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http://docs.python.org/py3k/library/string.html#format-specification-mini-language
The '#' option:
"For floats, complex and Decimal the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result."
1)
>>> import decimal
>>> '{0:#.5g}'.format(1.5)
'1.5000'
>>> '{0:.5f}'.format(decimal.Decimal(1.5))
'1.50000'
>>> '{0:.5g}'.format(decimal.Decimal(1.5))
'1.5'
>>> '{0:#.5g}'.format(decimal.Decimal(1.5))
'1.5'
>>>
no zeros with "#"
2)
>>> import decimal
>>> '{0:#.5g}'.format(decimal.Decimal('1.500000000000'))
'1.5000'
>>> '{0:.5g}'.format(decimal.Decimal('1.500000000000'))
'1.5000'
>>>
zeros without "#"

See issue #7098 for a discussion.
I propose to close this issue.

my question is about the "#" option
it is described as working with Decimal but it doesn't work with Decimal

IMO, the behaviour is fine; it's the docs that are unclear. The rules for Decimal are different mainly because trailing zeros have meaning for the Decimal type. (Decimal('1.250') and Decimal('1.25') are two distinct Decimal objects, unlike float('1.250') and float('1.25').)
See also issue #7094.

Ah no, I take it back. I think (2) is fine---this is the usual preservation of trailing zeros where possible. (1) needs to be fixed (and issue #7094 was left open waiting for this fix).