A remainder.

I can take a look at this if you like. But I don't want to spoil your fun
:)

Please, be my guest!

Thanks!

Here's a first attempt at Decimal.__format__; the patch is against the
trunk, and should be forward ported as usual to 3.0, with obvious minor
changes related to str/unicode. It still needs some cleanup and some
more tests, but I'm posting it now in the hope of getting some feedback.

I'm adding Raymond Hettinger and Eric Smith to the nosy list in case
they want to comment.

Some points of interest:

• I decided to make plain 'e', 'f' and 'g' formats (without an explicit
  precision) do no rounding: they return a string that captures the exact
  value of the Decimal instance (though it can lose information about
  significant zeros). So format(x, 'e') is basically a way to say 'give
  me str(x), but always include an exponent', and format(x, 'f') gives a
  way to print the value without ever including an exponent. format(x,
  'g') is identical to str(x), except that it always uses the character
  'e' for an exponent (instead of using 'e' or 'E' depending on the
  context).

There are other possible options here (have a default precision; use
the precision from the context), but this one seemed to make most sense.
I'd appreciate opinions.

• the integer format specifiers are not supported. After noticing that
  the integer format specifiers aren't supported for floats either, I
  think this is the right thing to do.

• the 'n' format specifier is not supported either; it's supposed to
  use the current locale. I can't find any easy way to do this---it seems
  as though the only real option is to manually fix the decimal point
  character, figure out where to place thousands separators, etc. This
  would add quite a lot of not-really-Decimal-related code to decimal.py,
  and I'm reluctant to do that. The right solution probably involves
  writing some support code and putting it somewhere else in the std. lib.

• on the subject of not-really-Decimal-related code, it would be great
  if the parse_format_specifier and format_align functions (near the end
  of decimal.py) could be moved somewhere else. I'm planning to reuse
  these functions for Fraction.__format__.

I've committed a reworked version of the patch in r61123.

Reopening this; I'd like to have a second go at implementing the 'n'
format specifier for the Decimal type.

See bpo-2802 for hints about how to go about this.

Adding support for the 'n' format specifier should be done before 3.1 goes
out.

Here's a patch to implement the 'n' format specifier for Decimals
(see also bpo-5481).

Raymond, could you give this a sanity check?

Sure, I will take a look.

Mark, this looks fine.

Can you add support for PEP-378?

New version of decimal_n_format.patch, with support for the thousands
separator (PEP-378). As discussed on python-dev, during zero-padding the
patched code adds an extra '0' on the left to avoid a leading ',' if
necessary. For example:

>>> format(Decimal('123456'), '08,')
'0,123,456'

The Decimal.__format__ method (and support code) had to be fairly
significantly reworked. However, I'm reasonably confident that this code
is correct: a patch review would be welcome if anyone has the time;
otherwise I'll commit this in a couple of days or so.

The tests you submitted are reassuring. I think you should go ahead and
commit this.

Committed, r70439 and r70440.