How can I construct a Python string that is 16 byt

Question

When I make a Python string of one character, i.e.

txt = "h"

and I check its size, I get

import sys
sys.getsizeof(txt)
34

If one character is 34 bytes, how do I make a String of 16 byes?

P.S. I'm playing around with PyCrypto AES, in OFB Mode, which required input of 16 bytes

Answer 1

getsizeof does not return the actual size of the string but it accounts for additional garbage collector overhead. As shown bellow test starts as an empty string but memory allocated for it is not zero due to reasons listed above.

>>> test = ""
>>> sys.getsizeof(test)
37
>>> test = "" + "h"
>>> sys.getsizeof(test)
38
>>> test = "" + "h" + "h"
>>> sys.getsizeof(test)
39
>>>

To create a 16 bytes string, one way would be:

>>> test = " " * 16
>>> sys.getsizeof(test)
53

53 - 37 = 16 bytes

Answer 2

sys.getsizeof(obj) returns the size of the object including internal values. This contains more than the characters or bytes of a string.

If you want to know how long a bytes object or a str is, then you can call len(byteString) or len(string).

I'm assuming that you want to use generate a random IV or key. This can be done in this way:

from Crypto import Random
iv = Random.new().read(16)

This invokes a CSPRNG if the system has enough entropy. You can also use AES.block_size instead of 16.