Great question! Let’s start by breaking down why this difference exists, then walk through simple fixes to replicate pytest’s real-time output behavior in your script.

First, the root cause: Python buffers standard output (stdout) by default. When you run python3 script.py, the buffering rules change based on where stdout is sent:

• If connected to an interactive terminal (your regular command line), it uses line buffering (flushes output when it hits a newline).
• If redirected to a file/pipe or run in a non-interactive environment, it switches to full buffering (only flushes when the buffer is full or the program exits).

Pytest’s -s flag disables output capturing and forces unbuffered stdout, which is why you see updates in real time. Here are 4 straightforward ways to get the same behavior for your script:

1. Use the -u Command-Line Flag #

The easiest fix—no code changes required. Run your script with the -u (unbuffered) flag, which forces stdout and stderr to skip buffering entirely:

python3 -u script.py

This matches exactly how pytest handles output when using -s.

2. Add flush=True to Specific print() Calls #

If you only need certain lines to show up in real time, modify your print statements to explicitly flush the buffer:

print("API call completed: 200 OK", flush=True)

This is perfect for granular control—you don’t have to change how you run the script, just target the outputs that matter most.

3. Set the PYTHONUNBUFFERED Environment Variable #

Enable unbuffered output by setting an environment variable before running your script:

PYTHONUNBUFFERED=1 python3 script.py

Or, if you want this to apply to all Python runs in your session, export the variable first:

export PYTHONUNBUFFERED=1
python3 script.py

This is handy if you don’t want to remember to add -u every time you run the script.

4. Force Unbuffered Stdout in the Script Itself #

If you want the script to enforce real-time output no matter how it’s run, add this at the very top of your code:

import sys
# Set stdout to line-buffered (flushes on newline)
sys.stdout = open(sys.stdout.fileno(), 'w', buffering=1)

Line buffering (buffering=1) works for most cases since print statements usually end with a newline. If you need output without newlines to show up immediately, use binary mode with buffering=0 (note: this requires writing bytes instead of strings).

内容的提问来源于stack exchange，提问作者Open Food Broker