Parsing Base64 encoded data containing an image from AWS Lambda with Python

user_4353740 0
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I have a Lambda function setup with a POST method that should be able to receive an image as multi-form data, load the image, do some calculations and return a simple array of numbers. The Lambda function sits behind a API Gateway with Lambda-Proxy integration on and multipart/form-data set as a Binary Media Type.

However, I can't for the life of me seem to figure out how to parse the multi-form data that is returned from AWS Lambda.

The event['body'] contains base64 encoded data that I can't post here because it takes up too much space.

I use the following snip of code to parse the multi-form data:

from requests_toolbelt.multipart import decoder
multipart_string = base64.b64decode(body)
content_type = data['event']['headers']['Content-Type']
multipart_data = decoder.MultipartDecoder(multipart_string, content_type)

where content_type is 'multipart/form-data; boundary=--------------------------881952313555430391739156'.

Running through the components of multipart_data like this..

for part in multipart_data.parts:
    print(part.content)
    print(part.headers)

gives this. The content (too long to post) looks like this:

b'\xff\xd8\xff\xe0\x00\x10JFIF\x00\ ... x00\x7f\xff\xd9'

and the headers:

{b'Content-Disposition': b'form-data; name="image"; filename="8281460-3x2-700x467.jpg"', b'Content-Type': b'image/jpeg'}

However, it still is not clear to me a) What part of the content is the actual image? b) How can I extract the image, and e.g. get it into PIL with Image.open?

Supplementary information:

Here is the small Flask app I use to POST the image and return the event data:

import json

from flask import Flask, request 

app = Flask(__name__)

@app.route('/', methods=['GET', 'POST'])
def hello(event, context):

    response = {
        "statusCode": 200,
        "event": event
    }

    return {
        "body": json.dumps(response),
    }

and here is the POSTMAN request as Python code:

import requests

url = "url-to-lambda-function"

payload = "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"image\"; filename=\"8281460-3x2-700x467.jpg\"\r\nContent-Type: image/jpeg\r\n\r\n\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--"
headers = {
    'content-type': "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW",
    'User-Agent': "PostmanRuntime/7.18.0",
    'Accept': "*/*",
    'Cache-Control': "no-cache",
    'Content-Type': "multipart/form-data; boundary=--------------------------881952313555430391739156",
    'Accept-Encoding': "gzip, deflate",
    'Content-Length': "30417",
    'Connection': "keep-alive",
    'cache-control': "no-cache"
    }

response = requests.request("POST", url, data=payload, headers=headers)

print(response.text)
aws-lambda base64 multipart image python
3 Answers

For anyone coming here, this is how I ended up solving it:

    body = event["body"]

    content_type = event["headers"]["Content-Type"]

    body_dec = base64.b64decode(body)

    multipart_data = decoder.MultipartDecoder(body_dec, content_type)

    binary_content = []

    for part in multipart_data.parts:
        binary_content.append(part.content)

    imageStream = io.BytesIO(binary_content[0])
    imageFile = Image.open(imageStream)
    imageArray = np.array(imageFile)

which will yield a array that you can work with, as you For me the difficulty was in understanding how multipart/form-data was stitched together again.

user_43537400
user_103206040

To add to tmo's answer: my multipart/form-data posts (to an AWS lambda with API gateway proxy integration) required that I access the content-type header instead with:

content_type = event['multiValueHeaders']['Content-Type'][0]

and then accessing the parts of the form-data from tmo's binary_content list with:

...
file_content = binary_content[0]
filename = str(binary_content[1].decode())
team_id = str(binary_content[2].decode())
user_16115030
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