__all__ attribute: bug and proposal

[Chris Angelico]

On Tue, Jun 28, 2016 at 6:56 AM, Pavel S <pavel at schon.cz> wrote:
> By a mistake, I forgot to put comma into '__all__' tuple of some module. Notice missing comma after 'B'.
>
> # module foo.py
> __all__ = (
>     'A',
>     'B'
>     'C',
> )
>
> class A: pass
> class B: pass
> class C: pass
>
> If you try to import * from the module, it will raise an error, because 'B' and 'C' will be concatenated into 'BC'.
>
>>>> from foo import *
> AttributeError: 'module' object has no attribute 'BC'
>
> The bug won't be found until someone imports *.

If you're primarily worried about classes and functions, here's a neat
trick you can use:

__all__ = []
def all(thing):
    __all__.append(thing.__name__)
    return thing

@all
class A: pass
@all
class B: pass
@all
class C: pass
@all
def d(): pass

del all # clean up the namespace (optional)


The decorator doesn't change anything (it returns its argument as-is),
but it captures the canonical name into __all__. Obviously you can't
use this if you want a non-canonical name, and you can't use it for
anything other than classes and functions (you can't decorate "pi =
3.14159"), but it might help with your actual problem.

ChrisA