Out-of-order stdout behavior from python buffering

test.py

import subprocess

print("=== Start of script")
subprocess.run(['ls', '-ld', '/'], check=True)
print("=== End of script")

# $ python3 test.py 
# === Start of script
# drwxr-xr-x 24 root root 4096 Jul  2  2025 /
# === End of script
#
#
# $ python3 test.py | cat
# drwxr-xr-x 24 root root 4096 Jul  2  2025 /
# === Start of script
# === End of script
#
#
# $ PYTHONUNBUFFERED=1 python3 test.py | cat
# === Start of script
# drwxr-xr-x 24 root root 4096 Jul  2  2025 /
# === End of script

More detail: Python buffering can be disabled, line-based, or full. If stdout is a TTY we get line-based, but otherwise it's full buffering.