>>> "{:<06}".format(1.2)
'1.2000'
>>> "{:<06}".format(decimal.Decimal(1.2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid format string

3.2 has a better error message:

>>> "{:<06}".format(Decimal("1.2"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.2/decimal.py", line 3632, in __format__
    spec = _parse_format_specifier(specifier, _localeconv=_localeconv)
  File "/usr/lib/python3.2/decimal.py", line 5600, in _parse_format_specifier
    "format specifier: " + format_spec)
ValueError: Alignment conflicts with '0' in format specifier: <06


That's because '0' already has a special meaning:

"Preceding the width field by a zero ('0') character enables sign-aware zero-padding for numeric types. This is equivalent to a fill character of '0' with an alignment type of '='."

The output is from Python 3.3. Why has Python 3.3 a less informative error message than 3.2?

I also wonder why it works with floats if it has a special meaning? Either float or Decimal is broken.

Christian Heimes <report@bugs.python.org> wrote:
> The output is from Python 3.3. Why has Python 3.3 a less informative error message than 3.2?

Because the error is discovered in libmpdec and it would require a significant
amount of work to provide fine-grained error messages.

> I also wonder why it works with floats if it has a special meaning?
> Either float or Decimal is broken.

Yes, IMO float should detect the ambiguity. You see that the zero implies left
padding:

>>> "{:06}".format(1.2)
'0001.2'

And in case of a conflict right padding wins:

>>> "{:<06}".format(1.2)
'1.2000'

The unambiguous way to get right padding is:

>>> "{:0<6}".format(Decimal("1.2"))
'1.2000'

>>> "{:X<6}".format(Decimal("1.2"))
'1.2XXX'

With int and float it's also possible to specify both conflicting
alignments and fill characters:


 >>> "{:x<06}".format(1.2)
'1.2xxx'


So I really think that the builtins should be changed to detect
the conflict.