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Your First S3 + Lambda Trigger: Build It, Test It, Clean It Up

A Lambda function that runs whenever a file is uploaded to a bucket and logs the file name.

Arvind kumar

a student of computer science and engineering
What we're building: A Lambda function that runs whenever a file is uploaded to a bucket and logs the file name.
  1. Create the bucket with public access blocked.
  2. Create the function (Python) with this handler:
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def lambda_handler(event, context):
for record in event["Records"]:
bucket = record["s3"]["bucket"]["name"]
key = record["s3"]["object"]["key"]
print(f"New object: s3://{bucket}/{key}")
  1. Add the S3 trigger for "All object create events" on that bucket.
  2. Test by uploading a file, then open the function's CloudWatch logs to see the printed line.
  3. Common pitfall: if your function writes output back to the same bucket that triggers it, it can invoke itself repeatedly. Use a separate output bucket or a prefix filter.
  4. Clean up: delete the trigger, the function, its log group, and the bucket so nothing keeps costing you.
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