pathwaycom/pathway · error · ValueError

Received `how` argument but was not expecting any. Consider

Error message

Received `how` argument but was not expecting any.
Consider using a generic join method that handles `how` to decide on a type of a join to be used.

What it means

Pathway splits join APIs into specific methods (join, join_left, join_right, join_outer, join_any, ...) that do not take a 'how' parameter, and generic methods (join_by / the generic entry points) that do. This handler runs with allow_how=False, so passing how=... to a specific join method raises this ValueError pointing you to the generic method. The check happens in the shared kwargs handler used by Table join methods.

Source

Thrown at python/pathway/internals/arg_handlers.py:79

        raise ValueError(
            "Table.windowby() received extra kwargs.\n"
            + "You probably want to use Table.windowby(...).reduce(**kwargs) to compute output columns."
        )
    return (self, time_expr), {
        "window": window,
        "behavior": behavior,
        "instance": instance,
    }


def join_kwargs_handler(*, allow_how: bool, allow_id: bool):
    def handler(self, other, *on, **kwargs):
        processed_kwargs = {}
        if "how" in kwargs:
            how = kwargs.pop("how")
            processed_kwargs["how"] = how
            if not allow_how:
                raise ValueError(
                    "Received `how` argument but was not expecting any.\n"
                    + "Consider using a generic join method that handles `how` "
                    + "to decide on a type of a join to be used."
                )
            elif isinstance(how, JoinMode):
                pass
            elif isinstance(how, str):
                raise ValueError(
                    "Received `how` argument of join that is a string.\n"
                    + "You probably want to use one of "
                    + "JoinMode.INNER, JoinMode.LEFT, JoinMode.RIGHT or JoinMode.OUTER values."
                )
            else:
                raise ValueError(
                    "How argument of join should be one of "
                    + "JoinMode.INNER, JoinMode.LEFT, JoinMode.RIGHT or JoinMode.OUTER values."
                )

View on GitHub (pinned to fa2f74a464)

Solutions

1. Drop the how kwarg from the specific method: t.join_left(t2, t1.k == t2.k).
2. Or switch to the generic form that accepts how: t.join(t2, t1.k == t2.k, how=pw.JoinMode.LEFT) via the generic entry point.
3. Pick the method matching the intended mode: join_inner/join_left/join_right/join_outer.

Example fix

# before
t1.join_left(t2, t1.k == t2.k, how="left")

# after
t1.join_left(t2, t1.k == t2.k)

def join_left_checked(t1, t2, *on, **kwargs):
    if "how" in kwargs:
        raise TypeError("join_left does not accept how; the method already fixes the mode")
    return t1.join_left(t2, *on, **kwargs)

When it happens

Trigger: t1.join_left(t2, t1.k == t2.k, how='left'); t1.join_inner(t2, how=pw.JoinMode.LEFT); any specific-arity join call that includes how.

Common situations: Copy-pasting from generic join examples into specific methods; pandas/sql habits where every join takes how=; refactoring from join(...how=...) to join_left and leaving the kwarg behind.

Related errors

• Received `id` argument but was not expecting any. Not every
• The id argument of a join has to be a ColumnReference.
• Received `how` argument of join that is a string. You probab
• How argument of join should be one of JoinMode.INNER, JoinMo
• Received `id` argument of join that is a string. Did you mea

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/a185f44f537f7106. Report an issue: GitHub.