This error is usually not a Python problem. It happens when LANG/LC_* points to a locale that isn’t available in the OS.

What happens

setlocale(..., '') means “use whatever locale LANG/LC_* says”. In slim containers and CI images, that locale is often not generated.

Concrete example: LANG=en_US.UTF-8, but en_US.UTF-8 isn’t generated on the system, so the OS can’t activate it.

Two fixes

Fix 1 (default): use C.UTF-8

If you don’t need locale-specific sorting/formatting and only need correct UTF-8 handling, C.UTF-8 is usually the easiest option for Docker and CI.

ENV LANG=C.UTF-8 \
    LC_ALL=C.UTF-8

If C.UTF-8 isn’t available in your image, use Fix 2.

Fix 2 (when you need a real locale): generate it

If you do need locale-aware formatting (or a specific locale like en_US.UTF-8), generate that locale in the OS image and set LANG/LC_ALL to match exactly.

Debian/Ubuntu example:

RUN apt-get update \
    && apt-get install -y --no-install-recommends locales \
    && sed -i 's/^# *\(en_US.UTF-8 UTF-8\)/\1/' /etc/locale.gen \
    && locale-gen en_US.UTF-8 \
    && rm -rf /var/lib/apt/lists/*

ENV LANG=en_US.UTF-8 \
    LC_ALL=en_US.UTF-8

Configure locale explicitly and this error goes away permanently.