add a note on extracting multiline processing in comprehensions

[graingert]

follow up to #135

[graingert]

how about also a note on how to combine cond and process?

Sometimes a condition or process may repeat computation unnecessarily, in those cases extract a function that returns an iterable with the result or an empty iterable:
.. code:: python hams = [x["ham"] for x in items if"ham"in x] # duplicate key lookup.. code:: pythondefprocess(x):try:return (x["ham"],)exceptKeyError:return () hams = [ham for x in items for ham in process(x)]

[graingert]

It's also often possible to inline this empty iterable into the comprehension
.. code:: python hams = [x["ham"] for x in items if x.get("ham") isnotNone] # duplicated key lookup.. code:: python hams = [ hamfor x in itemsfor ham in (x.get("ham"),)if ham isnotNone ]and alternative in python 3.8+ is
.. code:: python hams = [ hamfor x in itemsif (ham := x.get("ham")) isnotNone ]