List comprehension without [ ] in Python

The other respondents were correct in answering that you had discovered a generator expression (which has a notation similar to list comprehensions but without the surrounding square brackets).

In general, genexps (as they are affectionately known) are more memory efficient and faster than list comprehensions.

HOWEVER, it the case of ''.join(), a list comprehension is both faster and more memory efficient. The reason is that join needs to make two passes over the data, so it actually needs a real list. If you give it one, it can start its work immediately. If you give it a genexp instead, it cannot start work until it builds-up a new list in memory by running the genexp to exhaustion:

~ $ python -m timeit '"".join(str(n) for n in xrange(1000))'
1000 loops, best of 3: 335 usec per loop
~ $ python -m timeit '"".join([str(n) for n in xrange(1000)])'
1000 loops, best of 3: 288 usec per loop

The same result holds when comparing itertools.imap versus map:

~ $ python -m timeit -s'from itertools import imap' '"".join(imap(str, xrange(1000)))'
1000 loops, best of 3: 220 usec per loop
~ $ python -m timeit '"".join(map(str, xrange(1000)))'
1000 loops, best of 3: 212 usec per loop

>>>''.join( str(_) for _ in xrange(10) )

This is called a generator expression, and is explained in PEP 289.

The main difference between generator expressions and list comprehensions is that the former don't create the list in memory.

Note that there's a third way to write the expression:

''.join(map(str, xrange(10)))