What causes "Failed to get class {class_name}: {e}" in nautechsystems/nautilus_trader?

Check the class name in the config matches the exported class in the module exactly

Failed to get class {class_name}: {e}

Error message

Failed to get class {class_name}: {e}

What it means

Fired when the Python module imported successfully but does not expose an attribute with the given class name while resolving an actor or strategy specification ('module.path:ClassName'). The input at fault is a registration field naming a class that does not exist in the specified module.

Source

Thrown at crates/system/src/python/registration.rs:549

    if module_name.is_empty() || class_name.is_empty() || class_name.contains(':') {
        anyhow::bail!("{field} must be in format 'module.path:ClassName'");
    }

    Ok((module_name, class_name))
}

fn import_python_class<'py>(
    py: Python<'py>,
    module_name: &str,
    class_name: &str,
) -> anyhow::Result<Bound<'py, PyAny>> {
    let module = py
        .import(module_name)
        .map_err(|e| anyhow::anyhow!("Failed to import module {module_name}: {e}"))?;

    module
        .getattr(class_name)
        .map_err(|e| anyhow::anyhow!("Failed to get class {class_name}: {e}"))
}

fn create_config_instance<'py>(
    py: Python<'py>,
    config_path: &str,
    config: &HashMap<String, serde_json::Value>,
) -> anyhow::Result<Option<Bound<'py, PyAny>>> {
    if config_path.is_empty() && config.is_empty() {
        log::debug!("No config_path or empty config, using None");
        return Ok(None);
    }

    let Some((config_module_name, config_class_name)) = config_path.split_once(':') else {
        anyhow::bail!("config_path must be in format 'module.path:ClassName', was {config_path}");
    };

    if config_module_name.is_empty()
        || config_class_name.is_empty()

View on GitHub (pinned to 18893faf8b)

Solutions

Check the class name in the config matches the exported class in the module exactly
Open the module and confirm the class is defined (or re-exported) at the top level
Update stale configs after renames
Import the module interactively and `dir()` it to list available class names

Example fix

// before
{"strategy": "my_strategy.MomentumStrat"}  # class renamed
// after
{"strategy": "my_strategy.MomentumStrategy"}

Defensive patterns

Strategy: validation

Validate before calling

import importlib
mod = importlib.import_module(module_name)
assert hasattr(mod, class_name), f"{module_name} has no class {class_name}"

Try / catch

try:
    trader.add_strategy(strategy_config)
except Exception as e:
    if "Failed to get class" in str(e):
        logging.error("verify class name in config: %s", e)
    raise

Prevention

Grep the module for `class <Name>` when writing configs
Update configs whenever strategy classes are renamed
Use editor autocomplete on the module to copy exact class names

When it happens

Trigger: Registering a Python actor/strategy with a `module:Class` string where the class name is misspelled, the class was renamed/moved to another module, or only an instance (not the class) is exposed under that name.

Common situations: Refactors renaming strategy classes while old JSON/YAML configs still reference the old name; referencing a function instead of a class; case mismatch (e.g. `myactor` vs `MyActor`).

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/fe6cbbacf2128d4b. Report an issue: GitHub.