[Python-Dev] User conversions in custom string.Formatter

[Andrew Svetlov]

As PEP 3101 http://www.python.org/dev/peps/pep-3101/ says (and current
Python does) user can specify conversions like "{0!s}".
In custom formatters (derived from string.Formatter) he can override
convert_field method to add custom conversions.

I experimented with that last month and found it very convenient.
>From my perspective custom conversions are very close to 'filters'
from html template engines (jinja2, mako, django etc).
While I like to see custom conversions simple and easy I don't wan't
to bring 'cascading' and 'parametrization' to standard formatting.

But why don't relax spec a bit and allow to use any word (with
'identifier' semantix, I mean str.isidentifier()) as conversion name?
Proposed update doesn't make any backward incompatibility (both python
and public C API are not changed), it's clean and obvious.
And it's very funny to use words instead single characters for custom
user-specific conversions.

What do you think?