== Multilevel list comprehensions in Python

Python has recently (at least for some values of recently) grown
[['list comprehensions'|http://www.secnetix.de/~olli/Python/list_comprehensions.hawk]],
which let you easily iterate over a list to transform or select
entries (or both). List comprehensions can be thought of as syntactic
sugar for _map_ and _filter_ operations, but they're actually more
powerful.

One reason is that you can write a multilevel list comprehension,
which effectively iterates over multiple levels of lists. Take the
case where you have a list within a list and want to return all of the
low-level elements as a list:

>  l = []
>  for rr in qa:
>    for s in rr.strings:
>      l.append(s)

This can be rewritten as a two-level list comprehension:
>  l = [s for rr in qa for s in rr.strings]

This can't easily be done via _map_. (We would probably have to roll
in a _reduce_ to flatten the list of lists that _map_ would give us
into a single-level list.)

Multilevel list comprehensions work left to right; the leftmost 'for X
in Y' is the outermost one, and then we step inwards as we move right.
You can also use if conditions, so the correct version of the
list comprehension I wrote, [[in context|PythonDNSQueries]] and with
error checking, would be:

>  from dns.rdatatype import TXT
>  l = [s for rr in qa if rr.rdtype == TXT \
>       for s in rr.strings]

What impresses me about Python is that this works just the way I
thought it would work and both of these examples worked the first
time, just as I wrote them, and needed no debugging. (The first
version actually got used in a scratch program.)