Question

Joining a list:

>>> ''.join([ str(_) for _ in xrange(10) ])
'0123456789'

join must take an iterable.

Apparently, join‘s argument is [ str(_) for _ in xrange(10) ], and it’s a list comprehension.

Look at this:

>>>''.join( str(_) for _ in xrange(10) )
'0123456789'

Now, join‘s argument is just str(_) for _ in xrange(10), no [], but the result is the same.

Why? Does str(_) for _ in xrange(10) also produce a list or an iterable?

Answer 1

>>>''.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)))