add methods to get first and last elements of a range

[phr]

BPO	47257
Nosy	@mdickinson

^{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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assignee = None
closed_at = <Date 2022-04-08.10:34:44.377>
created_at = <Date 2022-04-08.08:03:40.803>
labels = ['type-feature']
title = 'add methods to get first and last elements of a range'
updated_at = <Date 2022-04-08.10:34:44.376>
user = 'https://bugs.python.org/phr'

bugs.python.org fields:

activity = <Date 2022-04-08.10:34:44.376>
actor = 'phr'
assignee = 'none'
closed = True
closed_date = <Date 2022-04-08.10:34:44.377>
closer = 'phr'
components = []
creation = <Date 2022-04-08.08:03:40.803>
creator = 'phr'
dependencies = []
files = []
hgrepos = []
issue_num = 47257
keywords = []
message_count = 3.0
messages = ['416962', '416971', '416973']
nosy_count = 2.0
nosy_names = ['phr', 'mark.dickinson']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue47257'
versions = []

[phr]

Inspired by a question on comp.lang.python about how to deal with an int set composed of integers and ranges. Range objects like range(1,5,2) contain start, stop, and step values, but it's messy and potentially tricky to get the actual first and last values of the range. Examples:

range(1,5,2) - first = 1, last = 3

range (5, 1, 2) - range is empty, first = last = None

range(5, 1, -1) - first is 5, last is 2

Note in the case where the range is not empty, you can get the "last" by a messy calculation but it's easier to pick the first element from the reverse iterator. But then you might forget to catch the stopiteration exception in the case that the list is empty. The same goes for the first element, roughly. And constructing the iterators just to pick one element seems like unnecessary overhead.

So it is better to have actual methods for these, with type Optional[int]. Then mypy should remind you to check for the empty case if you forget.

[mdickinson]

but it's messy and potentially tricky to get the actual first and last values of the range

Doesn't simple indexing already provide what you need here?

>>> range(1, 5, 2)[0]  # first element of range
1
>>> range(1, 5, 2)[-1]  # last element of range
3

[phr]

Oh nice, I didn't realize you could do that. len(range) and bool(range) (to test for empty) also work. Ok I guess this enhancement is not needed. I will close ticket, hope that is procedurally correct, otherwise feel free to fix. Thanks.