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add a note on extracting multiline processing in comprehensions #148

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add a note on extracting multiline processing in comprehensions #148

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add a note on extracting multiline processing in comprehensions
graingert Feb 28, 2020
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Update docs/readability/using_map_or_filter_where_list_comprehension_…
graingert Feb 28, 2020
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22 changes: 18 additions & 4 deletions docs/readability/using_map_or_filter_where_list_comprehension_is_possible.rst
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@@ -1,7 +1,8 @@
Using ``map()`` or ``filter()`` where list comprehension is possible
====================================================================
Using ``map()`` or ``filter()``
===============================

For transformations that can be expressed as a list comprehension, use list comprehensions over ``map()`` or ``filter()``. Although a ``map()`` or ``filter()`` expression may be functionally equivalent to a list comprehension, the list comprehension is more concise and easier to read. For expressions that are too long or complicated to express directly within a comprehension extract them into a function.

For simple transformations that can be expressed as a list comprehension, use list comprehensions over ``map()`` or ``filter()``. Although a ``map()`` or ``filter()`` expression may be functionally equivalent to a list comprehension, the list comprehension is generally more concise and easier to read.

Anti-pattern
------------
Expand All @@ -26,10 +27,23 @@ In the modified code below, the code uses a list comprehension to generate the s
values = [1, 2, 3]
doubles = [x * 2 for x in values]

Sometimes a condition or process is too complicated or impossible to express in a single expression, for those extract them to a function and use a comprehension:

.. code:: python

def process(v):
if v.ham == "spam":
return 4
if v.bacon == "eggs"
return 7

def cond(v): ...

[process(x) for x in items if cond(v)] # preferable to map(process, filter(cond, items))

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@graingert graingert Feb 28, 2020 •
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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)]

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@graingert graingert Feb 28, 2020 •
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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
    ]

References
----------

- PyLint - W0110, deprecated-lambda
- `Oliver Fromme - List Comprehensions <http://www.secnetix.de/olli/Python/list_comprehensions.hawk>`_