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From a3bbd7eeabee7c6b229e95e0e04af5b430ea32db Mon Sep 17 00:00:00 2001
From: Eric Eastwood <erice@element.io>
Date: Tue, 13 May 2025 10:40:49 -0500
Subject: [PATCH 06/34] Explain why we `flush_buffer()` for Python `print(...)`
output (#18420)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Spawning from using this code elsewhere and not knowing why it's there.
Based on this article and @reivilibre's experience mentioning
`PYTHONUNBUFFERED=1`,
> #### programming languages where the default “print” statement buffers
>
> Also, here are a few programming language where the default print
statement will buffer output when writing to a pipe, and some ways to
disable buffering if you want:
>
> - Python (disable with `python -u`, or `PYTHONUNBUFFERED=1`, or
`sys.stdout.reconfigure(line_buffering=False)`, or `print(x,
flush=True)`)
>
> _--
https://jvns.ca/blog/2024/11/29/why-pipes-get-stuck-buffering/#programming-languages-where-the-default-print-statement-buffers_
---
changelog.d/18420.misc | 1 +
docker/configure_workers_and_start.py | 5 +++++
docker/start.py | 5 +++++
3 files changed, 11 insertions(+)
create mode 100644 changelog.d/18420.misc
diff --git a/changelog.d/18420.misc b/changelog.d/18420.misc
new file mode 100644
index 0000000000..d52175af91
--- /dev/null
+++ b/changelog.d/18420.misc
@@ -0,0 +1 @@
+Explain why we `flush_buffer()` for Python `print(...)` output.
diff --git a/docker/configure_workers_and_start.py b/docker/configure_workers_and_start.py
index df34d51f77..102a88fad1 100755
--- a/docker/configure_workers_and_start.py
+++ b/docker/configure_workers_and_start.py
@@ -352,6 +352,11 @@ def error(txt: str) -> NoReturn:
def flush_buffers() -> None:
+ """
+ Python's `print()` buffers output by default, typically waiting until ~8KB
+ accumulates. This method can be used to flush the buffers so we can see the output
+ of any print statements so far.
+ """
sys.stdout.flush()
sys.stderr.flush()
diff --git a/docker/start.py b/docker/start.py
index 818a5355ca..0be9976a0c 100755
--- a/docker/start.py
+++ b/docker/start.py
@@ -22,6 +22,11 @@ def error(txt: str) -> NoReturn:
def flush_buffers() -> None:
+ """
+ Python's `print()` buffers output by default, typically waiting until ~8KB
+ accumulates. This method can be used to flush the buffers so we can see the output
+ of any print statements so far.
+ """
sys.stdout.flush()
sys.stderr.flush()
--
2.49.0
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