Ensure unwanted names removed in class definition
Peter Otten
__peter__ at web.de
Wed Aug 12 11:39:41 EDT 2015
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?
If you absolutely must: make sure that the names exist by setting them
explicitly:
class Parrot:
per = None
a = [... for per in ...]
del per
But I would probably use a generator expression. These don't leak names:
Python 2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Parrot:
... a = [per for per in "abc"]
... b = list(trans for trans in "def")
...
>>> Parrot.per
'c'
>>> Parrot.trans
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: class Parrot has no attribute 'trans'
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