What is the difference between !r and %r in Python?
%r is not a valid placeholder in the str.format() formatting operations; it only works in old-style % string formatting. It indeed converts the object to a representation through the repr() function.
In str.format(), !r is the equivalent, but this also means that you can now use all the format codes for a string. Normally str.format() will call the object.__format__() method on the object itself, but by using !r, repr(object).__format__() is used instead.
There are also the !s and (in Python 3) !a converters; these apply the str() and ascii() functions first.
The 0 in front indicates what argument to the str.format() method will be used to fill that slot; positional argument 0 is "Hello" in your case. You could use named arguments too, and pass in objects as keyword arguments:
"{greeting!r:20}".format(greeting="Hello") Unless you are using Python 2.6, you can omit this as slots without indices or names are automatically numbered; the first {} is 0, the second {} takes the second argument at index 1, etc.