Conversion to string: how do `s work?

[Chris Lasher]

I'm working my way through _Learning_Python_ 2nd ed., and I saw
something peculiar which is not explained anywhere in the text.

print " " + file + " size=" + `size`

The `s appear to somehow automagically convert the integer to a string
for concatenation. How does this work? Is this just a shortcut for
str(size)? Is it considered bad practice to use `s?