Pydantic `ByteSize.human_readable` with an optional

The Fix

pip install pydantic==2.6.2

Based on closed pydantic/pydantic issue #8668 · PR/commit linked

Open PR/Commit

@@ -1740,6 +1740,8 @@ class MyModel(BaseModel):
     print(m.size.human_readable(decimal=True))
     #> 50.0PB
+    print(m.size.human_readable(separator=' '))
+    #> 44.4 PiB

fix.md

Option A — Upgrade to fixed release\npip install pydantic==2.6.2\nWhen NOT to use: This fix should not be used if the default behavior of no separator is required.\n\n

Why This Fix Works in Production

Trigger: Currently, the return value of `ByteSize.human_readable` does not have any separator between the value and unit.
Mechanism: Adds an optional separator between the value and unit in the `ByteSize.human_readable` method.
Why the fix works: Adds an optional separator between the value and unit in the `ByteSize.human_readable` method. (first fixed release: 2.6.2).

Why This Breaks in Prod

Production symptom (often without a traceback): Currently, the return value of `ByteSize.human_readable` does not have any separator between the value and unit.

Proof / Evidence

GitHub issue: #8668
Fix PR: https://github.com/pydantic/pydantic/pull/8706
First fixed release: 2.6.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.73

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“@jks15satoshi, Go for it! Just makes sure that by default, the separator is '' so that this isn't a breaking change. Thanks for the suggestion.…”

@sydney-runkle · 2024-02-02 · source

“@sydney-runkle, Thanks. PR has been created :)”

@jks15satoshi · 2024-02-02 · source

Failure Signature (Search String)

Currently, the return value of `ByteSize.human_readable` does not have any separator between the value and unit.
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Currently, the return value of `ByteSize.human_readable` does not have any separator between the value and unit.
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Currently, the return value of `ByteSize.human_readable` does not have any separator between the value and unit.
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)

What Broke

The output format of byte sizes was visually unappealing without a space between value and unit.

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==2.6.2

When NOT to use: This fix should not be used if the default behavior of no separator is required.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/8706

First fixed release: 2.6.2

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the default behavior of no separator is required.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
2.6.2	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:serialization Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error Data consistency OpenAPI schema

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic `ByteSize.human_readable` with an optional separator (e.g. whitespace) between value and (Fix)