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unreachable-branch

Category: Unreachable Code
Severity: Warning
Triggered by: pyscn analyze, pyscn check

What it does

Flags an if, elif, or else branch that cannot be taken because every preceding branch terminates with return, raise, break, or continue.

Why is this a problem?

When each earlier branch already exits the function or loop, the remaining branch is logically dead. The guard still looks meaningful to a reader, which hides the real control flow.

This usually indicates:

  • Redundant conditions — the else exists only because the if used to fall through.
  • A subtle bug — the author expected the later branch to run in some case, but the earlier exits make it impossible.
  • Stale defensive code — a fallback that can no longer be reached.

Tests cannot cover the branch, and reviewers waste time reasoning about a path that never runs.

Example

def classify(payment):
    if payment.amount < 0:
        raise ValueError("negative amount")
    elif payment.amount == 0:
        return "empty"
    else:
        return "normal"
    return "unknown"   # ← unreachable branch

Use instead

Remove the dead branch, or restructure the earlier branches so the fallback is actually reachable.

def classify(payment):
    if payment.amount < 0:
        raise ValueError("negative amount")
    if payment.amount == 0:
        return "empty"
    return "normal"

Options

Option Default Description
dead_code.detect_unreachable_branches true Set to false to disable this rule.
dead_code.min_severity "warning" Raise to "critical" to hide these findings; lower to "info" to surface more.
dead_code.ignore_patterns [] Regex patterns matched against the source line; matches are suppressed.

References