pandas-dev/pandas · error · ValueError

cannot assign without a target object

Error message

cannot assign without a target object

What it means

Raised by visit_Assign() when an assignment expression is encountered but self.env.target is None — i.e., there is no target object to assign into. This is the parser-level counterpart of the eval-level checks; it fires when the expression contains '=' (assignment) but no target was provided to the eval call. The target is what receives the new column/value.

Solutions

1. Use DataFrame.eval for column assignments: df.eval('c = a + b').
2. If using pd.eval, pass a target: pd.eval('c = a + b', target=df) or target={}.

Example fix

// before
pd.eval('c = a + b')

// after
df.eval('c = a + b')
// or
pd.eval('c = a + b', target=df)

has_assign = isinstance(expr, str) and '=' in expr
if has_assign and target is None:
    raise ValueError('Assignment expressions require a target — pass target=df or use df.eval')
pd.eval(expr, target=target)

try:
    pd.eval(expr)
except ValueError as e:
    if 'cannot assign without a target' in str(e):
        result = df.eval(expr)
    else:
        raise

When it happens

Trigger: Calling pd.eval('c = a + b') without target= (target defaults to None). The expression contains an assignment but there is nowhere to put the result.

Common situations: Using pd.eval instead of df.eval for an assignment expression; forgetting to pass target when using the top-level eval function with an assignment.

Related errors

• Cannot assign expression output to target
• Cannot return a copy of the target
• Multi-line expressions are only valid if all expressions…
• can only assign a single expression
• Cannot operate inplace if there is no assignment

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/ad5c0bc6b8c8942e. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/computation/expr.py:628

        return slice(lower, upper, step)

    def visit_Assign(self, node, **kwargs):
        """
        support a single assignment node, like

        c = a + b

        set the assigner at the top level, must be a Name node which
        might or might not exist in the resolvers

        """
        if len(node.targets) != 1:
            raise SyntaxError("can only assign a single expression")
        if not isinstance(node.targets[0], ast.Name):
            raise SyntaxError("left hand side of an assignment must be a single name")
        if self.env.target is None:
            raise ValueError("cannot assign without a target object")

        try:
            assigner = self.visit(node.targets[0], **kwargs)
        except UndefinedVariableError:
            assigner = node.targets[0].id

        self.assigner = getattr(assigner, "name", assigner)
        if self.assigner is None:
            raise SyntaxError(
                "left hand side of an assignment must be a single resolvable name"
            )

        return self.visit(node.value, **kwargs)

    def visit_Attribute(self, node, **kwargs):
        attr = node.attr
        value = node.value

View on GitHub (pinned to 3b7651241d)