aosabook/500lines · error · ValueError

arguments to project tasks must be immutable and hashable, n

Error message

arguments to project tasks must be immutable and hashable, not the {}

What it means

Error "arguments to project tasks must be immutable and hashable, not the {}" thrown in aosabook/500lines.

Source

Thrown at contingent/code/contingent/projectlib.py:203

    return function.__name__, args

class Task(namedtuple('Task', ('task_function', 'args'))):
    """Turn a call to a function into a task 2-tuple.

    Given a task function and an argument list, returns a task 2-tuple
    that encapsulates the call as a single object. `Project` uses these
    task objects for consequence tracking and caching.

    Raises `ValueError` if `args` is not hashable.

    """
    __slots__ = ()

    def __new__(cls, task_function, args):
        try:
            hash(args)
        except TypeError as e:
            raise ValueError('arguments to project tasks must be immutable'
                             ' and hashable, not the {}'.format(e))

        return super().__new__(cls, task_function, args)

    def __repr__(self):
        "Produce a “syntactic,” source-like representation of the task."

        return '{}({})'.format(self.task_function.__name__,
                               ', '.join(repr(arg) for arg in self.args))

View on GitHub (pinned to fba689d101)

Solutions

1. Pass only immutable, hashable argument types (str, int, tuple, frozenset) to project task functions.
2. Convert mutable arguments like lists or dicts to tuples/frozensets before creating the task.
3. Do not pass objects whose __hash__ changes over time, since tasks are cached by hash.

Example fix

task(project.read_files, (tuple(filenames),))  # tuple(...) instead of the mutable list

When it happens

Trigger: Thrown at contingent/code/contingent/projectlib.py:203 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of aosabook/500lines@fba689d101 (2026-08-13). Data as JSON: /api/errors/5cca64d9d3bc73c5. Report an issue: GitHub.