fix: preserve original exception context in retry decorator

[dreamcreated]

Problem

The retry decorator in samcli/lib/utils/retry.py swallows the original exception when all retries are exhausted. This makes it impossible to diagnose the root cause of failures because the original traceback is lost.

Before this fix:

while remaining_attempts >= 1:
    try:
        return func(*args, **kwargs)
    except exc:
        ...
raise exc_raise(exc_raise_msg)  # original exception context lost

Fix

Capture the last exception and chain it using raise ... from last_exception:

last_exception = None
while remaining_attempts >= 1:
    try:
        return func(*args, **kwargs)
    except exc as e:
        last_exception = e
        ...
raise exc_raise(exc_raise_msg) from last_exception

This preserves the original traceback via Python exception chaining (__cause__), making it much easier to diagnose the root cause of failures.

Testing

The change is backwards-compatible — the raised exception type and message remain the same, only the __cause__ is now populated.

Related

• Issue tracking this bug: Bug: retry decorator swallows original exception context bnusunny/aws-sam-cli#29

[seshubaws]

@dreamcreated Thank you for raising this! You said "Fixes #29" but I think that may be a typo, what issue does this fix? Just so we can link it properly

[dreamcreated]

Hi @seshubaws, thank you for pointing that out! Yes, that was a typo — the original issue report is in the fork at bnusunny#29 (a good-first-issue tracking the same problem). The upstream aws/aws-sam-cli does not have a separate tracking issue for this, but the fix addresses the same bug described there.

The core problem: the retry decorator loses the original exception traceback by raising exc_raise(exc_raise_msg) without chaining it, which makes debugging failures very difficult. The one-line fix raise exc_raise(exc_raise_msg) from last_exception preserves the full traceback via Python exception chaining (__cause__).

Happy to open a dedicated tracking issue on aws/aws-sam-cli if that helps your process!

[vicheey]

Thanks for this fix @dreamcreated,

I traced the git history — the original retry.py was written in 2020 by @awood45 for Lambda Extensions support, and the missing exception chaining was a miss.

Code looks good.

[bnusunny]

@dreamcreated please also add a unit test in the PR.