pandas-dev/pandas · error · SyntaxError

can only assign a single expression

Error message

can only assign a single expression

What it means

Raised by visit_Assign() when the assignment node has more than one target (len(node.targets) != 1). pandas eval supports only a single assignment target like 'c = a + b'; chained or multiple-target assignments such as 'a = b = 1' or tuple unpacking 'a, b = 1, 2' are rejected because the assigner model tracks exactly one name.

Solutions

1. Split into one assignment per line: df.eval('a = 1\nb = 1').
2. Assign each target in a separate df.eval call or via regular DataFrame column assignment df['a'] = 1; df['b'] = 1.

Example fix

// before
df.eval('a = b = 1')

// after
df.eval('a = 1\nb = 1')

import re
def is_single_target_assign(expr: str) -> bool:
    # reject chained 'a = b ='
    return expr.count('=') == 1
if not is_single_target_assign(expr):
    raise ValueError('Use one assignment target per line')
df.eval(expr)

try:
    df.eval(expr)
except SyntaxError as e:
    if 'single expression' in str(e):
        # split chained assignments
        ...

When it happens

Trigger: df.eval('a = b = 1') (chained assignment); df.eval('a, b = 1, 2') (tuple unpacking, though this may hit a different error first); any expression with multiple '=' targets in one statement.

Common situations: Trying to initialize multiple columns in one eval line; porting Python multiple-assignment idioms into the eval DSL.

Related errors

• left hand side of an assignment must be a single name
• left hand side of an assignment must be a single resolvable…
• Cannot assign expression output to target
• cannot assign without a target object
• Cannot operate inplace if there is no assignment

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/c21a70eddc9a268a. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/computation/expr.py:624

            upper = self.visit(upper).value
        step = node.step
        if step is not None:
            step = self.visit(step).value

        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)

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