Receiving content-type: multipart/form-data from ReactJS Application to a Lambda Function

Greeting

Hi Champer!

Thanks for reaching out with your question. I understand you're new to Python and tackling a multipart/form-data request in an AWS Lambda function. Let’s break this down and provide clear, actionable steps to help you process the file data sent from your ReactJS application. 😊

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Clarifying the Issue

You're receiving a multipart/form-data payload in your Lambda function via API Gateway. The payload contains both text fields and files (uploaded using a file input control in your ReactJS app). While you've managed to process the text data, you're unsure how to handle and extract the file data, particularly from imagesList and coordinatorList, which include file objects.

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Key Terms

• Multipart/form-data: A format used for uploading files and form fields simultaneously. Each field is separated into "parts," which can include text and binary file data.
• API Gateway: The AWS service acting as the entry point for your Lambda function.
• AWS Lambda: A serverless compute service where you’re running your Python code to process the incoming request.

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The Solution (Our Recipe)

Steps at a Glance:

1. Parse the multipart/form-data payload using the multipart library.
2. Extract text fields and file data from the parsed payload.
3. Process the file data (e.g., save it to S3 or perform some operation).

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Step-by-Step Guide:

1. Parse the multipart/form-data payload using the multipart library:
   • Install the multipart library (pip install python-multipart).
   • Use the MultipartDecoder to decode the incoming payload.

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2. Extract text fields and file data from the parsed payload:
   • Iterate through the parts of the payload.
   • Separate text fields (e.g., groupId, groupName) from file data (e.g., file in imagesList and coordinatorList).

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3. Process the file data:
   • Convert the file data into a usable format (e.g., save it locally or upload it to AWS S3).
   • Optionally, add logic to validate or transform the file data.

Code Implementation:
Here’s an example Lambda function to handle your payload:

# Required libraries
import json  # To handle JSON responses
import boto3  # AWS SDK for Python to interact with S3
from multipart import MultipartDecoder  # To parse multipart/form-data payloads
from io import BytesIO  # For handling file content in memory

# Initialize the S3 client
s3_client = boto3.client("s3")

def lambda_handler(event, context):
    """
    Lambda function to handle multipart/form-data payloads from API Gateway.
    Processes both text fields and file uploads.
    """
    # Extract content type and body from the event
    content_type = event["headers"].get("content-type")
    body = event["body"]
    is_base64_encoded = event["isBase64Encoded"]
    
    # Decode the body if it is base64-encoded
    if is_base64_encoded:
        body = base64.b64decode(body)
    
    # Use MultipartDecoder to parse the payload
    decoder = MultipartDecoder(body, content_type)
    
    # Containers for extracted fields
    fields = {}  # To store text fields
    files = []   # To store file metadata and content

    # Iterate through the parts of the payload
    for part in decoder.parts:
        # Get the content disposition header to determine field type
        disposition = part.headers.get(b"content-disposition").decode("utf-8")
        
        if "filename" in disposition:
            # If a file is present, extract filename and file content
            filename = disposition.split("filename=")[1].strip('"')
            file_content = BytesIO(part.content)  # Use BytesIO to keep the content in memory
            files.append({"filename": filename, "content": file_content})
        else:
            # Handle text fields
            name = disposition.split("name=")[1].strip('"')
            fields[name] = part.content.decode("utf-8")
    
    # Example file processing: Upload files to S3
    for file in files:
        s3_client.upload_fileobj(
            Fileobj=file["content"],  # The file content as a file-like object
            Bucket="your-s3-bucket-name",  # Replace with your S3 bucket name
            Key=f"uploads/{file['filename']}",  # File path in the bucket
        )
    
    # Return a success response
    return {
        "statusCode": 200,
        "body": json.dumps({
            "message": "Data processed successfully!",
            "fields": fields,  # Include parsed text fields in the response
            "uploaded_files": [file["filename"] for file in files]  # Include uploaded filenames
        })
    }

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Closing Thoughts

This solution provides a straightforward way to handle multipart/form-data payloads in your AWS Lambda function. It decodes the payload, extracts both text and file data, and demonstrates an example of uploading files to S3. With this approach, you can expand your logic to handle any specific requirements, such as data validation or additional processing.

If you have any questions or run into issues, feel free to ask! We're here to help. 😊

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Farewell

Good luck with your project, Champer! I hope this solution makes it easier for you to handle multipart/form-data in your Lambda function. You've got this!

Cheers,

Aaron 🚀

I hope other don't spend hours trying to figure this out. Package is not python-multipart. It's requests-toolbelt

pip install requests-toolbelt 

import requests
from requests_toolbelt.multipart.decoder import MultipartDecoder