additional argument for str.join()

[f-brinkmann]

BPO	43280
Nosy	@stevendaprano, @sweeneyde, @f-brinkmann

^{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 2021-02-21.01:46:52.863>
created_at = <Date 2021-02-20.20:05:58.618>
labels = ['type-feature', 'library', '3.10']
title = 'additional argument for str.join()'
updated_at = <Date 2021-02-21.01:46:52.862>
user = 'https://github.com/f-brinkmann'

bugs.python.org fields:

activity = <Date 2021-02-21.01:46:52.862>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-21.01:46:52.863>
closer = 'steven.daprano'
components = ['Library (Lib)']
creation = <Date 2021-02-20.20:05:58.618>
creator = 'f-brinkmann'
dependencies = []
files = []
hgrepos = []
issue_num = 43280
keywords = []
message_count = 4.0
messages = ['387426', '387427', '387430', '387434']
nosy_count = 3.0
nosy_names = ['steven.daprano', 'Dennis Sweeney', 'f-brinkmann']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43280'
versions = ['Python 3.10']

[f-brinkmann]

It would be nice to have an additional argument for str.join() so this

", ".join(["a", "b", "c"], ", and ")

would output

"a, b, and, c"

[sweeneyde]

This seems like a very specific use case. Too specific IMO for a method on all string objects for anyone using Python anywhere in the world. Why not just write a function like this?

    def my_join(strings, sep=", ", last_sep=", and "):
        strings = list(strings)
        return sep.join(strings[:-1]) + last_sep + strings[-1]

    >>> my_join(["one", "two", "three"])
    'one, two, and three'

[stevendaprano]

Python 3.9 is in feature freeze, this couldn't be added before 3.10 now.

You have:

", ".join(["a", "b", "c"], ", and ")

outputing this:

"a, b, and, c"

So what are the semantics of this additional argument? To insert it before the last item?

[stevendaprano]

Looking more closely, I think that the semantics are to concatenate the extra argument to the second-last item:

", ".join(["a", "b", "c"])
# -> "a, b, c"

", ".join(["a", "b", "c"], ", and")
# -> "a, b, and, c"

which would be the same as:

", ".join(["a", "b, and", "c"])
# -> "a, b, and, c"

I'm not sure how this is useful. In English, there should never be a comma after the "and", and there possibly shouldn't be a comma after the "b" either, depending on context.

# Should be: "a, b and c" or "a, b, and c"

See the Oxford or serial comma:

https://thegrammargirls.wordpress.com/tag/oxford-comma/

I'm going to close this feature request. It's too specific and the semantics don't seem to be very useful. But if you would still like to propose this, or a similar change, please discuss it first either on the Python-Ideas mailing list:

https://mail.python.org/archives/list/python-ideas@python.org/

or at the Ideas topic on

https://discuss.python.org

so that we can determine the required semantics and get a sense for how useful it would be in general.