format r(repr) of print in python3

Question

>>>print('You say:{0:r}'.format("i love you"))
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    print('You say:{0:r}'.format("i love you"))
ValueError: Unknown format code 'r' for object of type 'str'

I just use %r(repr()) in python2, and it should work in python3.5. Why is it?

Besides, what format should I use?

Answer 1

What you are looking for is called conversion flag. And that should be specified like this

>>> print('you say:{0!r}'.format("i love you"))
you say:'i love you'

Quoting Python 3's official documentation,

Three conversion flags are currently supported: '!s' which calls str() on the value, '!r' which calls repr() and '!a' which calls ascii().

Please note that, Python 2 supports only !s and !r. As per the Python 2's official documentation,

Two conversion flags are currently supported: '!s' which calls str() on the value, and '!r' which calls repr().

---

In Python 2, you might have done something like

>>> 'you say: %r' % "i love you"
"you say: 'i love you'"

But even in Python 2 (also in Python 3), you can write the same with !r with format, like this

>>> 'you say: {!r}'.format("i love you")
"you say: 'i love you'"

Quoting example from official documentation,

Replacing %s and %r:

>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
"repr() shows quotes: 'test1'; str() doesn't: test2"