Life is never simple

For the last week of the community bonding period, I was finishing off the animation module for KivEnt. I added a JSON parser inside AnimationManager to read and store animation lists stored on disk in JSON format. Making this module was fairly simple. I had to use python’s json library to get a dict from the JSON file for reading, and convert a dict to a JSON string to store to a file. You must be thinking this would have gotten finished pretty easy, adding two functions in the AnimationManager and making a simple test JSON file in the twinkling stars example accompanying this module. But alas, things are never so simple in life. I mean, NEVER!

Every python user, if they have worked with python 2, will have faced this infamous error:

UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in \
position 0: ordinal not in range(128)

What this essentially means is that the python string we have has a byte of value 0x80 somewhere and our poor old ASCII. encoding, in which python strings are encoded, has no character for that value. The JSON parser module very smartly returns Unicode encoded strings, which is a character encoding different from ASCII. But the animation names in my module were beign stored as python strings, which are encoded in ASCII, and hence the error.

The JSON parser has to do this because the JSON files would almost always be in UTF-8 format which follows the Unicode encoding. Unicode allows support for a lot more characters than ASCII (The Euro symbol for example), hence is more widely used. Let me explain in detail what encodings are.

Encodings - Why do they exist?

Computers, being the stupid chips of silicon they are, do not understand text as we do. They love numbers, binary numbers, and hence store, interpret and communicate all kinds of data in that format. So humans need to devise a system to convert that binary data to an understandable format, which is the desired symbol of the alphabet in the case of text data. A very simple example is ASCII, which is basically a table mapping a 7-digit binary no. to an English letter or symbol.

ascii_table

This is still quite popular and in use (Python 2 uses this encoding for the str data type, as we saw above). ASCII originally was and still is only a 128-symbol encoding (7 bits) but there are a lot of extended versions to make use of the remaining 128 numbers left unassigned. So you have accented characters, currency signs and what not in those extended encodings (ISO Latin-1 is one such example). The problem is that there is no global standard which is followed for these extended encodings. So a file containing accented characters written in one standard, might be rendered as something else entirely on some other computer which uses some other standard.

That’s the problem python had with the number 128 (or 0x80) as one of the characters in the string. ASCII, encoding used by python 2, has no symbol mapped to that number. Or any number above 127.

The number 128 stands for the Euro sign(€) in the Latin-1 encoding. What if some European programmer wanted to print out the Euro sign in one of his very useful programs which calculates your monthly expenditure. Or what if a Japanese programmer wanted to output logs in Japanese and not English? ASCII doesn’t have characters for that!

Japanese people came up with their own standards to map japanese symbols, ended up with 4 different such standards, with no way to interchange between them and made a mess of the ‘Information Interchange’ problem ASCII had tried to solve. We need a standard which:

  • Has support for all possible symbols and languages in use
  • Is followed world-wide
  • Can represent any symbol as 8-bit bytes (called octets in the Unicode specification)

Enter Unicode

Unicode is like a mega mega version of ascii. In the simplest sense, what it does is assign a number to every possible symbol that you would ever want to display on a screen. These numbers are decided by the Unicode Consortium, a group of large tech companies who are concerened about text encodings. From Unicode’s Wikipedia entry:

The Unicode Standard, the latest version of Unicode contains a repertoire of more than 120,000 characters covering 129 modern and historic scripts, as well as multiple symbol sets.

Okay so our first 2 criterias are satisfied. We just need to convert these numbers into an octet format. We could just convert each number to binary, and split it into how many ever octets are required. Unicode is able to assign symbols to numbers upto 0x10FFFF which is 3 octets. This is a very wasteful way to convert Unicode to octets, because most of English text files would use 24 bits for each character when one only requires 7 bits. With each character we have 17 redundant bits. This is bad for storage and data-transfer bandwidths.

Most of the internet today uses a standard called ‘UTF-8’ for this conversion. It is an ingenious way which enables one to represent all of Unicode’s code points (numbers mappable to a symbol).

It can be defined using these very simple rules:

  • A byte starting with 0 is a single byte character (0XXXXXXX).
  • A byte starting with 110 denotes the starting of a two byte character (110XXXXX 10XXXXXX)
  • A byte starting with 10 is a continuation byte to a starting byte for a character.
  • 1110XXXX and 11110XXX denote starting bytes for 3 and 4 byte long characters. They will be followed by the required continuation bytes.
  • The character’s code point is determined by concatenating all the Xes and reading them as a single binary number.

It becomes immediately obvious by the single byte characters that such an encoding supports ASCII and represents it by default as a single byte. Any valid ASCII file is a valid UTF-8 file.

For an example lets try decoding an emoji character:

0xF0 0x9F 0x98 0x81

In binary:

11110000 10011111 10011000 10000001

Look how the first byte will tell its a four byte character, so three continuation bits are to be expected. Removing the UTF-8 header bits (highlighted ones) and concatenating the remaining bits:

000 011111 011000 000001 = 0x1f601

Looking up U-1f601 in the Unicode table we find that it is the grin emoji :D!

Most of the web has moved to using UTF-8. The problem with the python 2 data type str is that we need to explicitely declare it to be the unicode data type if we want support for special characters. And you need to be extra careful using one in place of the other, in which case you need to convert them. Else python throws you that headache of an error because it can’t decode a few characters in ascii.

But because UTF-8 is backwards compatible with ASCII, python 3 uses UTF-8 encoded Unicode as its standard string encoding. I wonder why we haven’t all dumped python 2 in some history archive and moved to python 3 yet :P! I have had too much of UnicodeDecode errors!

Credits

This awesome video by Computerphile <3: https://www.youtube.com/watch?v=MijmeoH9LT4

Other News

I’m anxious for the start of the coding period for GSoC! And I also just launched my club’s new website: https://stab-iitb.org/electronics-club/ which is getting awesome reviews from my freinds so I’m kinda happy and excited too!

I also recently read ‘The Martian’ by Andy Weir, after having watched the really great movie. The book is atleast 10 times more awesome than the movie! Its a must read for any hacker, tinkerer and automater!