pathwaycom/pathway · error · ValueError

TableSlice expects {repr(arg.name)} or this.{arg.name} argum

Error message

TableSlice expects {repr(arg.name)} or this.{arg.name} argument as column reference.

What it means

Raised by TableSlice._normalize() when a ColumnReference argument is bound to a 'this'-like object (a ThisMetaclass instance) that is not the global pathway.this. Slices accept plain column names or references of the form this.col (or this['col']) only; any other this-variant reference is rejected.

Source

Thrown at python/pathway/internals/table_slice.py:143

        )

    @trace_user_frame
    def ix_ref(self, *args, optional: bool = False, context=None) -> TableSlice:
        new_table = self._table.ix_ref(*args, optional=optional, context=context)
        return TableSlice(
            {name: new_table[colref._name] for name, colref in self._mapping.items()},
            new_table,
        )

    @property
    def slice(self):
        return self

    def _normalize(self, arg: str | ColumnReference):
        if isinstance(arg, ColumnReference):
            if isinstance(arg.table, ThisMetaclass):
                if arg.table != this:
                    raise ValueError(
                        f"TableSlice expects {repr(arg.name)} or this.{arg.name} argument as column reference."
                    )
            else:
                if arg.table != self._table:
                    raise ValueError(
                        "TableSlice method arguments should refer to table of which the slice was created."
                    )
            return arg.name
        else:
            return arg

View on GitHub (pinned to fa2f74a464)

Solutions

Pass the bare column name string instead of a foreign this reference: without('col') rather than without(other_this.col)
Use the global reference form: pathway.this.col or pathway.this['col']
Inside a join, first materialize the reference against the correct side (table.col) or wait until you are outside the join context

Example fix

# before
slice.without(result.this.col)  # ValueError: expects 'col' or this.col

# after
slice.without("col")
# or
import pathway as pw
slice.without(pw.this.col)

Defensive patterns

Strategy: type-guard

Validate before calling

import pathway as pw

def normalize_ref(ref):
    """Pass strings or global pw.this refs only."""
    if isinstance(ref, pw.ColumnReference):
        assert ref.table is pw.this or isinstance(ref, str), ref
    return ref

Type guard

def is_global_this_ref(ref) -> bool:
    import pathway as pw
    return isinstance(ref, str) or (
        getattr(ref, "table", None) is pw.this
    )

Try / catch

try:
    slice2 = table_slice.without(ref)
except ValueError as e:
    if "expects" in str(e):
        slice2 = table_slice.without(ref.name)  # fall back to bare name
    else:
        raise

Prevention

Prefer plain column-name strings when calling TableSlice methods
Never persist this-references from join contexts into later calls
Standardize on pw.this.col / pw.this['col'] inside expressions

When it happens

Trigger: Passing a column reference built from a custom/joining this object (e.g. this_from a join context, async ingest context this, or a subclass of ThisMetaclass) into TableSlice methods such as without(), rename(), or __getitem__; passing table.this.col where table.this is not the global this.

Common situations: Inside join contexts where pathway exposes a different this for the left/right side; using a ThisMetaclass subclass produced by table slicing machinery; code copied from a join example into plain slice operations.

Related errors

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/b186750a799338d1. Report an issue: GitHub.