encoding - Why declare unicode by string in python?

Question

I'm still learning python and I have a doubt:

In python 2.6.x I usually declare encoding in the file header like this (as in PEP 0263)

# -*- coding: utf-8 -*-

After that, my strings are written as usual:

a = "A normal string without declared Unicode"

But everytime I see a python project code, the encoding is not declared at the header. Instead, it is declared at every string like this:

a = u"A string with declared Unicode"

What's the difference? What's the purpose of this? I know Python 2.6.x sets ASCII encoding by default, but it can be overriden by the header declaration, so what's the point of per string declaration?

Addendum: Seems that I've mixed up file encoding with string encoding. Thanks for explaining it :)

Answer 1

Those are two different things, as others have mentioned.

When you specify # -*- coding: utf-8 -*-, you're telling Python the source file you've saved is utf-8. The default for Python 2 is ASCII (for Python 3 it's utf-8). This just affects how the interpreter reads the characters in the file.

In general, it's probably not the best idea to embed high unicode characters into your file no matter what the encoding is; you can use string unicode escapes, which work in either encoding.

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When you declare a string with a u in front, like u'This is a string', it tells the Python compiler that the string is Unicode, not bytes. This is handled mostly transparently by the interpreter; the most obvious difference is that you can now embed unicode characters in the string (that is, u'u2665' is now legal). You can use from __future__ import unicode_literals to make it the default.

This only applies to Python 2; in Python 3 the default is Unicode, and you need to specify a b in front (like b'These are bytes', to declare a sequence of bytes).