Stream output from a Python script

Run your script with python3 -u. The -u flag tells Python to write each line to stdout the moment your code produces it, rather than collecting lines in a buffer and writing them out in one block.

websocketd --port=8080 python3 -u ./count.py

import time

for count in range(1, 6):
    print(count)
    time.sleep(0.5)

Connect a client and the five numbers arrive half a second apart. Remove the -u and nothing arrives until the script exits, at which point all five appear together. The output is identical either way. Only its timing changes. Output buffering explains why the runtime does this.

If you do not control the command line

Sometimes you cannot add -u. The script may be launched by its shebang line, or through --dir, or by a wrapper you did not write. In that case flush from inside the script.

Pass flush=True to each print you want delivered immediately:

import time

for count in range(1, 6):
    print(count, flush=True)
    time.sleep(0.5)

Or call sys.stdout.flush() after the writes that matter, which also works if you are producing output with sys.stdout.write rather than print:

import sys
import time

for count in range(1, 6):
    print(count)
    sys.stdout.flush()
    time.sleep(0.5)

Both are equivalent to -u for the lines you apply them to. -u is safer, because it cannot be forgotten on the one print that mattered.

Setting PYTHONUNBUFFERED takes two steps

Python also stops buffering when the environment variable PYTHONUNBUFFERED holds any non-empty value. Exporting it in your shell is not enough on its own.

websocketd builds a fresh environment for every process it launches. It copies across only the variables named by --passenv, so a variable you did not name never reaches your script. Export the variable and name it:

export PYTHONUNBUFFERED=1
websocketd --port=8080 --passenv=PATH,PYTHONUNBUFFERED ./count.py

--passenv replaces the default list rather than adding to it. The default is PATH,LD_LIBRARY_PATH on Linux and PATH,DYLD_LIBRARY_PATH on macOS, so writing --passenv=PYTHONUNBUFFERED on its own hands your script an environment with no PATH in it at all. Name every variable you need, including PATH, on the one flag.

A script that never launches another program may not notice a missing PATH. One that does will: any lookup by bare command name fails with a file-not-found error, which looks nothing like an environment problem from the inside.

Prefer -u unless you specifically need the environment variable, for example because the same script has to behave the same way under a container runtime that sets it.

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