list().index() should provide better error reporting

[djc]

BPO	7252
Nosy	@benjaminp, @djc, @merwok

^{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 2009-11-02.16:15:37.160>
created_at = <Date 2009-11-02.11:59:12.201>
labels = ['library']
title = 'list().index() should provide better error reporting'
updated_at = <Date 2011-11-07.16:49:31.989>
user = 'https://github.com/djc'

bugs.python.org fields:

activity = <Date 2011-11-07.16:49:31.989>
actor = 'eric.araujo'
assignee = 'none'
closed = True
closed_date = <Date 2009-11-02.16:15:37.160>
closer = 'benjamin.peterson'
components = ['Library (Lib)']
creation = <Date 2009-11-02.11:59:12.201>
creator = 'djc'
dependencies = []
files = []
hgrepos = []
issue_num = 7252
keywords = []
message_count = 6.0
messages = ['94825', '94826', '94830', '94831', '94834', '147239']
nosy_count = 3.0
nosy_names = ['benjamin.peterson', 'djc', 'eric.araujo']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue7252'
versions = ['Python 2.6']

[djc]

>>> a = 'b'
>>> [].index(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.index(x): x not in list

This is suboptimal. IMO it would be much more useful if the ValueError
reported the actual value that wasn't in the list, like this:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.index('b'): 'b' not in list

The error in general doesn't really seem to fit in, repeating the code
but with a fake variable name in it. In real contexts, it's mostly just
repeating what's there on a previous line:

File "/home/watt/src/dawkins/ttlib.py", line 86, in shift
bits.append(SHIFTS.index(rest.split('_')[0]))
ValueError: list.index(x): x not in list

So maybe just make it "'b' not in list"? Or do we really need a
reference to the index() method in there?

[djc]

FWIW, quickly grepping through the raises of ValueErrors in the 2.6
stdlib doesn't bring up any other usage of repeat-with-fake-variable-x.

[benjaminp]

How's "object is not in list"?

[djc]

I want the actual value in there, though! So I can spot the bug.

[benjaminp]

r76058 should do the trick.

[merwok]

FWIW, quickly grepping through the raises of ValueErrors in the 2.6
stdlib doesn't bring up any other usage of repeat-with-fake-variable-x.

bpo-13349 begs to differ :)