(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. As an SQL query doesn't know in advance if it will produce any row the class doesn't implement __len__(). Moreover, users of the class sometimes write
if sqlQuery: for row in sqlQuery: ... else: # no rows
which is a bug (the query doesn't know if it's True or False; to find it out the user have to execute the query by trying to iterate over it). To prevent users from writing such code the class implements __nonzero__() that always raises an exception. Unfortunately, I found some libraries test the object in boolean context before iterating over it and that, of course, triggers the exception from __nonzero__(). Even worse, some libraries test the object in boolean context regardless of iterating over it. For example, logging module (this is where my question becomes "developing for Python") triggers the exception in such simple case:
logginig.debug("Query: %s", sqlQuery)
Funny, the code
logginig.debug("Query: %s, another: %s", sqlQuery, another_value)
doesn't trigger the exception. This is due to the code in logginig/__init__.py:
if args and (len(args) == 1) and args and (type(args) == types.DictType): args = args
(class LogRecord, method __init__). "and args" triggers the exception.
My questions are:
1. Should I consider this a bug in the logging module (and other libraries) and submit patches? 2. Or should I stop raising exceptions in __nonzero__()?
In this particular case with logging the fix is simple - do "and args" after type check.