[Python-ideas] new format spec for iterable types

[Stephen J. Turnbull]

Wolfgang Maier writes:

 > But the proposed simple version saves me from mistakenly writing:
 > 
 > '{}\t{}'.format(head, '\t'.join(data))
 > 
 > when some of the elements in data aren't strings and I should have written:
 > 
 > '{}\t{}'.format(head, '\t'.join(str(e) for e in data))
 > 
 > , a mistake that I seem to never learn to avoid :)

(Note: I don't suffer from that particular mistake, so I may be
biased.)  I think it's a nice trick but doesn't clear the bar for
adding to the standard iterables yet.

A technical comment: you don't actually need the '*' for myList
(although I guess you find it useful to get an error rather than line
noise as a separator if it isn't present?)

On the basic idea: if this can be generalized a bit so that

    head = 99 
    data = range(10)                # optimism!
    s = '{:.1f}, {:.1f*, }'.format(head, data)

produces

    s == '99.0, 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0'

then I'd be a lot more attracted to it.  I would think the simple
version is likely to produce rather ugly output if you have a bunch of
floats in data.  (BTW, that string was actually generated with

    '{:.2f}, {}'.format(99, ', '.join('{:.2f}'.format(x) for x in range(10)))

which doesn't win any beauty contests.)

Bikeshedding in advance, now you pretty much need the '*' (and have to
hope that the types in the iterable don't use it themselves!), because
'{:.1f, }' really does look like line noise!  I might actually prefer
'|' (or '/') which is "heavier" and "looks like a separator" to me:

    s = '{:.1f}, {:.1f|, }'.format(head, data)

Finally, another alternative syntax would be the same in the
replacement field, but instead of iterables implementing it, the
.format method would (using your syntax and example for easier
comparison):

    s = '{}, {:*, }'.format(head, *data)

I'm afraid this won't work unless restricted to be the last
replacement field, where it just consumes all remaining positional
arguments.  I think that restriction deserves a loud "ugh", but maybe
it will give somebody a better idea.

Steve