nautechsystems/nautilus_trader · error

config_path must be in format 'module.path:ClassName', was {

Error message

config_path must be in format 'module.path:ClassName', was {config_path}

What it means

Config class loading expects `config_path` in the exact `module.path:ClassName` form (module and class separated by exactly one colon). If splitting on ':' does not yield exactly 2 parts, the path is malformed and the loader bails. This guards against paths with no colon or with extra colons.

Source

Thrown at crates/live/src/python/node.rs:1787

/// This constructor is shared by `add_actor_from_config` and `add_strategy_from_config`.
/// It handles:
/// 1. Importing the config class from the module path
/// 2. Converting the `HashMap<String, serde_json::Value>` to a Python dict
/// 3. Trying kwargs-first construction, falling back to default + setattr
/// 4. Calling `__post_init__` for dataclasses when using the setattr path
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 config_parts: Vec<&str> = config_path.split(':').collect();
    if config_parts.len() != 2 {
        anyhow::bail!("config_path must be in format 'module.path:ClassName', was {config_path}");
    }
    let (config_module_name, config_class_name) = (config_parts[0], config_parts[1]);

    log::debug!(
        "Importing config class from module: {config_module_name} class: {config_class_name}"
    );

    let config_module = py
        .import(config_module_name)
        .map_err(|e| anyhow::anyhow!("Failed to import config module {config_module_name}: {e}"))?;
    let config_class = config_module
        .getattr(config_class_name)
        .map_err(|e| anyhow::anyhow!("Failed to get config class {config_class_name}: {e}"))?;

    // Convert config dict to Python dict
    let py_dict = PyDict::new(py);

    for (key, value) in config {

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Use the format 'package.module:ClassName', e.g. 'my.strategies.config:MyConfig'.
  2. Ensure exactly one colon separates module path from class name.
  3. Verify the module is importable from the working directory / PYTHONPATH after fixing the format.
  4. Trim stray whitespace or quotes around config_path.

Example fix

// before
config_path = "my.strategies.MyStrategyConfig"
// after
config_path = "my.strategies:MyStrategyConfig"
Defensive patterns

Strategy: validation

Validate before calling

def check_config_path(p: str):
    parts = p.split(':')
    assert len(parts) == 2 and parts[0] and parts[1], f"config_path must be 'module.path:ClassName', was {p!r}"

check_config_path(config_path)

Type guard

def is_dotted_path(p: str) -> bool:
    parts = p.split(':')
    return len(parts) == 2 and all(parts)

Try / catch

try:
    node.build(...)  # triggers config class loading
except Exception as e:
    if "must be in format 'module.path:ClassName'" in str(e):
        print("use 'package.module:Class' form")
    raise

Prevention

When it happens

Trigger: Passing a config_path like "my.module.MyConfig" (no colon), "my.module:My:Config" (extra colon), or an empty/whitespace path to the node builder.

Common situations: Windows-style path confusion, copy-paste dropping the colon, or using a class name with a nested module path including extra colons.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/a2b40d6038419413. Report an issue: GitHub.