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 "#"