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

The "exception-vs-error value" debate is partly about code clarity. Consider code with an error value:

idx = sequence.index(x)
if idx == ERROR:
    # do error processing
else:
    print '%s occurred at position %s.' % (x, idx)

The error handling ends up stuffed in the middle of our algorithm, obscuring program flow. On the other hand:

try:
    idx = sequence.index(x)
    print '%s occurred at position %s.' % (x, idx)
except IndexError:
    # do error processing

In this case, the amount of code is effectively the same, but the main algorithm is unbroken by error handling.