IndexError from ipaddress._BaseNetwork.__getitem__ has no message

[gareth-rees]

BPO	20508
Nosy	@ncoghlan, @gareth-rees, @berkerpeksag
Files	ipaddress.patch ipaddress.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-06-11.19:30:19.147>
created_at = <Date 2014-02-04.13:09:31.540>
labels = ['easy', 'type-feature', 'library']
title = 'IndexError from ipaddress._BaseNetwork.__getitem__ has no message'
updated_at = <Date 2016-06-11.19:58:16.111>
user = 'https://github.com/gareth-rees'

bugs.python.org fields:

activity = <Date 2016-06-11.19:58:16.111>
actor = 'gdr@garethrees.org'
assignee = 'none'
closed = True
closed_date = <Date 2016-06-11.19:30:19.147>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2014-02-04.13:09:31.540>
creator = 'gdr@garethrees.org'
dependencies = []
files = ['33903', '43341']
hgrepos = []
issue_num = 20508
keywords = ['patch', 'easy', 'needs review']
message_count = 7.0
messages = ['210224', '235955', '268080', '268219', '268233', '268256', '268261']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'pmoody', 'python-dev', 'gdr@garethrees.org', 'berker.peksag']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20508'
versions = ['Python 3.6']

[gareth-rees]

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.)

[BreamoreBoy]

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.

[berkerpeksag]

+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.

[gareth-rees]

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.

[berkerpeksag]

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

[python-dev]

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

[gareth-rees]

Thank you for applying this patch.