[Python-Dev] bool(iter([])) changed between 2.3 and 2.4 (original) (raw)

Terry Reedy tjreedy at udel.edu
Wed Sep 21 01:55:26 CEST 2005

"Guido van Rossum" <guido at python.org> wrote in message news:ca471dc20509201449652f11d at mail.gmail.com...

I just finished debugging some code that broke after upgrading to Python 2.4 (from 2.3). Turns out the code was testing list iterators for their boolean value (to distinguish them from None).

This seem unnecessarily indirect. Why not 'it != None' or now, 'it is not None'?

In 2.3, a list iterator (like any iterator) is always true. In 2.4, an exhausted list iterator is false; probably by virtue of having a len() method that returns the number of remaining items.

According to 2.4 News, dict iterators also got len method, though I saw no mention of list.

I realize that this was a deliberate feature,

I presume there were two reasons: internal efficiency of preallocations (list(some_it) for example) and letting people differentiate iterator with something left to return versus nothing, just as we can differentiate collections with something versus nothing.

and that it exists in 2.4 as well as in 2.4.1 and will in 2.4.2; yet, I'm not sure I like it. Was this breakage (which is not theoretical!) considered at all?

Searching the gmane archive with the link given on the python site for 'iterator len', I could not find any discussion of the len method addition itself.

Terry J. Reedy

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