What do backticks mean to the Python interpreter? Example: `num`

Question

I'm playing around with list comprehensions and I came across this little snippet on another site:

return ''.join([`num` for num in xrange(loop_count)])

I spent a few minutes trying to replicate the function (by typing) before realising the `num` bit was breaking it.

What does enclosing a statement in those characters do? From what I can see it is the equivalent of str(num). But when I timed it:

return ''.join([str(num) for num in xrange(10000000)])

It takes 4.09 seconds whereas:

return ''.join([`num` for num in xrange(10000000)])

takes 2.43 seconds.

Both give identical results, but one is a lot slower. What is going on here?

Oddly... repr() gives slightly slower results than `num`. 2.99 seconds vs 2.43 seconds. I am using Python 2.6 (haven't tried 3.0 yet).

Answer 1

Backtick quoting is generally non-useful and is gone in Python 3.

For what it's worth, this:

''.join(map(repr, xrange(10000000)))

is marginally faster than the backtick version for me. But worrying about this is probably a premature optimisation.

Answer 2

My guess is that num doesn't define the method __str__(), so str() has to do a second lookup for __repr__.

The backticks look directly for __repr__. If that's true, then using repr() instead of the backticks should give you the same results.