It's not built in, but if you want unusual length numbers then you could use the bitstring module.
>>> from bitstring import Bits
>>> a = Bits(bin='111111111111')
>>> a.int
-1
The same object can equivalently be created in several ways, including
>>> b = Bits(int=-1, length=12)
It just behaves like a string of bits of arbitrary length, and uses properties to get different interpretations:
>>> print a.int, a.uint, a.bin, a.hex, a.oct
-1 4095 111111111111 fff 7777