Learn Python the hard way, exercise 10.4

#Combine %r with double-quote and single-quote escapes and print them out. 
#Compare %r with %s. Notice how %r prints it the way you'd write it in your file, 
#but %s  prints it the way you'd like to see it?

This is what I wrote for the exercise:

1) mybugs1 = "Guido said: %r and moved on." %'I \'love\' \"my\" bugs'

2) mybugs2 = "Stallman said: %s and moved on." % 'I \'love\' \"my\" bugs'

3) print mybugs1

4) print mybugs2

Output

Guido said: 'I \'love\' "my" bugs' and moved on.

Stallman said: I 'love' "my" bugs and moved on.

Question

%r doesn't print everything the way I'd write it in my .py file (for example it prints "my" instead of \"my\" at line 1). Why?

Solution

there are several ways to write string literals in python, with single or double quotes, single-line or multiline, raw or normal. None of that is preserved in the string itself, though; once the parser parses it, no information remains about how it was presented in the source; it doesn't even need to be in the source, you could generate the string dynamically, say, by reading it from a file, asking the user, or turning a number into a string.

So when you repr() a string, python guesses which way to format it to look like a literal. The rules it uses are simple, if the string contains single quotes but no double quotes, it uses a single-line, double-quote, non-raw literal; In all other cases, it uses a single-line, single-quote, non-raw literal; Stated another way, python prefers single quotes, but if it's formatting a string that has single quotes, but not double quotes, it can repr() that string without backslash escaping the quotes by using a double quoted string.

remember, repr() doesn't return what you typed, because it doesn't know what you typed; you might never have typed it all. It returns something that can be parsed back into the same value. The identity^* is:

x == eval(repr(x))

not

x == repr(eval(x))

^{* repr() is not magical for this either, not all objects implement __repr__ in a way that preserves this constraint. repr is mostly used for providing useful debugging information, not for generating python code}