Carl Banks wrote: > Patrick Maupin wrote: > >>PTY wrote: >> >> >>>It looks like there are two crowds, terse and verbose. I thought terse >>>is perl style and verbose is python style. BTW, lst = [] was not what >>>I was interested in :-) I was asking whether it was better style to >>>use len() or not. >> >>It's not canonical Python to use len() in this case. From PEP 8: >> >>- For sequences, (strings, lists, tuples), use the fact that empty >> sequences are false. >> >> Yes: if not seq: >> if seq: >> >> No: if len(seq) >> if not len(seq) >> >>The whole reason that a sequence supports testing is exactly for this >>scenario. This is not an afterthought -- it's a fundamental design >>decision of the language. > > > That might have made sense when Python and string, list, tuple were the > only sequence types around. > > Nowadays, Python has all kinds of spiffy types like numpy arrays, > interators, generators, > etc., for which "empty sequence is false" just > doesn't make sense.
Iterators and generators are *not* sequences types. wrt/ non-builtin container types, I suggest you re-read section 3.3.1 of the language references: """ __nonzero__( self) Called to implement truth value testing, and the built-in operation bool(); should return False or True, or their integer equivalents 0 or 1. When this method is not defined, __len__() is called, if it is defined (see below). If a class defines neither __len__() nor __nonzero__(), all its instances are considered true. """ http://docs.python.org/ref/customization.html > If Python had been designed with these types in > mind, I'm not sure "empty list is false" would have been part of the > language, let alone recommend practice. FWIW, this magic method already existed in 1.5.2 : http://www.python.org/doc/1.5.2p2/ref/customization.html -- bruno desthuilliers python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for p in '[EMAIL PROTECTED]'.split('@')])" -- http://mail.python.org/mailman/listinfo/python-list
