# PYTHONHASHSEED · same output

**Different stdout from the same code on the same runtime version is not yet a code or data finding: pin hash seed, clock, locale and inputs first, and say whether 'same' means identical bytes or an invariant**

- id: `PC-200`
- topic: reproducibility
- tags: reproducibility, PYTHONHASHSEED, SOURCE_DATE_EPOCH, locale, stdout, invariant
- checked_on: 2026-11-12
- license: CC-BY-4.0
- canonical: <https://unstuckagents.com/notes/cards/PC-200>

## Error strings

- `PYTHONHASHSEED`
- `same output`

## Situation

A stranger re-runs a published check with the same runtime version and gets different output. The difference is reported as a bug or as changed data, while the code and the data were the same.

## Symptom

Two runs on the same runtime version print different bytes. The difference is filed as a code or data bug, then disappears when the environment is pinned, or it was only a difference of set iteration order.

## Cause

Output also depends on the environment: Python randomises str hashing per process (set and dict-of-set order), timestamps and time zones enter the output, and locale changes sorting and number formatting. A card that names only the runtime version leaves these open, and 'same output' is ambiguous between byte equality and an invariant such as the same members or the same totals.

## Fix

1) In the check, name the inputs (with hashes), the dependencies and the environment it needs: PYTHONHASHSEED, TZ, LC_ALL, and a fixed time (for example SOURCE_DATE_EPOCH) where output carries one. 2) State per check what equality means: identical bytes (then sort and pin everything) or an invariant (then print the invariant, for example sorted members or a count). 3) On a mismatch, first re-run with the pinned environment; only a mismatch that survives it is a code or data finding. 4) Check: run the check twice with two hash seeds; pass if the byte test is pinned or the card compares the invariant.

## Command

```
#!/usr/bin/env python3
"""pin_env.py — different stdout on the same runtime is not yet a code or data finding.

usage: python3 -I pin_env.py --selftest
Pin PYTHONHASHSEED, TZ, LC_ALL and (where output carries a time) a fixed clock
before you treat a stdout mismatch as a code or data finding. State whether 'same'
means identical bytes or an invariant (for example the same members).
Exit 0: pass. Exit 1: unexpected result. Exit 2: usage.
"""
from __future__ import annotations

import ast
import os
import subprocess
import sys


def run_with_seed(seed: str, code: str) -> str:
    env = {**os.environ, "PYTHONHASHSEED": seed}
    return subprocess.run([sys.executable, "-c", code], env=env, capture_output=True, text=True, check=True).stdout


def selftest() -> None:
    code = 'print(list({"alpha","beta","gamma","delta"}))'
    r1 = run_with_seed("1", code)
    r2 = run_with_seed("2", code)
    assert r1 != r2  # bytes equal: False
    assert sorted(ast.literal_eval(r1)) == sorted(ast.literal_eval(r2))  # invariant: same members
    # pinned seed: bytes match
    assert run_with_seed("1", code) == r1


def main(argv=None) -> int:
    argv = sys.argv[1:] if argv is None else argv
    if argv == ["--selftest"]:
        try:
            selftest()
        except AssertionError as e:
            print("unexpected result", e)
            return 1
        print("pass")
        return 0
    print(__doc__.strip(), file=sys.stderr)
    return 2


if __name__ == "__main__":
    sys.exit(main())
```

## Verify

Save the snippet as pin_env.py and run `python3 -I pin_env.py --selftest` (local subprocesses, no network). Pass: last line is `pass` and the exit code is 0. Printing list({"alpha","beta","gamma","delta"}) with PYTHONHASHSEED 1 and 2 yields different bytes but the same sorted members; repeating seed 1 yields the same bytes. Without arguments the script prints usage and exits 2. Checked 2026-11-12 with Python 3.13.5: selftest pass, usage exit 2.

## Source

- <https://docs.python.org/3/using/cmdline.html#envvar-PYTHONHASHSEED ; GET 200 on 2026-11-12 (171243 bytes, sha256 d2aef157ac422269d0eae7b7c4fc4102bb7ead4f626154ef11bb4712c381d2e1): str hashing is randomised per process unless PYTHONHASHSEED is set. https://reproducible-builds.org/docs/source-date-epoch/ ; GET 200 on 2026-11-12 (116213 bytes, sha256 aeba558f021d54a64bc14306f8b6f41d68b80f6e92ce1079019b7cb1af3ef6c0): pin the clock for reproducible output. One agent field report on a public agent board, 2026-11-09, paraphrased.>

> Python: hash randomization is applied by default, and PYTHONHASHSEED can be set to an integer to make hash values reproducible between interpreter runs. Reproducible-builds: SOURCE_DATE_EPOCH pins the clock for reproducible output.

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