Should we make dict not accept a sequence of sets?

[DeepSpace]

BPO	39178
Nosy	@rhettinger, @stevendaprano, @methane, @serhiy-storchaka

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2020-01-01.15:44:30.164>
created_at = <Date 2020-01-01.14:00:21.874>
labels = ['type-feature', '3.9']
title = 'Should we make dict not accept a sequence of sets?'
updated_at = <Date 2020-01-01.15:44:30.161>
user = 'https://bugs.python.org/DeepSpace'

bugs.python.org fields:

activity = <Date 2020-01-01.15:44:30.161>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-01.15:44:30.164>
closer = 'steven.daprano'
components = []
creation = <Date 2020-01-01.14:00:21.874>
creator = 'DeepSpace'
dependencies = []
files = []
hgrepos = []
issue_num = 39178
keywords = []
message_count = 3.0
messages = ['359155', '359157', '359161']
nosy_count = 5.0
nosy_names = ['rhettinger', 'steven.daprano', 'methane', 'serhiy.storchaka', 'DeepSpace']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39178'
versions = ['Python 3.9']

[DeepSpace]

While writing this SO answer (https://stackoverflow.com/a/59552970/1453822) I came to think, should dict preemptively make sure it doesn't accept a sequence of sets (given that it may lead to wrong output in the good case, and miserably fail in the worst case)?

[serhiy-storchaka]

This is a very special case. I think it is better to leave it to third-party linters.

Actually it is so uncommon, that I have doubts that even linters will want to handle it.

[stevendaprano]

"should dict preemptively make sure it doesn't accept a sequence of sets"

No. The person on StackOverflow made a mistake in their code: they used an unordered data structure (set) instead of an ordered data structure (tuple):

    dict({i,j} for i,j in enumerate(lst))

It is a waste of time to slow down the dict constructor to check for something so unusual as this. Everyone will pay the cost of the checks and virtually no-one will get any benefit.

I'm closing this as "rejected", but if anyone disagrees they can reopen it.

By the way, the best way to initialise a dictionary in this situation is to avoid the unnecessary generator expression and just go straight to enumerate:

    dict(enumerate(lst))