[Python-Dev] Iterating over objects of unknown length (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Sep 27 03:33:47 CEST 2007

Oleg Broytmann wrote:

Hello!

(This seems like a "developing with Python" question and partially it is but please read on.) I have a class that represents SQL queries. Instances of the class can be iterated over. ... users of the class sometimes write if sqlQuery: for row in sqlQuery: ... else: # no rows To prevent users from writing such code the class implements nonzero() that always raises an exception.

I'm not sure I like that idea. It's common practice to write 'if x:' as a shorthand for 'if x is not None:' when it's known that x is an object that doesn't have a notion of emptiness. A nonzero that always raises an exception just to spite you interferes with that.

Another thing is that any code doing "if x" to test for emptiness is clearly expecting x to be a sequence, not an iterator, and you've violated the contract by passing it one. This is what you may be running into with the libraries you mention.

Generally I think it's a bad idea to try to protect people from themselves when doing so can interfere with legitimate usage.

-- Greg Ewing, Computer Science Dept, +--------------------------------------+ University of Canterbury, | Carpe post meridiem! | Christchurch, New Zealand | (I'm not a morning person.) | greg.ewing at canterbury.ac.nz +--------------------------------------+

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