pathwaycom/pathway · error · TypeError

You cannot instantiate `this` class.

Error message

You cannot instantiate `this` class.

What it means

Pathway's global this object is a singleton proxy used to build column references; it is not a class you can instantiate. __call__ is explicitly blocked with a TypeError so mistakes like pw.this() fail fast instead of producing a broken object.

Source

Thrown at python/pathway/internals/thisclass.py:119

    @trace_user_frame
    def __iter__(self):
        class subclass(self, iter_guard):  # type: ignore[valid-type,misc]
            @classmethod
            def __iter__(self):
                raise TypeError("You cannot iterate over mock class.")

        subclass.__qualname__ = self.__qualname__ + "." + "__iter__" + "(...)"
        subclass.__name__ = "__iter__"
        return iter([subclass])

    def keys(self):
        # _key_guard_counter is necessary, otherwise key-collisions happen
        return [f"{KEY_GUARD}_{next(_key_guard_counter)}"]

    @trace_user_frame
    def __call__(self):
        raise TypeError("You cannot instantiate `this` class.")

    def pointer_from(
        self, *args: Any, optional=False, instance: expr.ColumnReference | None = None
    ):
        return expr.PointerExpression(self, *args, optional=optional, instance=instance)  # type: ignore[arg-type]

    def _base_this(self) -> ThisMetaclass:
        raise NotImplementedError

    def _eval_table(self, table: Joinable) -> Joinable:
        raise NotImplementedError

    def _eval_substitution(
        self, substitution: dict[ThisMetaclass, Joinable]
    ) -> Joinable:
        base_this: ThisMetaclass = self._base_this()
        if base_this not in substitution:
            raise TypeError(f"Usage of {base_this} not supported here.")

View on GitHub (pinned to fa2f74a464)

Solutions

1. Remove the call: use pw.this.column_name or pw.this['column_name'] directly
2. If you meant to define a schema, declare a class inheriting pw.Schema and instantiate that instead
3. If a callable is required by an API, wrap it: lambda: pw.this.x

Example fix

# before
value = pw.this().amount  # TypeError: cannot instantiate `this`

# after
value = pw.this.amount

import pathway as pw

# pw.this must never be called; guard callables at the boundary
def ensure_not_this(obj):
    assert obj is not pw.this, "pw.this is not callable/instantiable"

def is_pathway_this(obj) -> bool:
    import pathway as pw
    return obj is pw.this

try:
    value = pw.this()
except TypeError as e:
    if "instantiate" in str(e):
        value = pw.this  # use the proxy itself
    else:
        raise

When it happens

Trigger: Writing pw.this() (usually a typo when intending pw.this.col or a schema class); attempting PathwayThis(); passing this to APIs that call their argument, e.g. map(this) or functools.partial patterns.

Common situations: Confusion between this and schema classes (users coming from pandas/ORM APIs who reflexively call constructors); copy-paste from code that instantiates custom Schema classes next to this usage.

Related errors

• You cannot iterate over mock class.
• Usage of {base_this} not supported here.
• TableSlice expects {repr(arg.name)} or this.{arg.name} argum
• {name} is a method name. It is discouraged to use it as a co
• Pathway does not support using unary operator {operator_fun.

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/ebbcba08718a7900. Report an issue: GitHub.