Explanation of 'or' in exception syntax, why is it valid syntax and how does it work?

Question

A colleague of mine mistakenly typed this (simplified) code, and was wondering why his exception wasn't getting caught:

>>> try:
...     raise ValueError
... except IndexError or ValueError:
...     print 'Caught!'
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError

Now I know that the correct syntax to catch both types of exception should be except (IndexError, ValueError):, but why is the above considered valid syntax? And how does it work?

For example, the above code will throw a ValueError and it won't get caught. But take this code:

>>> try:
...     raise IndexError
... except IndexError or ValueError:
...     print 'Caught!'
... 
Caught!

The IndexError will get caught. How is the or evaluated, and what is it evaluated to?!

Thanks for any light you can shed!

Answer 1

That's because IndexError or ValueError is evaluated to IndexError.

>>> IndexError or ValueError
<type 'exceptions.IndexError'>

The or operator returns the first expression that evaluates to True (In this case IndexError), or the last expression, if none of them are True.
So, your except statement is actually equivalent to:

except IndexError:

Answer 2

The result of the Boolean operations or and and is always one of the operands, so foo or bar will evaluate to foo if foo is truthy, or bar if foo if falsy.

In this case both IndexError and ValueError are truthy so IndexError or ValueError evaluates to IndexError, and your except statement is equivalent to except IndexError.