TheAlgorithms/Python · error · ValueError

{var_name} must be a list

Error message

{var_name} must be a list

What it means

Raised by viterbi's internal _validate_list (dynamic_programming/viterbi.py:278) when observations_space or states_space is not a Python list. The Viterbi implementation indexes and iterates these collections assuming list semantics, and the validator deliberately rejects other types (tuples, strings, sets, ints) with a message naming the offending parameter via var_name.

Source

Thrown at dynamic_programming/viterbi.py:278

    _validate_list(observations_space, "observations_space")
    _validate_list(states_space, "states_space")


def _validate_list(_object: Any, var_name: str) -> None:
    """
    >>> _validate_list(["a"], "mock_name")
    >>> _validate_list("a", "mock_name")
    Traceback (most recent call last):
            ...
    ValueError: mock_name must be a list
    >>> _validate_list([0.5], "mock_name")
    Traceback (most recent call last):
            ...
    ValueError: mock_name must be a list of strings
    """
    if not isinstance(_object, list):
        msg = f"{var_name} must be a list"
        raise ValueError(msg)
    else:
        for x in _object:
            if not isinstance(x, str):
                msg = f"{var_name} must be a list of strings"
                raise ValueError(msg)


def _validate_dicts(
    initial_probabilities: Any,
    transition_probabilities: Any,
    emission_probabilities: Any,
) -> None:
    """
    >>> _validate_dicts({"c":0.5}, {"d": {"e": 0.6}}, {"f": {"g": 0.7}})
    >>> _validate_dicts("invalid", {"d": {"e": 0.6}}, {"f": {"g": 0.7}})
    Traceback (most recent call last):
            ...
    ValueError: initial_probabilities must be a dict

View on GitHub (pinned to f5988cc097)

Solutions

1. Convert with list(...) at the call site: viterbi(list(observations_space), list(states_space), ...).
2. For numpy arrays use .tolist() to get native Python str elements.
3. Keep the HMM config format aligned with the API (JSON arrays deserialize to lists naturally).

Example fix

# before
viterbi(('walk','shop'), ['rainy','sunny'], initial_p, trans_p, emit_p)  # ValueError

# after
viterbi(['walk','shop'], ['rainy','sunny'], initial_p, trans_p, emit_p)

def as_str_list(x) -> list[str] | None:
    return list(x) if isinstance(x, (list, tuple, set)) and all(isinstance(i, str) for i in x) else None

def is_str_list(x: object) -> TypeGuard[list[str]]:
    return isinstance(x, list) and all(isinstance(i, str) for i in x)

When it happens

Trigger: Calling viterbi with observations_space=('rainy','sunny') (a tuple) or a numpy array instead of a list; passing a string like 'abcd' (a str is not a list); passing an int. The message interpolates the parameter name, e.g. 'observations_space must be a list'.

Common situations: Passing numpy array columns or pandas Series directly from a data pipeline without .tolist(); using tuples for 'immutability' in config; assuming duck typing accepts any iterable.

Related errors

• {var_name} must be a list of strings
• {var_name} must be a dict
• {var_name} all keys must be strings
• {var_name} {nested_text}all values must be {value_type.__nam
• There's an empty parameter

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/65242cad372ce84a. Report an issue: GitHub.