PyO3/pyo3 · error

Python stdout len didn't return expected number of lines: {}

Error message

Python stdout len didn't return expected number of lines: {}

What it means

Sanity check after running a small sysconfig query script with the target interpreter: the number of lines captured from the interpreter's stdout does not match the number of print statements the script emitted. The faulty input is the interpreter's output — truncated, buffered, or polluted (e.g. site banner noise) — so the build flags cannot be parsed reliably.

Source

Thrown at pyo3-build-config/src/impl_.rs:1919

        if cfg!(windows) {
            let script = String::from("import sys;print(sys.version_info < (3, 13))");
            let stdout = run_python_script(interpreter.as_ref(), &script)?;
            if stdout.trim_end() == "True" {
                return Ok(Self::new());
            }
        }

        let mut script = String::from("import sysconfig\n");
        script.push_str("config = sysconfig.get_config_vars()\n");

        for k in &BuildFlags::ALL {
            use std::fmt::Write;
            writeln!(&mut script, "print(config.get('{k}', '0'))").unwrap();
        }

        let stdout = run_python_script(interpreter.as_ref(), &script)?;
        let split_stdout: Vec<&str> = stdout.trim_end().lines().collect();
        ensure!(
            split_stdout.len() == BuildFlags::ALL.len(),
            "Python stdout len didn't return expected number of lines: {}",
            split_stdout.len()
        );
        let flags = BuildFlags::ALL
            .iter()
            .zip(split_stdout)
            .filter(|(_, flag_value)| *flag_value == "1")
            .map(|(flag, _)| flag.clone())
            .collect();

        Ok(Self(flags).fixup())
    }

    fn fixup(mut self) -> Self {
        if self.0.contains(&BuildFlag::Py_DEBUG) {
            self.0.insert(BuildFlag::Py_REF_DEBUG);
        }

View on GitHub (pinned to ac9b6899d3)

Solutions

1. Run the failing interpreter manually with -c "import sysconfig; ..." and inspect its stdout for extra output or truncation
2. Disable site banners/user site packages (e.g. python -S) or print statements in sitecustomize that pollute stdout
3. Use a clean, unmodified interpreter installation for the build

When it happens

Trigger: Thrown at pyo3-build-config/src/impl_.rs:1919 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of PyO3/pyo3@ac9b6899d3 (2026-09-05). Data as JSON: /api/errors/65757b7cb557cd61. Report an issue: GitHub.