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:: python
    def process(x):
        try:
            return (x["ham"],)
        except KeyError:
            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") is not None]  # duplicated key lookup

.. code:: python
    hams = [
        ham
        for x in items
        for ham in (x.get("ham"),)
        if ham is not None
    ]

and alternative in python 3.8+ is

.. code:: python
    hams = [
        ham
        for x in items
        if (ham := x.get("ham")) is not None
    ]