What's the python all module level variable for?

Question:

I’ve seen it a lot in python/Lib source code but I don’t know what it is for.

I thought it was used to limit accessible members of of a module. So only the elements at __all__ will show up when dir(module).

I did a little example and saw it was not working as I expected.

So… What’s the python __all__ module level variable for?

Asked By: Juanjo Conti

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Answers:

It controls what you get pulled into your namepsace when you

from blah import *

See Importing * from a Package

Answered By: John La Rooy

It has two purposes:

Anybody who reads the source will know what the exposed public API is. It doesn’t prevent them from poking around in private declarations, but does provide a good warning not to.
When using from mod import *, only names listed in __all__ will be imported. This is not as important, in my opinion, because importing everything is a really bad idea.

Answered By: John Millikin

http://docs.python.org/tutorial/modules.html#importing-from-a-package

Now what happens when the user writes
from sound.effects import *? Ideally,
one would hope that this somehow goes
out to the filesystem, finds which
submodules are present in the package,
and imports them all. This could take
a long time and importing sub-modules
might have unwanted side-effects that
should only happen when the sub-module
is explicitly imported.

The only solution is for the package
author to provide an explicit index of
the package. The import statement uses
the following convention: if a
package’s __init__.py code defines a
list named __all__, it is taken to be
the list of module names that should
be imported when from package import *
is encountered. It is up to the
package author to keep this list
up-to-date when a new version of the
package is released. Package authors
may also decide not to support it, if
they don’t see a use for importing *
from their package.

Answered By: Jochen Ritzel