pathwaycom/pathway · error · ValueError

Table.groupby() cannot have id argument when grouping by mul

Error message

Table.groupby() cannot have id argument when grouping by multiple columns.

What it means

In Table.groupby(), an id= keyword names the single column whose values become the group ids. Combining id= with more than one positional grouping column is contradictory (len(args) > 1 after instance folding), so ValueError is raised telling you not to pass id together with multiple columns.

Source

Thrown at python/pathway/internals/table.py:1239

        ... 10  | Alice | dog
        ... 9   | Bob   | dog
        ... 8   | Alice | cat
        ... 7   | Bob   | dog
        ... ''')
        >>> t2 = t1.groupby(t1.pet, t1.owner).reduce(t1.owner, t1.pet, ageagg=pw.reducers.sum(t1.age))
        >>> pw.debug.compute_and_print(t2, include_id=False)
        owner | pet | ageagg
        Alice | cat | 8
        Alice | dog | 10
        Bob   | dog | 16
        """
        if instance is not None:
            args = (*args, instance)
        if id is not None:
            if len(args) == 0:
                args = (id,)
            elif len(args) > 1:
                raise ValueError(
                    "Table.groupby() cannot have id argument when grouping by multiple columns."
                )
            elif args[0]._column != id._column:
                raise ValueError(
                    "Table.groupby() received id argument and is grouped by a single column,"
                    + " but the arguments are not equal.\n"
                    + "Consider using <table>.groupby(id=...), skipping the positional argument."
                )

        for arg in args:
            if not isinstance(arg, expr.ColumnReference):
                if isinstance(arg, str):
                    raise ValueError(
                        f"Expected a ColumnReference, found a string. Did you mean <table>.{arg}"
                        + f" instead of {repr(arg)}?"
                    )
                else:
                    raise ValueError(

View on GitHub (pinned to fa2f74a464)

Solutions

1. Drop the id= argument when grouping by multiple columns
2. If you need custom ids, group by exactly one column and pass it via id= only: t.groupby(id=t.a)
3. For instance-based windows, use t.groupby(..., instance=t.x) without id

Example fix

# before
result = t.groupby(t.owner, t.pet, id=t.id).reduce(...)

# after
result = t.groupby(t.owner, t.pet).reduce(...)

def groupby_args_ok(args, id) -> bool:
    return id is None or len(args) <= 1

When it happens

Trigger: t.groupby(t.a, t.b, id=t.c); t.groupby(t.a, instance=..., id=...) where instance plus a positional arg already makes args longer than one; id combined with several columns for windowing-by-instance patterns.

Common situations: Adapting instance/window groupby examples and keeping id from a simpler example; forgetting that instance also appends to args.

Related errors

• You cannot use {dep.to_column_expression()} in this reduce s
• You cannot use {dep.to_column_expression()} in this context.
• Expected a ColumnReference, found a string. Did you mean thi
• In JoinResult.groupby() all arguments have to be a ColumnRef
• Table.groupby() received id argument and is grouped by a sin

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/722ccd7ee61cc509. Report an issue: GitHub.