Python

The Python programming language is used for the majority of user-facing code in NautilusTrader. Python provides a rich ecosystem of libraries and frameworks, making it ideal for strategy development, data analysis, and system integration.

Code style

PEP-8

The codebase generally follows the PEP-8 style guide. One notable departure is that Python truthiness is not always taken advantage of to check if an argument is None for everything other than collections.

As per the Google Python Style Guide, it's discouraged to use truthiness to check if an argument is/is not None, when there is a chance an unexpected object could be passed into the function or method which will yield an unexpected truthiness evaluation (which could result in a logical error type bug).

"Always use if foo is None: (or is not None) to check for a None value. E.g., when testing whether a variable or argument that defaults to None was set to some other value. The other value might be a value that's false in a boolean context!"

:::note Use truthiness to check for empty collections (e.g., if not my_list:) rather than comparing explicitly to None or empty. :::

We welcome all feedback on where the codebase departs from PEP-8 for no apparent reason.

Type hints

All function and method signatures must include type annotations:

def __init__(self, config: EMACrossConfig) -> None:
def on_bar(self, bar: Bar) -> None:
def on_save(self) -> dict[str, bytes]:
def on_load(self, state: dict[str, bytes]) -> None:

Union syntax: Use PEP 604 union syntax for optional types:

# Preferred
def get_instrument(self, id: InstrumentId) -> Instrument | None:

# Avoid
def get_instrument(self, id: InstrumentId) -> Optional[Instrument]:

Generic types: Use TypeVar for reusable components:

T = TypeVar("T")
class ThrottledEnqueuer(Generic[T]):

Docstrings

The NumPy docstring spec is used throughout the codebase. This needs to be followed consistently so the docs build correctly.

Python docstrings should be written in the imperative mood – e.g. "Return a cached client."

This convention aligns with the prevailing style of the Python ecosystem and makes generated documentation feel natural to end-users.

Private methods

Do not add docstrings to private methods (prefixed with _):

• Docstrings generate public-facing API documentation.
• Docstrings on private methods incorrectly imply they are part of the public API.
• Private methods are implementation details not intended for end-users.

Exceptions where docstrings are acceptable:

• Very complex methods with non-trivial logic, multiple steps, or important edge cases.
• Methods requiring detailed parameter or return value documentation due to complexity.

When a private method needs context (such as a tricky precondition or side effect), prefer a short inline comment (#) near the relevant logic rather than a docstring.

Properties vs methods (PyO3 bindings)

When exposing Rust types to Python via PyO3, use #[getter] (property) or a plain method based on what the call site communicates, not whether the value can change:

• Property (#[getter]): cheap, side-effect-free, attribute-like view of current state. Scalar fields, predicates, and lightweight derived values belong here even if they change over the object's lifetime. Examples: status, side, quantity, price, is_open, has_inputs, realized_pnl, venue_order_id.
• Method (no #[getter]): actions, mutations, nontrivial work, allocations/copies, I/O, or anything that takes arguments. Examples: apply(fill), unrealized_pnl(price), calculate_pnl(...).
• Gray area (prefer method): getters that clone or allocate a collection each call. Using a method signals the cost to the caller. Examples: events(), adjustments(), client_order_ids(), trade_ids().

Test naming

Descriptive names explaining the scenario:

def test_currency_with_negative_precision_raises_overflow_error(self):
def test_sma_with_no_inputs_returns_zero_count(self):
def test_sma_with_single_input_returns_expected_value(self):

Ruff

ruff is used to lint the codebase. Ruff rules can be found in the top-level pyproject.toml, with ignore justifications typically commented.

Cython (legacy)

:::note This section covers Cython conventions for .pyx and .pxd files. :::

For .pyx and .pxd files, make sure all functions and methods returning void or a primitive C type (such as bint, int, double) include the except * keyword in the signature. Without it, Python exceptions are silently ignored.

For more information, see the Cython docs.