__nonzero__ of iterators

[Christian Eder]

Hi,

I just discovered the following pitfall in Python 2.3.
Consider the following code :

 >>> a = {}
 >>> bool (a.keys ())
False
 >>> bool (a.iterkeys ())
True

So, an "empty" iterator evaluates to True in boolean context.
At a first glance, this is not what one would expect. This causes 
several problems, e.g. if you operate on something expected to be 
sequence, and you guard your code with "if seq :" to avoid crashing into 
an empty sequence, you still crash if you get an empty iterator, even if 
the rest of your code is able to deal with an iterator as well as with 
any other sequence type.

At a second glance, the behaviour is clear, since the iterator object 
does not know wheter it is able to provide the next element, before 
having done so.

Anyway, although I don't know how the iterator protocol is implemented 
in Python 2.3, I suppose the __nonzero__ could be changed to check 
whether a next element can be provided without actually consuming it.

I would like to read some comments on this topic as I'm sure that I'm 
not the first one wondering whether the current behaviour should be that 
way.

thanks
chris