AWS Lambda Error - Handler 'index.handler' missing on module 'index'
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This error simply means the entry point Lambda was told to load does not exist.
Problem
Your Lambda function fails with the following error during deployment or execution:
Handler 'index.handler' missing on module 'index'
Lambda cannot find the entry point you configured. It looks for a specific Python file and a specific function inside that file. If either one is missing or misnamed, Lambda fails before running any of your code.
Clarifying the Issue
This error means Lambda is searching for two things:
- A file named
index.py - Inside it, a function named
handler
If the file name or function name is incorrect, mis-capitalized, or located in the wrong folder, Lambda cannot initialize the function. This is a configuration mismatch, not an IAM or dependency issue.
Why It Matters
The handler is the starting point of your Lambda. If it cannot be found, the entire function fails before execution. This stops API requests, event triggers, Step Functions workflows, and CI/CD pipelines. Correct handler alignment ensures your Lambda boots cleanly and reliably.
Key Terms
- Handler: The function Lambda runs as the entry point.
- Module: The Python file containing the handler.
- Entry Point: The module and function combination used by Lambda, such as
index.handler.
Steps at a Glance
- Confirm the file name matches the module name.
- Confirm the handler function name exists in the file.
- Ensure the correct handler string is configured.
- Redeploy the code package.
- Test the function and verify successful execution.
- Add a simple fallback log message to confirm handler loading.
Step 1: Confirm the file name matches the module name
Lambda expects a file named index.py if your handler is configured as index.handler.
List your files:
ls -l
Example output:
app.py
index_backup.py
lambda_function.py
If index.py is missing, rename or create it:
mv lambda_function.py index.py
This aligns the module name with the handler configuration.
Step 2: Confirm the handler function name exists in the file
Open index.py and make sure it contains a function named handler:
def handler(event, context):
return {"statusCode": 200, "body": "Hello from Lambda"}
If the function name is different, rename it or update the handler configuration.
Step 3: Ensure the correct handler string is configured
Check the handler value in the Lambda console or your Infrastructure as Code template.
For index.py with a function named handler, use:
index.handler
Incorrect examples include:
index.handle
lambda_function.handler
app.handler
Fixing the handler string ensures Lambda looks in the correct file and function.
Step 4: Redeploy the code package
If you are packaging your function manually:
zip -r lambda.zip .
aws lambda update-function-code \
--function-name MyFunction \
--zip-file fileb://lambda.zip
Expected output:
{
"FunctionName": "MyFunction",
"LastModified": "2025-11-14T19:40:00.123+0000"
}
Lambda will now use the updated file and handler.
Step 5: Test the function and verify successful execution
Invoke the function:
aws lambda invoke \
--function-name MyFunction \
response.json
cat response.json
Expected output:
{"statusCode": 200, "body": "Hello from Lambda"}
CloudWatch should show no handler missing errors.
Step 6: Add a fallback log message for future debugging
Add a simple startup message at the top of index.py:
print("Lambda handler loaded successfully")
This produces a quick indicator in CloudWatch that confirms your handler loaded correctly.
Pro Tips
- The handler must be a top-level function. Lambda will not search inside classes or nested functions.
- Avoid uppercase letters in file names; certain tools and ZIP operations are case-sensitive.
- Lambda layers cannot provide the handler file. Only the function ZIP can contain the entry point.
- If using a folder structure like
src/, be explicit:src.index.handler.
Conclusion
This error simply means the entry point Lambda was told to load does not exist. By aligning your file name, function name, and handler configuration, you ensure Lambda boots cleanly and your integrations run without interruption. Once everything matches, your function becomes fully operational and dependable.
Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.
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