If you try to look up an out-of-range address from an object returned
by ipaddress.ip_network, then ipaddress._BaseNetwork.__getitem__
raises an IndexError with no message:

    Python 3.4.0b3 (default, Jan 27 2014, 02:26:41) 
    [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import ipaddress
    >>> ipaddress.ip_network('2001:db8::8/125')[100]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/ipaddress.py", line 601, in __getitem__
        raise IndexError
    IndexError

Normally an IndexError is associated with a message explaining the
cause of the error. For example:

    >>> [].pop()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IndexError: pop from empty list

It would be nice if the IndexError from
ipaddress._BaseNetwork.__getitem__ included a message like this.

With the attached patch, the error message looks like this in the
positive case:

    >>> ipaddress.ip_network('2001:db8::8/125')[100]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/Users/gdr/hg.python.org/cpython/Lib/ipaddress.py", line 602, in __getitem__
        % (self, self.num_addresses))
    IndexError: 100 out of range 0..7 for 2001:db8::8/125

and like this in the negative case:

    >>> ipaddress.ip_network('2001:db8::8/125')[-100]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/Users/gdr/hg.python.org/cpython/Lib/ipaddress.py", line 608, in __getitem__
        % (n - 1, self.num_addresses, self))
    IndexError: -100 out of range -8..-1 for 2001:db8::8/125

(If you have a better suggestion for how the error message should
read, I could submit a revised patch. I suppose it could just say
"address index out of range" for consistency with list.__getitem__ and
str.__getitem__. But I think the extra information is likely to be
helpful for the programmer who is trying to track down the cause of an
error.)

Could someone review the attached patch please.  I've looked at the test code and there is one assertRaises for IndexError which I'm assuming covers this case.

+1 for "address index out of range". The current test only covers the first IndexError. We also need to add another one for the else branch.

I've attached a revised patch that addresses Berker Peksag's concerns:

1. The message associated with the IndexError is now "address out of range" with no information about which address failed or why.

2. There's a new test case for an IndexError from an IPv6 address lookup.

Thank you Gareth. I will commit ipaddress.patch this weekend.

New changeset bc758c62bc4f by Berker Peksag in branch 'default':
Issue #20508: Improve exception message of IPv{4,6}Network.__getitem__
https://hg.python.org/cpython/rev/bc758c62bc4f

Thank you for applying this patch.