[Python-ideas] Have a "j" format option for lists

Steven D'Aprano steve at pearwood.info
Wed May 9 09:06:10 EDT 2018

On Wed, May 09, 2018 at 09:39:08AM -0300, Facundo Batista wrote:
> This way, I could do:
> >>> authors = ["John", "Mary", "Estela"]
> >>> "Authors: {:, j}".format(authors)
> 'Authors: John, Mary, Estela'

Looks interesting, but I think we need to know the semantics in more 
detail. For example:

- if the items of the list aren't already strings, how are they

- do you truly mean lists *only*, or is any iterable acceptible?

Here's a tiny proof-of-concept for the feature:

import string
class Template(string.Formatter):
    def format_field(self, value, spec):
        if spec.endswith('j'):
            value = ', '.join(map(str, value))
            spec = spec[:-1] + 's'
        return super(Template, self).format_field(value, spec)

Template().format('{:j} => {:d}', ['alpha', 'beta', 42, 'delta'], 99)

# returns 'alpha, beta, 42, delta => 99'


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