A slightly unobvious trap with '
from module import *'
If you are already being lazy, it is easy to
drift into the habit of doing '
from yourmodule import *' in your
own code, especially in situations where you really are going to use
everything from your module in the code that you're writing (for
example, importing your Django app's models into your view code for
it). As it happens, there is a little trap for you waiting here.
When they write '
from <X> import *', most people are probably thinking
that it imports everything they've defined in module X into their
current module. But this is not quite what it does. What it actually
does is that it imports everything in module X's namespace into your
namespace. Now, most of what's in module X's namespace is what you
defined in it. But the namespace also includes whatever you imported in
module X; all of those things have now been silently imported into
your namespace too.
__all__ can cut this off. I don't think people use
__all__ very much for internal modules and internal imports like
this, unless they are the sort of person who eschews '
from <X> import
*' in the first place.)
At this point it's quite easy to just start using some of these silently
imported things in your code without realizing that you haven't
explicitly imported them. When dealing with a bunch of directly imported
things (eg, '
from what.util import SomeThing') and a bunch of your own
code, it's easy to lose track of what you have and haven't imported yet.
When you write the code and it works, you're of course going to assume
that you explicitly imported whatever you're using, because that's the
only way the code could work, right?
Now the problem is that in your code in module Y is quietly depending
on the inner workings of module X. If you revise module X so that it
no longer needs the
SomeThing utility bit and so remove the import,
X will still work but suddenly this other module Y breaks. Oops.
(The extreme case of this is forgetting to import an entire module.
Then you might wind up with all sorts of code in module Y making free
use of, say, 'socket.<whatever>', despite there being nary an '
socket' to be seen. This is the sort of thing that leaves you starting
at the revision history in your version control system and wondering how
the code ever worked in the first place.)
A more perverse version of this has intermediate steps; module X is imported wholesale into module Y, which is in turn imported wholesale into module Z, which is where you accidentally use SomeThing without importing it. After this happens, there's a whole raft of changes that can be done that break module Z.
(I'm sure that your imagination can come up with even more extreme and odd scenarios.)