astral-sh/ruff · error

{value:?} cannot be converted to EmptyStringCmpOp

Error message

{value:?} cannot be converted to EmptyStringCmpOp

What it means

Pylint's PRM-style rule checking comparisons against empty strings ('' or "") only understands a subset of comparison operators (is, is not, ==, !=). The `TryFrom<&CmpOp>` conversion bails for any other operator, indicating the rule's checker reached a comparison with an unsupported operator — a diagnostic/fix invariant violation.

Source

Thrown at crates/ruff_linter/src/rules/pylint/rules/compare_to_empty_string.rs:132

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum EmptyStringCmpOp {
    Is,
    IsNot,
    Eq,
    NotEq,
}

impl TryFrom<&CmpOp> for EmptyStringCmpOp {
    type Error = anyhow::Error;

    fn try_from(value: &CmpOp) -> Result<Self, Self::Error> {
        match value {
            CmpOp::Is => Ok(Self::Is),
            CmpOp::IsNot => Ok(Self::IsNot),
            CmpOp::Eq => Ok(Self::Eq),
            CmpOp::NotEq => Ok(Self::NotEq),
            _ => bail!("{value:?} cannot be converted to EmptyStringCmpOp"),
        }
    }
}

impl EmptyStringCmpOp {
    fn into_unary(self) -> &'static str {
        match self {
            Self::Is | Self::Eq => "not ",
            Self::IsNot | Self::NotEq => "",
        }
    }
}

impl std::fmt::Display for EmptyStringCmpOp {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let repr = match self {
            Self::Is => "is",
            Self::IsNot => "is not",

View on GitHub (pinned to 15f3fe6b15)

Solutions

1. Update Ruff to the latest release
2. Rewrite the comparison to use `==`/`!=` against '' if the diagnostic is intended
3. File an issue with the offending comparison if reproducible

Example fix

# before
if s == '':
    pass
# after
if not s:
    pass

# Only ==/!=/is/is not comparisons against '' are supported by the rule:
if value == '':
    ...  # flagged; rewrite as `if not value:`

When it happens

Trigger: `try_from` was called with a `CmpOp` other than Is/IsNot/Eq/NotEq (e.g. `<`, `>=`, `in`) during PLC0202/E710-ish empty-string comparison analysis.

Common situations: Effectively unreachable with stock Ruff since the rule only matches supported operators; appears with modified builds or AST-level changes.

Related errors

• `else` is expected to be on its own line
• indented block to start with indentation
• Empty `else` clause
• Compound statement cannot be inlined
• Failed to collapse `with`: {err}

AI-assisted analysis of astral-sh/ruff@15f3fe6b15 (2026-09-05). Data as JSON: /api/errors/05fe19337553de47. Report an issue: GitHub.