Python list.index throws exception when index not found

Question

Why does list.index throw an exception, instead of using an arbitrary value (for example, -1)? What\'s the idea behind this?

To me it looks cleaner to d

Answer 1

def GetListIndex(list, searchString):
    try:
        return list.index(searchString)

    except ValueError:
        return False

    except Exception:
        raise

Answer 2

Because -1 is itself a valid index. It could use a different value, such as None, but that wouldn't be useful, which -1 can be in other situations (thus str.find()), and would amount simply to error-checking, which is exactly what exceptions are for.

Answer 3

I agree with Devin Jeanpierre, and would add that dealing with special values may look good in small example cases but (with a few notable exceptions, e.g. NaN in FPUs and Null in SQL) it doesn't scale nearly as well. The only time it works is where:

• You've typically got lots of nested homogeneous processing (e.g. math or SQL)
• You don't care about distinguishing error types
• You don't care where the error occurred
• The operations are pure functions, with no side effects.
• Failure can be given a reasonable meaning at the higher level (e.g. "No rows matched")

Answer 4

One simple idea: -1 is perfectly usable as an index (as are other negative values).