In python you can specify the format inside the curved brackets.

You can do things such as

>>> '{:b}'.format(2)
'10'

In your case, d prints as decimal integer.

You can find all the doc here

https://docs.python.org/2/library/string.html#format-specification-mini-language

They are used for formatting strings. %s acts a placeholder for a string while %d acts as a placeholder for a number. Their associated values are passed in via a tuple using the % operator.

name = 'marcog'
number = 42
print '%s %d' % (name, number)

will print marcog 42. Note that name is a string (%s) and number is an integer (%d for decimal).

See https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting for details.

In Python 3 the example would be:

print('%s %d' % (name, number))

It is a formatting character. It tells the formatter to treat the argument as an integer number and format it as such. Other valid formatters could be x to format it as a hexadecimal number, or b for binary, etc.

See the Format String Syntax, and more specifically, the Format Specification Mini-Language:

'd'
Decimal Integer. Outputs the number in base 10.

Each {...} part is a slot, and the positional arguments to the str.format() method are slotted in in the same order.

Argument unpacking seems to be what you're looking for.