This error is almost never a Python problem. It’s a mismatch between what your environment variables say and what the OS actually has installed.

What’s going on

Somewhere in the Python code, setlocale(..., '') gets called, which means “use whatever locale LANG/LC_* says”. In minimal environments (slim containers, some CI images, freshly set up servers) those locales are often not installed.

So you end up with LANG=en_US.UTF-8 set, but no en_US.UTF-8 locale definition on disk.

Two fixes

1) Use C.UTF-8

If you don’t need locale-specific sorting or formatting and only need correct UTF-8 handling, C.UTF-8 is the easiest option. It works reliably in containers and CI.

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

2) Install the locale you need

If you do need locale-aware formatting, install the locale in your OS image and set LANG/LC_ALL to match. The key is consistency: the environment variables need to match what’s actually installed.

Where to apply the fix

• In containers: use C.UTF-8 unless you have a specific reason not to.
• On VMs: install locales once during provisioning.
• In CI: don’t rely on defaults, set LANG/LC_ALL explicitly.

Once you treat locale as something you configure explicitly rather than assume is there, this error stops coming back.