pathwaycom/pathway · error · ValueError

Table.groupby() received id argument and is grouped by a sin

Error message

Table.groupby() received id argument and is grouped by a single column, but the arguments are not equal.
Consider using <table>.groupby(id=...), skipping the positional argument.

What it means

When Table.groupby() receives both a single positional column and id=, Pathway compares the underlying column objects (args[0]._column != id._column). If they are not the very same column, ValueError is raised: passing two different single columns (one positional, one as id) is ambiguous — the fix is to pass the intended column only via id=.

Source

Thrown at python/pathway/internals/table.py:1243

        ... ''')
        >>> 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(
                        "All Table.groupby() arguments have to be a ColumnReference."
                    )

        self._check_for_disallowed_types(*args)

View on GitHub (pinned to fa2f74a464)

Solutions

Remove the positional argument and keep only id=: t.groupby(id=t.a)
Or remove id= and keep the plain positional column if custom ids are not needed

Example fix

# before
g = t.groupby(t.key, id=t.user_id)  # two different columns -> ValueError

# after
g = t.groupby(id=t.user_id)

Defensive patterns

Strategy: validation

Validate before calling

def id_matches_single_arg(args, id) -> bool:
    return id is None or len(args) != 1 or args[0]._column == id._column

Prevention

Pass the grouping id column exclusively via id=, never also positionally
Clean up leftover positional args when refactoring to id= form

When it happens

Trigger: t.groupby(t.a, id=t.b) with a != b; t.groupby(id=t.id, *args left over from a refactor) where the positional column is a transform of the id column rather than the identical ColumnReference.

Common situations: Refactoring a call from t.groupby(t.id) to t.groupby(id=t.id) and leaving the positional argument in; passing a copy/derived reference that is a different column object.

Related errors

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/e782c4d5cfac3c94. Report an issue: GitHub.