[Python-ideas] new format spec for iterable types

Chris Angelico rosuav at gmail.com
Tue Sep 8 19:58:49 CEST 2015

On Wed, Sep 9, 2015 at 3:49 AM, Sven R. Kunze <srkunze at mail.de> wrote:
> On 08.09.2015 12:00, Wolfgang Maier wrote:
>> head = 99
>> data = myList(range(10))
>> s = '{}, {:*, }'.format(head, data)
>> # or
>> s2 = '{}{sep}{:*{sep}}'.format(head, data, sep=', ')
>> print(s)
>> print(s2)
>> # 99, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
>> Thoughts?
> I like it and I agree this is an oft-used pattern. From my experience I can
> tell patterns are workarounds if a language cannot handle it properly.
> I cannot tell what a concrete syntax would exactly look like but I would
> love to see an easy-to-read solution.

It looks tempting, but there's a reason Python has join() as a
*string* method, not a method on any sort of iterable. For the same
reason, I think it'd be better to handle this as a special case inside
str.format(), rather than as a format string of the iterables; it
would be extremely surprising for code to be able to join a list, a
tuple, a ListIterator, or a generator, but not a custom class with
__iter__ and __next__ methods. (Even more surprising if it works with
some standard library types and not others.) Plus, it'd mean a lot of
code duplication across all those types, which is unnecessary.

It'd be rather cool if it could be done as a special format string,
though, which says "here's a separator, here's a format string, now
iterate over the argument and format them with that string, then join
them with that sep, and stick it in here". It might get a bit verbose,


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