Bypass Accumulation Signal (BAT)

Signal ID: BAT
Full name: Bypass Accumulation
Type: Scoring signal (contributes to drift score)
Default weight: 0.03
Scope: file_local

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What BAT detects

BAT detects files where quality-bypass markers accumulate beyond a density threshold. These markers include # type: ignore, # noqa, # pragma: no cover, typing.Any, cast(), @pytest.mark.skip, and TODO/FIXME comments. Each marker is acceptable in isolation, but accumulation signals systemic quality evasion.

Before — bypass accumulation

from typing import Any, cast

def process(data: Any) -> Any:  # type: ignore[no-any-return]
    result = cast(dict, data)  # noqa: S101
    if result.get("value"):
        return result["value"]  # type: ignore
    return None  # pragma: no cover
    # TODO: handle edge cases
    # FIXME: this sometimes crashes

Seven bypass markers in a 10-line function. The code is more bypass than logic.

After — addressed bypasses

from typing import TypedDict

class ProcessInput(TypedDict):
    value: str | None

def process(data: ProcessInput) -> str | None:
    if data.get("value"):
        return data["value"]
    return None

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Why bypass accumulation matters

• Each bypass is a deferred problem — they represent conscious decisions to skip quality checks.
• AI-generated code often needs bypasses — type stubs are incomplete, so # type: ignore is common.
• Accumulation signals systemic issues — a file with one noqa is fine; a file with ten suggests the code doesn't fit the project's quality standards.
• Quality drift is measurable — BAT makes bypass density visible and trackable over time.

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How the score is calculated

BAT counts bypass markers and calculates density:

\[ \text{bypass\_density} = \frac{\text{bypass\_marker\_count}}{\text{total\_lines}} \]

Files exceeding the density threshold are flagged.

Tracked markers: - # type: ignore (with or without error codes) - # noqa (with or without rule codes) - # pragma: no cover - typing.Any usage - cast() calls - @pytest.mark.skip / @pytest.mark.xfail - TODO / FIXME / HACK / XXX comments

Severity thresholds:

Score range	Severity
≥ 0.7	HIGH
≥ 0.5	MEDIUM
≥ 0.3	LOW
< 0.3	INFO

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How to fix BAT findings

1. Address the underlying issue — each bypass exists for a reason. Fix the root cause.
2. Add proper types instead of Any and cast().
3. Fix linting issues instead of adding noqa.
4. Write tests instead of pragma: no cover.
5. Resolve TODOs — if a TODO has been there for months, either do it or remove it.

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Configuration

# drift.yaml
weights:
  bypass_accumulation: 0.03   # default weight (0.0 to 1.0)

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Detection details

1. Scan all source lines for known bypass markers (regex-based).
2. Count markers per file — total and by category.
3. Calculate bypass density (markers / total lines).
4. Flag files exceeding the density threshold.

BAT is deterministic and text-based (does not require AST parsing).

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Related signals

• TPD (Test Polarity Deficit) — detects incomplete tests. BAT detects skipped/incomplete quality checks.
• BEM (Broad Exception Monoculture) — detects poor error handling. BAT detects the markers that acknowledge poor quality.