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Cross post from reddit.<pre><code>List comprehensions are wonderful, but some of these examples are
less clear than they could be.

>>> [a.lower() for a in [b[i] for b in x for i in range(len(b))]]
['h', 'e', 'l', 'l', 'o', 'w', 'o', 'r', 'l', 'd', 'h', 'o', 'w', 'a', 'r', 'e', 'y', 'o', 'u', '?']

would be clearer as

>>> [char.lower() for word in mylist for char in word]

9 times out of 10, you don't need to use range or len for your list
comprehensions.

One cool trick is to use generators instead of list comprehensions.
They're almost the same, but generators use parentheses instead of
brackets. The advantage of generators is that you can pass them to
functions or loops without wasting time building up a list object that
will then be GC'd one line down. Here's something cool you can do
with a generator:

>>> any(mytest(i) for i in mylist)

This will tell you if any member of your list meets your test criteria.
Since it uses a generator instead of a list comprehension, it stops
testing as soon as it gets back an item that tests true. A list
comprehension would go all the way through the list even if it
matched on the first test. Here's another good one:

>>> mycounter = sum(1 for i in mylist if cond(i))

This will tell you the number of items in the list that match your
condition. It's the same as:

>>> mycounter = 0
>>> for i in mylist:
... if cond(i):
... mycounter = 1
...

OK, finally, here are some insane list comprehensions from the
Python cookbook:

# sum
nl = [2, 3, 6, 18]
[j for j in [0] for i in nl for j in [j i]][-1]

# product
nl = [2, 3, 6, 18]
[j for j in [1] for i in nl for j in [j * i]][-1]

# factorial
fac = 6
[j for j in [1] for i in xrange(2, fac 1) for j in [j*i]][-1]

While you should never use these in production code (too cryptic!),
they are really insanely cool.</code></pre>