Trying to understand binary conversion script

Question

I have the following python code, which converts a binary string into plain-text:

import bin ascii
n = int('01100001011000100110001101100100', 2)
binascii.unhexlify('%x' % n)

This code functions properly, but I don't understand what's going on here.

What is the purpose of the "2" at the end of the int declaration?
What is the purpose of the "'%x' % n" argument?

The first creates an integer out of string and the '2' marks the string as integer representation in base 2. — Nir Alfasi
– Nir Alfasi, Commented Oct 6, 2017 at 0:43

Zach Gates · Accepted Answer · 2017-10-06 00:43:57Z

1

What is the purpose of the 2 at the end of the int declaration?

According to the documentation, int can take a second argument: the base of the first argument.

What is the purpose of the '%x' % n argument?

%x in a string indicates that an item will be formatted to hexadecimal with lowercase letters. '%x' % n. It is similar to the builtin hex function, except it doesn't have the leading 0x in the string. An equivalent expression would be format(n, 'x').

answered Oct 6, 2017 at 0:43

Zach Gates

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1 Comment

ShadowRanger Over a year ago

Note: It's only coincidence this works. If the int value hadn't happened to require an even number of hex characters to represent, binascii.unhexlify would have barfed (it won't try zero padding on either end). On Python 3, you could get faster, correct behavior even when the int formats as odd-length hex, without binascii, by replacing the unhexlify line with: n.to_bytes((n.bit_length() + 7) // 8, 'big') (which directly converts from int to bytes without producing an intermediate str at all, avoiding reliance on the hex representation entirely)

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