pathwaycom/pathway · error · ValueError

Duplicate expression value given for {name}

Error message

Duplicate expression value given for {name}

What it means

Pathway's select/with_columns-style APIs merge positional column references and keyword arguments into a single name->expression mapping. This ValueError is raised when two inputs resolve to the same output column name, because a column cannot be defined twice in one operation. The name for positional args comes from expr.smart_name(), so a positional column and a kwarg (or two columns from different tables with the same name) can silently collide.

Source

Thrown at python/pathway/internals/desugaring.py:304

            evaled_table = arg._eval_substitution(substitution)
            new_kwargs.update(evaled_table)
    return {
        name: _desugar_this_arg(substitution, arg) for name, arg in new_kwargs.items()
    }


def combine_args_kwargs(
    args: Iterable[expr.ColumnReference],
    kwargs: Mapping[str, Any],
    exclude_columns: set[str] | None = None,
) -> dict[str, expr.ColumnExpression]:
    all_args = {}

    def add(name, expression):
        if exclude_columns is not None and name in exclude_columns:
            return
        if name in all_args:
            raise ValueError(f"Duplicate expression value given for {name}")
        if name == "id":
            raise ValueError("Can't use 'id' as a column name")
        if not isinstance(expression, expr.ColumnExpression):
            expression = expr.ColumnConstExpression(expression)
        all_args[name] = expression

    for expression in args:
        add(expr.smart_name(expression), expression)
    for name, expression in kwargs.items():
        add(name, expression)

    return all_args


class DesugaringContext:
    _substitution: dict[thisclass.ThisMetaclass, table.Joinable] = {}

    @property

View on GitHub (pinned to fa2f74a464)

Solutions

1. Remove the duplicate: keep either the positional reference or the kwarg, not both, e.g. use select(a=t.b) to rename instead of passing both t.a and a=...
2. Rename colliding columns explicitly with kwarg syntax (select(left_value=t1.a, right_value=t2.a)) when selecting same-named columns from multiple tables
3. If building kwargs dynamically from a dict, filter out names already supplied positionally (or pass exclude_columns where the API supports it) before calling select

Example fix

// before
table.select(table.a, a=table.b)
// after
table.select(a=table.b)

from pathway.internals import expression as expr

def safe_select_kwargs(args, kwargs):
    names = [expr.smart_name(a) for a in args]
    for name in kwargs:
        if name in names:
            raise ValueError(f"duplicate column name '{name}' in select arguments")
    return kwargs

try:
    table.select(*args, **kwargs)
except ValueError as e:
    if "Duplicate expression value given" in str(e):
        # dedupe names and retry with explicit renames
        ...

When it happens

Trigger: Calling table.select(t.a, a=t.b) or table.select(t1.a, t2.a) where smart_name of a positional reference equals a kwarg key or another positional's name; also select(*t1.columns, **{'a': t2.a}) patterns and with_columns variants that route through combine_args_kwargs.

Common situations: Renaming via kwargs while also passing the original column positionally; selecting same-named columns from two joined tables without renaming; copying a dict comprehension into kwargs that duplicates an existing column name; exclude_columns not covering the duplicate.

Related errors

• Join received extra kwargs. You probably want to use TableLi
• synchronization_group can only be set once
• Can't use 'id' as a column name
• Cannot use expression as boolean.
• unit has to be one of s, ms, us, ns but is {unit}.

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/54b9347e7001505c. Report an issue: GitHub.