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To: python-list@python.org
From: Ben Finney <ben+python@benfinney.id.au>
Subject: Ensure unwanted names removed in class definition
Date: Wed, 12 Aug 2015 19:01:05 +1000
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Xref: csiph.com comp.lang.python:95274

How can I ensure incidental names don't end up in the class definition,
with code that works on both Python 2 and Python 3?

With the following class definition, the incidental names `foo` and
`bar`, only needed for the list comprehension, remain in the `Parrot`
namespace::

    __metaclass__ = object

    class Parrot:
        """ A parrot with beautiful plumage. """

        plumage = [
                (foo, bar) for (foo, bar) in feathers.items()
                if bar == "beautiful"]

    assert hasattr(Parrot, 'plumage')  # ← okay, has the wanted name
    assert not hasattr(Parrot, 'foo')  # ← FAILS, has an unwanted name
    assert not hasattr(Parrot, 'bar')  # ← FAILS, has an unwanted name

So I can remove those names after using them::

    __metaclass__ = object

    class Parrot:
        """ A parrot with beautiful plumage. """

        plumage = [
                (foo, bar) for (foo, bar) in feathers.items()
                if bar == "beautiful"]
        del foo, bar

    assert hasattr(Parrot, 'plumage')  # ← okay, has the wanted name
    assert not hasattr(Parrot, 'foo')  # ← okay, no unwanted name
    assert not hasattr(Parrot, 'bar')  # ← okay, no unwanted name

But that fails on Python 3, since the names *don't* persist from the
list comprehension:

    __metaclass__ = object

    class Parrot:
        """ A parrot with beautiful plumage. """

        plumage = [
                (foo, bar) for (foo, bar) in feathers.items()
                if bar == "beautiful"]
        del foo, bar  # ← FAILS, “NameError: name 'foo' is not defined”

How can I write the class definition with the list comprehension and
*not* keep the incidental names — in code that will run correctly on
both Python 2 and Python 3?

-- 
 \       “Pinky, are you pondering what I'm pondering?” “Well, I think |
  `\   so, but *where* do you stick the feather and call it macaroni?” |
_o__)                                           —_Pinky and The Brain_ |
Ben Finney