pathwaycom/pathway · error · AttributeError

transformer has no attribute `{table_name}`

Error message

transformer has no attribute `{table_name}`

What it means

Raised by TransformerReference.__getattr__ when a row transformer accesses self.transformer.<name> but <name> is not among the transformer's class_args (the tables declared in the @pw.table_transformer decorator). It is an AttributeError thrown during trace building, before any data flows. The check is `table_name not in self.row_reference._class_arg.transformer.class_args`.

Source

Thrown at python/pathway/internals/graph_runner/row_transformer_operator_handler.py:237

            self.table_name
        ]
        return table_class_arg(self.row_reference, index)  # type: ignore[call-arg]

    def pointer_from(self, *args, optional=False):
        # XXX verify
        return api.ref_scalar(*args, optional=optional)


@dataclass
class TransformerReference:
    """Helper class to support `self.transformer.foo_class[id] access."""

    row_reference: RowReference

    @trace.trace_user_frame
    def __getattr__(self, table_name: str):
        if table_name not in self.row_reference._class_arg.transformer.class_args:
            raise AttributeError(f"transformer has no attribute `{table_name}`")
        return TransformerReferenceIndexer(self.row_reference, table_name)


class RowReference:
    """Represents a context for particular row.

    Holds all needed information allowing to translate reference to a column by name into
    proper indexes understood by backend.
    """

    id: api.Pointer

    _context: api.Context
    _class_arg: rt.ClassArgMeta
    _mapping: TransformerColumnIndexMapping
    _operator_id: int
    """Identifies an instance of the row transformer."""

View on GitHub (pinned to fa2f74a464)

Solutions

1. Add the missing table name to the @pw.table_transformer decorator's class-argument list so class_args contains it
2. Fix the name in self.transformer.<name> to match an already-declared table argument
3. Check for typos between the decorator declaration and the transformer body

Example fix

# before
@pw.table_transformer
def my_transformer(source):
    return self.transformer.lookup[source.id]  # 'lookup' not declared

# after
@pw.table_transformer
 def my_transformer(source, lookup):
    return self.transformer.lookup[source.id]

import inspect

def transformer_tables_declared(transformer_func) -> set[str]:
    sig = inspect.signature(transformer_func)
    return set(sig.parameters) - {"self"}

# before calling: assert the accessed table is declared
assert "lookup" in transformer_tables_declared(my_transformer), "declare 'lookup' in the transformer signature"

When it happens

Trigger: Calling a @pw.table_transformer-decorated function whose body does self.transformer.some_table[...] where 'some_table' was not listed in the decorator's class arguments (e.g. decorator declares input_tables=['t'] but the body uses self.transformer.other).

Common situations: Renaming a transformer input table in the decorator but not in the body; copy-pasting a transformer implementation and forgetting to update the decorator; typos in the table name.

Related errors

• `{self.name}` has no attribute `{name}`
• Join received extra kwargs. You probably want to use TableLi
• synchronization_group can only be set once
• Duplicate expression value given for {name}
• Cannot use expression as boolean.

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/8023dc86403a364f. Report an issue: GitHub.