pytorch/pytorch · error · ValueError

'{_ellipsis}' is not an allowed axis identifier

Error message

'{_ellipsis}' is not an allowed axis identifier

What it means

Thrown in functorch/einops `_parsing.py` when the user-supplied `axes_lengths` mapping contains the ellipsis symbol (`'…'`, or `'...'` converted during parsing) as a key. The ellipsis stands for 'all remaining axes' and cannot be assigned a fixed length.

Source

Thrown at functorch/einops/_parsing.py:230

) -> tuple[ParsedExpression, ParsedExpression]:
    """Parse an `einops`-style pattern into a left-hand side and right-hand side `ParsedExpression` object.

    Args:
        pattern (str): the `einops`-style rearrangement pattern
        axes_lengths (Mapping[str, int]): any additional length specifications for dimensions

    Returns:
       tuple[ParsedExpression, ParsedExpression]: a tuple containing the left-hand side and right-hand side expressions
    """
    # adapted from einops.einops._prepare_transformation_recipe
    # https://github.com/arogozhnikov/einops/blob/230ac1526c1f42c9e1f7373912c7f8047496df11/einops/einops.py
    try:
        left_str, right_str = pattern.split("->")
    except ValueError:
        raise ValueError("Pattern must contain a single '->' separator") from None

    if _ellipsis in axes_lengths:
        raise ValueError(f"'{_ellipsis}' is not an allowed axis identifier")

    left = ParsedExpression(left_str)
    right = ParsedExpression(right_str)

    if not left.has_ellipsis and right.has_ellipsis:
        raise ValueError(
            f"Ellipsis found in right side, but not left side of a pattern {pattern}"
        )
    if left.has_ellipsis and left.has_ellipsis_parenthesized:
        raise ValueError(
            f"Ellipsis is parenthesis in the left side is not allowed: {pattern}"
        )

    return left, right


def validate_rearrange_expressions(
    left: ParsedExpression, right: ParsedExpression, axes_lengths: Mapping[str, int]

View on GitHub (pinned to dcd2ecae77)

Solutions

1. Remove the ellipsis entry from `axes_lengths`
2. If you need to fix the number of remaining axes, name them explicitly in the pattern instead of using `...`

Example fix

# before
y = rearrange(x, '... h -> h', **{'...': 3})

# after
y = rearrange(x, 'b c h -> h')  # name the axes explicitly

_ELLIPSIS = "\u2026"

def axes_lengths_ok(axes_lengths) -> bool:
    return _ELLIPSIS not in axes_lengths and "..." not in axes_lengths

When it happens

Trigger: Calling `rearrange(t, '... h -> h', **{'...': 3})` or otherwise passing an ellipsis key/keyword in the length specification.

Common situations: Programmatically forwarding a config dict of axis lengths that includes a placeholder for the ellipsis; misunderstanding that `...` is a wildcard, not a nameable axis.

Related errors

• Expression may contain dots only inside ellipsis (...)
• Expression may contain dots only inside ellipsis (...); only
• Ellipsis found in right side, but not left side of a pattern
• Ellipsis is parenthesis in the left side is not allowed: {pa
• Anonymous axis should have positive length, not {self.value}

AI-assisted analysis of pytorch/pytorch@dcd2ecae77 (2026-08-14). Data as JSON: /api/errors/5ebbed70e2cd6ad5. Report an issue: GitHub.