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Subject: Re: Ensure unwanted names removed in class definition
To: python-list@python.org
References: <85fv3oj2y6.fsf@benfinney.id.au>
From: MRAB <python@mrabarnett.plus.com>
Date: Wed, 12 Aug 2015 16:14:06 +0100
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On 2015-08-12 10:01, Ben Finney wrote:
> 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?
>
Have you thought about catching the NameError?