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Lambda Function Debugging Demo

Mutable Default Argument Bug

This Lambda function demonstrates a classic Mutable Default Argument Bug

What the bug does

  • The process_user_data() function has metadata=[] as a default parameter.
  • In Python, default arguments are evaluated only once when the function is defined.
  • The same list object is reused across all function calls, causing data to accumulate unexpectedly.
  • In case of Lambda this continues as long as the same execution environment is serving the requests.

Demo behavior

When you call your Lambda function URL:

  1. First call: result will have 1 item
  2. Second call: result will have 2 items (including the first call's data!)
  3. Each subsequent Lambda invocation: The list keeps growing within that container instance

Testing

  • Call your function URL: https://your-function-url/?user_id=john
  • Refresh the page multiple times - watch the list grow!
  • The bug persists until the Lambda container is recycled

The Fix (for the debugging demo)

def process_user_data(user_id, metadata=None):
    if metadata is None:
        metadata = []
    # ... rest of function

Deployment instructions

  1. Deploy the SAM template:

    aws cloudformation deploy --template-file template-bug.yaml --stack-name lambda-debug-demo --capabilities CAPABILITY_AUTO_EXPAND CAPABILITY_IAM CAPABILITY_NAMED_IAM

  2. Get the function URL from the outputs and test the bug by calling it multiple times

  3. Observe how the metadata list grows with each invocation within the same container lifecycle