # Copy the environment dict before mutating it
            environment = environment.copy()
            environment.update(secrets_environment)

        # Add in permission grants from the secrets parameter
        secrets_grants = self._grants_from_secrets(secrets)
        if secrets_grants:
            # Copy the grants list before mutating it
            grants = grants.copy()
            grants.extend(secrets_grants)

        # Select the default VPC based on the secrets passed
        vpc = vpc or self._vpc_from_secrets(secrets)

        # Set the security groups for this lambda
        security_groups = security_groups or []

        # Make / retrieve the code bundle
        bundle = self.code_bundle()

        # Create the lambda primitive
        self.fn = aws_lambda.Function(
            self,
            id=construct_id if ff_tenant_provisioning else "Fn",
            # Specify the runtime for executing the lambda
            runtime=RUNTIME,
            # The bundled code which ultimately provides a zip
            code=bundle,
            # The entry point of the lambda
            handler=handler,
            # The environment variables for the lambda
            environment=environment,
            # The maximum runtime of the lambda after which it is cancelled
            # Warning: API gateway has its own maximum request processing
            # time which will impact the ability to utilize this parameter
            # for lambdas called via API gateway
            timeout=Duration.seconds(timeout_seconds),
            # The memory size and indirectly the amount of CPU resources
            # available to the lambda at runtime
            memory_size=memory_size_mb,
            # Retain logs from the lambda executions
            log_retention=log_retention,
            # The VPC (AWS EC2 Network) which the lambda is connected to at runtime
            vpc=vpc,
            # The security groups attached to this lambda
            security_groups=security_groups,
            architecture=aws_lambda.Architecture.X86_64,
            # The number of simultaneous executions to reserve for the function
            reserved_concurrent_executions=reserved_concurrent_executions,
            allow_public_subnet=allow_public_subnet,
        )

        # Set the function_arn property directly so that it is
        # convenient to access
        self.function_arn = self.fn.function_arn

        # Grant permissions as requested by the caller
        for grant_target in grants:
            grant_target(role.get_asserted_role(self.fn.role))

    def code_bundle(self) -> aws_lambda.Code:
        """
        Creates and caches one code bundle per stack.
        Note: Code bundles may not be shared across stacks in CDK =(
        """
        # If there is an existing bundle for this stack, return it
        stack_name = Stack.of(self).stack_name
        if stack_name in self._BUNDLE_CACHE:
            return self._BUNDLE_CACHE[stack_name]

        bundle = aws_lambda.Code.from_asset(CODE_BUNDLE_PATH)

        # Update the cache entry
        self._BUNDLE_CACHE[stack_name] = bundle

        # Return the newly created code bundle
        return bundle

    def _environment_variables_from_secrets(self, secrets: list):
        environment = {}

        # Provide environment variables with the ARN of every
        # secret which is in use
        for identifier, secret in self.stack().secrets.items():
            # Do not include environment variables for secrets not in use
            if identifier not in secrets:
                continue

            # Provide an environment variable with the ARN of the
            # specified secret
            environment.update(
                {
                    f"{identifier.name.upper()}_SECRET_ARN": secret.secret_arn,
                }
            )

        return environment

    def _grants_from_secrets(self, secrets: list):
        grants = []

        # Grant secret read permissions as necessary
        for identifier, secret in self.stack().secrets.items():
            # Do not provide environment variables for secrets not in use
            if identifier not in secrets:
                continue

            # Provide read access to the specified secret
            grants.append(secret.grant_read)

        return grants

    def _vpc_from_secrets(self, secrets: list):
        # If any of the secrets require legacy bridge vpc then return that
        for identifier in LEGACY_BRIDGE_SECRETS:
            if identifier in secrets:
                return self.stack().legacy_bridge_vpc

        # Since no special secrets are associated with this lambda no
        # bridge VPC is necessary
        return None


def allow_lambda_assume_role(fn: aws_lambda.Function):
    assume_statement = iam.PolicyStatement(actions=["sts:AssumeRole"], effect=iam.Effect.ALLOW, resources=["*"])
    role.get_asserted_role(fn.role).add_to_principal_policy(assume_statement)


def allow_cross_account_invoke(fn: aws_lambda.Function, account_id: str):
    fn.grant_invoke(iam.AccountPrincipal(account_id))


def allow_lambda_get_bucket_policy(fn: aws_lambda.Function, bucket_name_contains: str) -> None:
    fn.add_to_role_policy(
        iam.PolicyStatement(
            actions=["s3:GetObject"],
            effect=iam.Effect.ALLOW,
            resources=[f"arn:aws:s3:::*{bucket_name_contains}*/*"],
        )
    ) which uses from aws_cdk import Stack
from constructs import Construct
from platform_sdk import EnvironmentType

# TODO: This EnvironmentType datatype is dangerous and needs to be removed.
# The very idea that CDK code would know whether it was creating
# a dev, test, uat, or prod environment and make decisions
# accordingly is something we promised infra we would never do
# as it is quite dangerous and a very bad practice


class CDKConstruct(Construct):
    def __init__(self, *args, **kwargs):
        # Initialize the construct base class
        super().__init__(*args, **kwargs)

    def stack(self):
        """The stack object in which a construct has been placed"""
        return Stack.of(self)

    @property
    def account(self) -> str:
        """The AWS account of the stack"""
        return self.stack().account

    @property
    def region(self) -> str:
        """The AWS region of the stack"""
        return self.stack().region

    @property
    def tenant(self) -> str:
        """The name of the tenant for a stack"""
        return "-".join(self.stack().tenant.split()).lower()

    @property
    def environment_type(self) -> EnvironmentType:
        """The environment type of the stack"""
        return self.stack().environment_type

    @property
    def ff_tenant_provisioning(self) -> bool:
        return self.stack().ff_tenant_provisioning

    @property
    def storage_tenant(self) -> str:
        """The storage tenant of the stack"""
        return self.stack().storage_tenant

    @property
    def stack_name(self) -> str:
        """The name of the stack"""
        return self.stack().stack_name

    def prefix(self) -> str:
        """A prefix naming conventions used in externally managed resources"""
        # TODO: This seems quite fragile and would be better as an input for the stack,
        # which is also a much more verbose way to integrate with externally managed
        # resources that could otherwise be overlooked in a stack setup
        return f"{self.tenant}-{self.environment_type}".lower()
 I want you to explain me all AWS concepts behind it and what each line does in AWS terms and why does it do it ?