Lightning-AI/pytorch-lightning · warning · NotImplementedError

__setitem__ is not supported

Error message

__setitem__ is not supported

What it means

_Monotonic overrides list to enforce strictly increasing values; item assignment cannot be validated cheaply, so __setitem__ deliberately raises NotImplementedError. Users are meant to only append values, never mutate history.

Solutions

1. Rebuild the container by appending values in increasing order instead of assigning by index
2. If you need a mutable history, copy the values into a plain list: list(monitor._elapsed_interval)
3. Avoid touching private attributes prefixed with underscore

Example fix

# before
monitor._elapsed_interval[0] = 1.0

# after
values = sorted(list(monitor._elapsed_interval))
values[0] = 1.0
new_list = _Monotonic(maxlen=len(values))
for v in sorted(values):
    new_list.append(v)

# nothing to validate: simply never assign into the container
values = list(monotonic_list)  # copy out if you need to edit

When it happens

Trigger: Directly indexing the internal list used by ThroughputMonitor (e.g. monitor._elapsed_interval[i] = value) or any code that treats the container as a plain list and assigns by index or slice.

Common situations: Test helpers or monkeypatching that try to rewrite recorded values; copy/deepcopy or serialization code that repopulates via item assignment rather than append; debugging code that edits history.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

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• The ` ` does not use the `CheckpointIO` plugin interface.
• A dataloader should be passed only once to the…

AI-assisted analysis of Lightning-AI/pytorch-lightning@9fed5c27d2 (2026-08-28). Data as JSON: /api/errors/8225afa7790e0f72. Report an issue: GitHub.

Appendix: source

Thrown at src/lightning/fabric/utilities/throughput.py:719

    def last(self) -> Optional[T]:
        if len(self) > 0:
            return self[-1]
        return None

    @override
    def append(self, x: T) -> None:
        last = self.last
        if last is not None and last >= x:
            raise ValueError(f"Expected the value to increase, last: {last}, current: {x}")
        list.append(self, x)
        # truncate excess
        if len(self) > self.maxlen:
            del self[0]

    @override
    def __setitem__(self, key: Any, value: Any) -> None:
        # assigning is not implemented since we don't use it. it could be by checking all previous values
        raise NotImplementedError("__setitem__ is not supported")

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