SHAKE128 is an extendable-output function (XOF) in the SHA-3 family, as specified in FIPS 202.
As a XOF, SHAKE128 is a generalization of a cryptographic hash function. Instead of creating a fixed-length digest (e.g. 32 bytes like SHA-2/256), it can produce outputs of any desired length.
Output bits do not depend of the output length.
The 128 in its name indicates its maximum security level (in bits), as described in Sections A.1 and A.2 of FIPS 202.
In the following example, the output is 26 bytes (208 bits) long:
>>> from Crypto.Hash import SHAKE128
>>> from binascii import hexlify
>>>
>>> shake = SHAKE128.new()
>>> shake.update(b'Some data')
>>> print hexlify(shake.read(26))Crypto.Hash.SHAKE128