Cannot list python versions, is uv (0.5.19 and newer) instal

Error message

Cannot list python versions, is uv (0.5.19 and newer) installed? Err:
{}

What it means

Windmill uses `uv python list` to discover installable Python versions for worker-side Python environments. If the uv command exits non-zero (uv missing, too old, not on PATH, or broken), the stderr is surfaced wrapped in this message asking whether uv >= 0.5.19 is installed.

Source

Thrown at backend/windmill-worker/src/python_versions.rs:369

                        )
                    }
                })
                .collect::<Result<Vec<PyV>, Error>>()?
                .into_iter()
                .unique()
                .sorted()
                .filter(|pyv| filter.contains(&*pyv))
                .rev()
                .collect_vec();

            *LAST_CHECKED.write().await = Utc::now();
            CACHED_VERSIONS.write().await.replace(list.clone());

            Ok(list)
        } else {
            // If the command failed, print the error
            let stderr = String::from_utf8(output.stderr)?;
            bail!(
                "Cannot list python versions, is uv (0.5.19 and newer) installed? Err:\n{}",
                stderr
            );
        }
    }

    /// Parse lockfile for assigned python version.
    /// If not found returns 3.11
    pub fn parse_from_requirements<S: AsRef<str>>(requirements_lines: &[S]) -> Self {
        Self::try_parse_from_requirements(requirements_lines).unwrap_or(
            // If there is no assigned version in lockfile we automatically fallback to 3.11
            // In this case we have dependencies or other metadata, but no associated python version
            // This is the case for old deployed scripts
            PyVAlias::default().into(),
        )
    }

    /// Parse lockfile for assigned python version.

View on GitHub (pinned to e474e8803c)

Solutions

Install uv >= 0.5.19 on the worker, e.g. `curl -LsSf https://astral.sh/uv/install.sh | sh`, or bump the pinned uv in the worker Dockerfile
Ensure the uv binary is on the PATH of the worker process (system-wide, e.g. /usr/local/bin, not only a user shell's ~/.local/bin)
Run `uv python list` manually as the worker's user and read the raw stderr from the message to fix the underlying failure
Restart the worker after installing/upgrading uv

Example fix

# before (Dockerfile)
RUN apt-get install -y python3
# after
RUN curl -LsSf https://astral.sh/uv/install.sh | sh && uv --version # >= 0.5.19

Defensive patterns

Strategy: try-catch

Validate before calling

#!/bin/sh
command -v uv >/dev/null 2>&1 || { echo 'uv not installed'; exit 1; }
[ "$(uv --version | awk '{print $2}' | tr -d .)" -ge 519 ] || { echo 'uv too old (<0.5.19)'; exit 1; }

Try / catch

match result {
    Err(e) if e.to_string().contains("Cannot list python versions") => {
        // provision uv >= 0.5.19 on the worker, then retry
    }
    other => other?,
}

Prevention

Bake uv >= 0.5.19 into worker images at a system PATH location
Pin and verify the uv version in CI before deploying workers
Smoke-test Python version listing on worker startup health checks
Avoid installing tools only in interactive shells the worker never sees

When it happens

Trigger: Listing available Python versions (worker startup / Python version picker) when `uv` is not installed, is older than 0.5.19 (which added `python list`), is not on the worker process's PATH, or otherwise exits non-zero — the stderr appears after `Err:`.

Common situations: Self-hosted worker image without uv; uv installed via a version manager not visible to the worker's PATH; an old uv pinned in a Dockerfile; a corrupted or permission-broken uv binary.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/d0edf600be60181b. Report an issue: GitHub.