Eric Smith <report@bugs.python.org> wrote:
> The test as written will always give an error for None. I think the
> better fix is to change it to be:
> 
> if format_dict['type'] is None or format_dict['type'] in 'gG':
> 
> That "fixes" this particular exception, but since the format specifier
> is invalid, it produces nonsense. I think Decimal's format parser needs
> to detect this as an error.

Do you mean that zero significant digits are invalid or that the conversion
specifier cannot be omitted? If I look at the following, the intentions are
not quite clear to me:

'0.1    '

'0.1    '

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.1/decimal.py", line 3611, in __format__
    spec = _parse_format_specifier(specifier, _localeconv=_localeconv)
  File "/usr/lib/python3.1/decimal.py", line 5595, in _parse_format_specifier
    if format_dict['type'] in 'gG' or format_dict['type'] is None:
TypeError: 'in <string>' requires string as left operand, not NoneType

'    0.1'

1. Conversion specifier can be omitted if significant digits != 0.

2. Significant digits can be zero if conversion specifier is present.

3. Exception

4. float() accepts the format.

I would be in favour of making the conversion specifier mandatory - and
perhaps disallow zero significant digits in the case of 'g'.  If the
intention is to mimic the C Standard, zero significant digits should
probably be silently promoted to one.

In any case, float() and Decimal() should preferably behave identically.

P.S.:

Unrelated, but I just see that float() has the wrong default alignment.