[refurb] Fix false negative for underscores before sign in Decimal constructor (FURB157)

[danparizher]

Summary

Fixes FURB157 false negative where Decimal("_-1") was not flagged as verbose when underscores precede the sign character. This fixes #21186.

Problem Analysis

The verbose-decimal-constructor (FURB157) rule failed to detect verbose Decimal constructors when the sign character (+ or -) was preceded by underscores. For example, Decimal("_-1") was not flagged, even though it can be simplified to Decimal(-1).

The bug occurred because the rule checked for the sign character at the start of the string before stripping leading underscores. According to Python's Decimal parser behavior (as documented in CPython's _pydecimal.py), underscores are removed before parsing the sign. The rule's logic didn't match this behavior, causing a false negative for cases like "_-1" where the underscore came before the sign.

This was a regression introduced in version 0.14.3, as these cases were correctly flagged in version 0.14.2.

Approach

The fix updates the sign extraction logic to:

1. Strip leading underscores first (matching Python's Decimal parser behavior)
2. Extract the sign from the underscore-stripped string
3. Preserve the string after the sign for normalization purposes

This ensures that cases like Decimal("_-1"), Decimal("_+1"), and Decimal("_-1_000") are correctly detected and flagged. The normalization logic was also updated to use the string after the sign (without underscores) to avoid double signs in the replacement output.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[ntBre]

Thanks, this makes sense to me!