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Why does Python use str.encode and str.decode conventions?

  • Concerning str.encode and str.decode within the Python 2.7 language - Why do we decode a string to make it unicode? It seems more logical to encode a string to unicode considering a string is an array of 8-bit characters. I'm therefore encoding, in some cases, two or more characters, into one unicode character. Likewise, if I encode a unicode character, I'm - according to Python 2.7 - converting one character into one or more. It seems more logical here to decode a unicode character. I just don't get the logic of these conventions and was hoping someone had an explanation to make this concept a little easier to grasp.

  • Answer:

    There's two separate concepts: "byte string" and "character string". The former is a sequence of bytes (8-bit values); whereas the latter is a sequence of characters (an abstract concept which represents a unit of text). The former is ideal for representing raw binary data; whereas the latter is ideal for representing text. In Python 2.x, character strings are implemented by the unicode type, and byte strings are implemented by the str type. Despite its name, "str" is not for representing general character strings. Part of your confusion may arise from this. In Python 3.x, character strings are implemented by the str type, and byte strings are implemented by the bytes type. The conversion between byte string and character string is called an "encoding". Regardless of the encoding, converting from character string to binary data is always called "encoding", and converting from binary data back into a character string is always called "decoding".

Xuan Luo at Quora Visit the source

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Unicode characters are not things with a fixed "physical" representation. There are a number of schemes to convert unicode charaters into actual bits and bytes, which are called encodings. When you use encode() you are encoding a unicode string into some bytes. When you decode, you are decoding the unicode string that was encoded in those bytes. Encoding is "To format (electronic data) according to a standard format" so, converting the unicode code point to bytes according to, say, UTF-8 is encoding, while converting UTF-8 bytes into whatever your internal representation for unicode is, is decoding. So, yes, you could bend over backwards and say "I am decoding these UTF-8 bytes into the UTF-16 (or whatever) the python interpreter is using internally", but it's not reasonable, because you are not even specifying the output format when you decode :-)

Roberto Alsina

Unicode is the preferred representation of text.  From that point of view, encoding means converting the text to another character encoding; for example to print it to a console or send the text over a network connection.  Decoding is the opposite; i.e., converting a sequence of bytes to Unicode. This is consistent with the general use of the "encode" and "decode". For example, if you want to save a Python data structure to a file or send it over the network, you first encode it as JSON/YAML/pickle format/... and decode it to get back the data structure.

Larry Rosenstein

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