Allow operator 'getter' methods to take a list and return a tuple

[bitdancer]

BPO	16457
Nosy	@rhettinger, @ncoghlan, @bitdancer

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2012-11-13.15:14:30.520>
created_at = <Date 2012-11-12.00:17:38.997>
labels = ['type-feature', 'library']
title = "Allow operator 'getter' methods to take a list and return a tuple"
updated_at = <Date 2012-11-13.15:14:30.518>
user = 'https://github.com/bitdancer'

bugs.python.org fields:

activity = <Date 2012-11-13.15:14:30.518>
actor = 'r.david.murray'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2012-11-13.15:14:30.520>
closer = 'r.david.murray'
components = ['Library (Lib)']
creation = <Date 2012-11-12.00:17:38.997>
creator = 'r.david.murray'
dependencies = []
files = []
hgrepos = []
issue_num = 16457
keywords = []
message_count = 4.0
messages = ['175412', '175415', '175486', '175500']
nosy_count = 3.0
nosy_names = ['rhettinger', 'ncoghlan', 'r.david.murray']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue16457'
versions = ['Python 3.4']

[bitdancer]

Consider this code snippet, simplified from a real application:

def display(self, *columns)
    getter = attrgetter(*columns)
    toprint = [[str(x) for x in getter(row)] for row in self._rows]

This works great...as long as there are two or more columns to print.
If there is only one column, it bombs because the getter returns a value
instead of a tuple.

This would not be a problem in and of itself, but there is no way to
tell attrgetter that I want a tuple even if there is only one value.

I believe it would be backward compatible to allow:

    attrgetter(['a'])
    itemgetter(['a', 'b'])

to return a tuple, since a list cannot be an attribute name. The same
would apply to itemgetter, since a list cannot be a dictionary key.

[ncoghlan]

Lists can't be dictionary keys, but they can appear in other mappings (e.g. an id-keyed mapping). So, while you could add this capability to attrgetter without breaking backwards compatibility, it's not possible for itemgetter.

[rhettinger]

It's unfortunate that the automatic scalar/tuple switchover design doesn't play well with start-args. We have the same issue arising in a number of places (for example, min(*args) isn't happy when args is of length 1).

While inconvenient for variable length argument lists, I don't think the proposed solution is clean. As it currently stands, the signature is reasonably simple, easy to explain, and doesn't depend on exact type checks (like %-formatting does with tuple/dict arguments).

Instead of contorting the signature for itemgetter(), it would be better if the use case were to be addressed with the existing language features:

>>> t = tuple('abcdefghi')
>>> for columns in ([], [2], [2,4], [2,4,6]):
	print(tuple(t[i] for i in columns))
	
()
('c',)
('c', 'e')
('c', 'e', 'g')

[bitdancer]

Given Nick's point about itemgetter, I agree this isn't worth doing. I wouldn't want the signatures of attrgetter and itemgetter to no longer be parallel.

Min isn't a problem, by the way, since it accepts an iterator as a single argument.