---
title: The DynamoDB crash that only appears on your second write
date: 2026-07-31
tags: dynamodb, python, testing, aws-lambda, boto3
type: success
reproducible: true
---

# The DynamoDB crash that only appears on your second write

My publish endpoint worked perfectly — once. The first story went in clean. The
second one returned Internal Server Error, and the log said:
"TypeError: Float types are not supported. Use Decimal types instead."

DynamoDB does not accept Python floats. Everyone learns this eventually; the
interesting part is why the first write survived. My API computes similarity
scores between the incoming story and the existing ones, and stores the top
matches. On the very first publish the table was empty — no matches, no scores,
no floats, no crash. The bug did not exist until the data did.

The fix took one line (scores became integer percentages — DynamoDB is happy with
ints and Decimals). The lesson took longer to sink in: my "it works" test had only
ever exercised the empty-database path. Any code whose behavior depends on
existing state — scoring, deduplication, aggregation, "related items" — has a
second code path that first-write tests never touch.

Now every write-path test seeds the table with one record before testing the
write. The populated path is the real path; the empty path happens exactly once
in production, ever.

## Reproduce this

Prerequisites: any Python service writing to DynamoDB via boto3.

Your agent will need from you: the location of your put_item / update_item calls,
and your test setup.

Steps: trace every value that feeds a DynamoDB write and flag anything computed —
divisions, averages, rounded scores — because in Python those are floats; convert
to int or decimal.Decimal at the boundary; then add one integration test that
seeds a record and writes a second one, so state-dependent code (similarity,
dedup, counters) actually executes under test.

Verify: grep your codebase for round( and / feeding item dicts; run the
seeded-table test; publish twice against a real table — the second write is the
one that tells the truth.