Listing Lambda function

We will create a Lambda function that reads all the data in the DynamoDB table.

Prerequisites - Install Python

Why is this required?

AWS SAM CLI uses your local Python installation to build the Lambda function’s deployment package. During sam build, SAM installs the dependencies listed in requirements.txt using your local Python runtime. If the Python version on your machine does not match the runtime version specified in template.yaml, the build may fail or produce incompatible packages that cause errors when the Lambda function runs on AWS.

How to install Python 3.13 on Windows:

1. Go to the official Python download page: https://www.python.org/downloads/

2. Download Python 3.13.x (the latest 3.13 release).

3. Run the installer:

   • ✅ Check “Add python.exe to PATH” at the bottom of the installer.

   • Click “Install Now” or “Customize installation” if needed.

4. After installation, verify the version by opening a terminal and running:

   python --version
   

   You should see output like: Python 3.13.x

For macOS/Linux, you can use pyenv to install and manage multiple Python versions:

# Install pyenv (macOS with Homebrew)
brew install pyenv

# Install Python 3.13
pyenv install 3.13

# Set it as the local version
pyenv local 3.13

Verify your Python version before proceeding:

python --version

Edit SAM Template

1. Open the template.yaml file in the fcj-book-shop folder.

2. Add the following resource under the Resources section:

   BooksList:
     Type: AWS::Serverless::Function
     Properties:
       CodeUri: fcj-book-shop/books_list
       Handler: books_list.lambda_handler
       Runtime: python3.13
       FunctionName: books_list
       Environment:
         Variables:
           TABLE_NAME: !Ref BooksTable
       Architectures:
         - x86_64
       Policies:
         - Statement:
             - Sid: ReadDynamoDB
               Effect: Allow
               Action:
                 - dynamodb:Scan
                 - dynamodb:Query
               Resource:
                 - !Sub arn:aws:dynamodb:${AWS::Region}:${AWS::AccountId}:table/${booksTableName}
                 - !Sub arn:aws:dynamodb:${AWS::Region}:${AWS::AccountId}:table/${booksTableName}/index/*
   

   

This resource defines a Lambda function named books_list that:

• Uses Python 3.13 runtime on x86_64 architecture
• Has an environment variable TABLE_NAME referencing the DynamoDB BooksTable
• Is granted dynamodb:Scan and dynamodb:Query permissions on the Books table

Create Lambda Function Code

3. Create the following directory structure:

   fcaj-book-shop
   ├── fcaj-book-shop
   │   └── books_list
   │       └── books_list.py
   └── template.yaml
   

   • Create the fcaj-book-shop/books_list folder inside the fcaj-book-shop directory.
   • Create the books_list.py file and copy the following code into it:

   import json
   import os
   import boto3
   from decimal import *
   from boto3.dynamodb.types import TypeDeserializer
   
   client = boto3.client('dynamodb') 
   serializer = TypeDeserializer()
   
   table_name = os.environ['TABLE_NAME']
   
   class DecimalEncoder(json.JSONEncoder):
       def default(self, obj):
           if isinstance(obj, Decimal):
               return str(obj)
           return json.JSONEncoder.default(self, obj)
   
   def deserialize(data):
       if isinstance(data, list):
           return [deserialize(v) for v in data]
   
       if isinstance(data, dict):
           try:
               return serializer.deserialize(data)
           except TypeError:
               return {k: deserialize(v) for k, v in data.items()}
       else:
           return data
   
   def lambda_handler(event, context):
       data_books = client.scan(
           TableName=table_name,
           IndexName='name-index'
       )
       format_data_books = deserialize(data_books["Items"])
       for book in format_data_books:
           data_comment = client.query(
               TableName=table_name, 
               KeyConditionExpression="id = :id AND rv_id > :rv_id", 
               ExpressionAttributeValues={
                   ":id": {"S": book['id']}, 
                   ":rv_id": {"N": "0"}
               }
           )
           format_data_comment = deserialize(data_comment['Items'])
           print(data_comment['Items'])
           book["comments"] = format_data_comment
   
       print (format_data_books)
       return {
           "statusCode": 200,
           "headers": {
               "Content-Type": "application/json",
               "Access-Control-Allow-Origin": "*",
               "Access-Control-Allow-Methods": "GET,PUT,POST,DELETE, OPTIONS",
               "Access-Control-Allow-Headers": "Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method,X-Access-Token,XKey,Authorization"
           },
           "body": json.dumps(format_data_books, cls=DecimalEncoder)
       }
   

   

Build and Deploy

4. Run the following commands to build and validate the SAM template:

   sam validate
   sam build
   

   

5. Deploy the updated stack:

   sam deploy
   

   The changeset will show two new resources being added:

   • BooksListRole (AWS::IAM::Role)
   • BooksList (AWS::Lambda::Function)

   Enter y to confirm the deployment.

   

6. Wait for the deployment to complete. You should see Successfully created/updated stack.

   

Verify on AWS Console

7. Open the CloudFormation console, click on the fcaj-book-shop stack, and switch to the Resources tab. You should now see 5 resources including BooksList and BooksListRole.

   

8. Open the Lambda console. Click on the books_list function.

   

9. At the books_list function page, you can review:

   • The Code source section showing the deployed books_list.py code.

   

   • The Function overview section showing the function diagram and details.

   

10. Click the Configuration tab, then select Environment variables from the left menu. Verify that TABLE_NAME is set to Books.

    

11. Select Permissions from the left menu. Click on the execution role name (e.g., fcaj-book-shop-BooksListRole-...).

    

12. At the IAM Role page, check the Permissions policies section. You should see:

    • AWSLambdaBasicExecutionRole (AWS managed) - for CloudWatch Logs
    • BooksListRolePolicy0 (Customer inline) - the custom policy we defined

    

13. Click on BooksListRolePolicy0 to expand it. Review the policy JSON that grants dynamodb:Scan and dynamodb:Query permissions on the Books table.