format r(repr) of print in python3

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().

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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"

In python3's f-string formatting, you can also use:

print(f"You say:{'i love you'!r}")
You say:'i love you'
print(f'You say:{"i love you"!r}')
You say:'i love you'

Noted that both return 'i love you' enclosed in single-quotes.