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Why Your Python Logs Vanish in Docker (and How PYTHONUNBUFFERED=1 Saves the Day)

Your Python app works fine locally, but in Docker, the logs are MIA. Here's why Python's output buffering drives container users crazy—and how one environment variable fixes everything.

Posted
By Wewake
5 min read

The Mystery: Why are my logs disappearing ?

This has unfortnately happened almost every time I have come back to using python in my little side projects, or experiments.

I create a neat little Python app. It runs perfectly, printing helpful logs at every step:

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# app.py
print("Starting application...")
print("Connecting to database...")
print("Processing request...")
print("All done!")

I run it locally:

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$ python app.py
Starting application...
Connecting to database...
Processing request...
All done!

All good! I containerize it:

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FROM python:3.12-slim
COPY app.py .
CMD ["python", "app.py"]

Build and run it in Docker:

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$ docker build -t myapp .
$ docker run myapp
$ docker logs <container-id>
#

But the logs are nowhere to be seen. Where did it go ? If running k8, I check kubectl logs , same thing. No logs.

The logs, like a magician’s trick, have vanished into thin air. What sorcery is this?

Spoiler: It’s Python’s output buffering, and it’s been frustrating developers since Docker became popular.

What is Output Buffering?

Output buffering is when your program doesn’t immediately write output to its destination (like your terminal or Docker logs). Instead, it collects output in a temporary buffer (a chunk of memory) and writes it all at once.

Think of it like this:

Without buffering:

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[Your code] ---> "Hello" ---> [Terminal]
            ---> "World" ---> [Terminal]
            ---> "!!!"    ---> [Terminal]

With buffering :

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[Your code] ---> "Hello" --\
            ---> "World" ---}--[Buffer]---> [Terminal] (when full or flushed)
            ---> "!!!"   --/

Why Buffering Exists ?

Buffering is a performance optimization. Writing to output (terminal, files, network) is slow compared to memory operations.

The Three Types of Buffering

Python (and most languages) uses three buffering modes:

  1. Unbuffered: Write immediately (no buffer)
    • Used for: stderr by default
    • Behavior: Every print() appears instantly
  2. Line-buffered: Write after each newline (\n)
    • Used for: stdout when connected to a terminal (TTY)
    • Behavior: Output appears after each line
  3. Block-buffered: Write when buffer is full (default: 8KB)
    • Used for: stdout when NOT connected to a terminal (pipes, files, Docker)
    • Behavior: Output appears in chunks, or when program exits

Note: Python detects if stdout is connected to a terminal. If yes, it line-buffers. If no (like in Docker), it block-buffers.

Want to see the actual buffer size? You can check it yourself:

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import io
print(f"Default buffer size: {io.DEFAULT_BUFFER_SIZE} bytes")
# Output: Default buffer size: 8192 bytes

This io.DEFAULT_BUFFER_SIZE constant (defined in Python’s io module) is what Python uses for block-buffering. It’s typically 8192 bytes (8KB), though it can vary based on the system’s block size.

Why Docker Makes This Worse

When you run Python in a Docker container:

  1. stdout is NOT a TTY (it’s a pipe to Docker’s logging driver)
  2. Python switches to block-buffering (8KB buffer by default)
  3. The logs sit in memory until:
    • The buffer fills up, OR
    • The program exits, OR
    • You explicitly flush

It’s a classic case of “works on my machine” syndrome—because your machine is a TTY!

The Solution: PYTHONUNBUFFERED=1

Set the PYTHONUNBUFFERED environment variable to any non-empty value. This forces Python to use unbuffered mode for stdout and stderr.

In Dockerfile:

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FROM python:3.12-slim

# Add this line!
ENV PYTHONUNBUFFERED=1

COPY app.py .
CMD ["python", "app.py"]

In docker-compose.yml:

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version: '3.8'
services:
  app:
    build: .
    environment:
      - PYTHONUNBUFFERED=1

In Kubernetes deployment:

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apiVersion: apps/v1
kind: Deployment
metadata:
  name: python-app
spec:
  template:
    spec:
      containers:
      - name: app
        image: myapp:latest
        env:
        - name: PYTHONUNBUFFERED
          value: "1"

At runtime:

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docker run -e PYTHONUNBUFFERED=1 myapp

Now our logs will appear instantly:

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$ docker logs myapp
Starting application...
Connecting to database...
Processing request...
All done!

Fun fact: The value doesn’t matter. PYTHONUNBUFFERED=1, PYTHONUNBUFFERED=true, even PYTHONUNBUFFERED=banana all work. Python just checks if the variable is set and non-empty.

Alternative Solutions

While PYTHONUNBUFFERED=1 is the easiest fix, here are other approaches:

Use python unbuffered commandline option

The -u flag forces unbuffered mode:

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FROM python:3.12-slim
COPY app.py .
CMD ["python", "-u", "app.py"]  # Note the -u flag

This is equivalent to PYTHONUNBUFFERED=1, just more explicit.

Pros: No environment variable needed
Cons: Have to remember to add -u everywhere you run Python

Flush Manually

Call flush() explicitly when you want output to appear:

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import sys

print("Critical log message", flush=True)  # Appears immediately

# Or flush everything:
sys.stdout.flush()

Pros: Fine-grained control over when to flush
Cons:

  • Easy to forget
  • Clutters your code
  • Doesn’t help with third-party libraries that use print()

Use stderr in Python’s logging module

stderr is unbuffered by default.

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import logging
import sys

# Configure logging to stderr (which is unbuffered by default)
logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(levelname)s - %(message)s',
    stream=sys.stderr  # stderr is unbuffered!
)

logger = logging.getLogger(__name__)

logger.info("Application starting")
logger.info("Connected to database")
logger.error("Something went wrong!")

Why this works:

  1. stderr is unbuffered by default (even in Docker)
  2. Structured logging is better for production anyway
  3. You get timestamps, log levels, and better formatting

Conclusion

The PYTHONUNBUFFERED=1 environment variable is a tiny fix that solves a huge headache. It forces Python to stop buffering output, so your logs appear immediately in Docker and Kubernetes.

References

Python
python docker kubernetes logging debugging containers pythonbuffered tty terminal
This post is licensed under CC BY 4.0 by the author.

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