It may happen that you execute a Python script and nothing happenes, i.e. it seems that it “hangs” without showing any output.

Even if you explicitly include some print() statements to debug the issue, it may still not print anything to a terminal.

One of the reasons of why your Python script does not show any output is because it buffers stdout and stderr steams instead of printing them.

This also can happen if you execute the Python script from a CI/CD pipeline (e.g. using Jenkins, Gitlab-CI, TeamCity, etc.) or if you run it using a Dockerfile.

This short note shows how to solve the issue when the Python script “hangs” and doesn’t show any output.

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Python Script Not Showing Output

To force the stdout and stderr streams to send the output of a Python script straight to a terminal (or a log file), you can set the PYTHONUNBUFFERED environment variable (to any value different from 0) or execute the python command with the -u option.

The PYTHONUNBUFFERED environment variable can be set as follows:

# Shell
$ export PYTHONUNBUFFERED=true
$ python script.py

# Dockerfile
ENV PYTHONUNBUFFERED=true
CMD [ "python", "script.py" ]

Or you can simply run the python command with the -u option:

# Shell
$ python -u script.py

# Dockerfile
CMD [ "python", "-u", "script.py" ]

After using any of the methods above, your Python script should start printing the output.

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