pathwaycom/pathway · error · ValueError

Table.from_columns() cannot have empty arguments list

Error message

Table.from_columns() cannot have empty arguments list

What it means

Table.from_columns(*args, **kwargs) builds a new table from column references passed positionally and by keyword. combine_args_kwargs merges both; if the merged dict is empty (no positional and no keyword arguments), ValueError('Table.from_columns() cannot have empty arguments list') is raised because there is no source table to select from.

Source

Thrown at python/pathway/internals/table.py:300

        Returns:
            Table: Created table.


        Example:

        >>> import pathway as pw
        >>> t1 = pw.Table.empty(age=float, pet=float)
        >>> t2 = pw.Table.empty(foo=float, bar=float).with_universe_of(t1)
        >>> t3 = pw.Table.from_columns(t1.pet, qux=t2.foo)
        >>> pw.debug.compute_and_print(t3, include_id=False)
        pet | qux
        """
        all_args = cast(
            dict[str, expr.ColumnReference], combine_args_kwargs(args, kwargs)
        )
        if not all_args:
            raise ValueError("Table.from_columns() cannot have empty arguments list")
        else:
            arg = next(iter(all_args.values()))
            table: Table = arg.table
            for arg in all_args.values():
                if not table._universe.is_equal_to(arg.table._universe):
                    raise ValueError(
                        "Universes of all arguments of Table.from_columns() have to be equal.\n"
                        + "Consider using Table.promise_universes_are_equal() to assert it.\n"
                        + "(However, untrue assertion might result in runtime errors.)"
                    )
            return table.select(*args, **kwargs)

    @trace_user_frame
    @check_arg_types
    def concat_reindex(self, *tables: Table) -> Table:
        """Concatenate contents of several tables.

        This is similar to PySpark union. All tables must have the same schema. Each row is reindexed.

View on GitHub (pinned to fa2f74a464)

Solutions

1. Ensure at least one column reference is passed before calling from_columns
2. If the column set may legitimately be empty, branch and create pw.Table.empty() with the desired schema instead
3. Log the generated argument list when constructing calls dynamically

Example fix

# before
t = pw.Table.from_columns(*cols)  # cols == [] -> ValueError

# after
if not cols:
    t = pw.Table.empty()
else:
    t = pw.Table.from_columns(*cols)

def has_columns(args, kwargs) -> bool:
    return len(args) + len(kwargs) > 0

When it happens

Trigger: Calling pw.Table.from_columns() with zero arguments; programmatically building the argument list from a loop/dict that turns out empty; passing an empty dict via **kwargs expansion.

Common situations: Dynamic pipelines that assemble columns from configuration or data-driven lists which can be empty; guard clauses that skip appending arguments entirely.

Related errors

• You cannot use {dep.to_column_expression()} in this reduce s
• join condition should be of form <left_table>.<column> == <r
• Schema.with_types() argument name has to be an existing colu
• Schema.without() argument {name!r} has to refer to an existi
• 'topic' must be a non-empty string; got an empty list. Kafka

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/22687c09b7008436. Report an issue: GitHub.