Replace Exception with Test

Replace Exception with Test checks a predictable condition before an operation instead of deliberately causing an exception to discover that condition. Here test means a conditional check in the program, not a unit test. The modern name is Replace Exception with Precheck. 1

Original version: indexing discovers an empty list

The function returns the first name or a fallback for an empty, ordinary in-memory list.

public static class Names
{
    public static string FirstOrDefault(List<string> names)
    {
        try { return names[0]; }
        catch (ArgumentOutOfRangeException) { return "Nobody"; }
    }
}

def first_or_default(names):
    try:
        return names[0]
    except IndexError:
        return "Nobody"

An empty list is expected and easy to inspect. Using an exception here can make the ordinary decision less visible.

Refactored version: state the empty-list case

public static class Names
{
    public static string FirstOrDefault(List<string> names)
    {
        if (names.Count == 0) return "Nobody";
        return names[0];
    }
}

def first_or_default(names):
    if len(names) == 0:
        return "Nobody"
    return names[0]

Both versions return Nobody for an empty list and Mira for a list beginning with Mira. The revised branch names the ordinary condition instead of relying on a failed access.

A precheck does not make the later operation safe forever

The examples assume no concurrent mutation between checking the count and reading the element. With shared mutable data, that gap can create a race. Prefer an atomic operation or appropriate synchronization when the state can change.

Likewise, checking that a file exists does not guarantee it will still exist—or be readable—when opened. The open operation can still fail, so exception handling remains necessary. Remote services and permissions introduce similar gaps.

Python often uses direct operations with exception handling intentionally, especially for mappings and protocol-based objects. Do not turn that idiom into a universal violation. Choose a precheck when it accurately expresses a local, stable condition without duplicating work or creating a check-then-act bug.

Sources

1. Martin Fowler — Replace Exception with Precheck.
2. Python — Errors and exceptions.