Question

What is the pythonic way for checking a user provided vector index?

def get_value(vector, index):
    try:
        return vector[index]
    except IndexError:
        raise ValueError('bad index')

or

def get_value(vector, index):
   if -1 < index < len(vector):
       return vector[index]
   else:
       raise ValueError('bad index')

Answer 1

Trying first is more Pythonic. From the glossary:

EAFP

Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.

Although I wouldn’t even put that around a try…except since you’re just throwing an IndexError, which it already does.