On Tue, Apr 28, 2009 at 2:22 PM, Colin J. Williams <c...@ncf.ca> wrote: > Lie Ryan wrote: >> >> Aaron Brady wrote: >>> >>> What is the rationale for considering all instances true of a user- >>> defined type? >> >> User-defined objects (or type) can override .__len__() [usually container >> types] or .__nonzero__() to make bool() returns False. >> >>> Is it strictly a practical stipulation, or is there >>> something conceptually true about objects? >> >> Objects are true unless they define themself as false. The practical >> implication is we can do this: >> >> def foo(args = None): >> if args: >> ... >> >> In python all objects are true except: None, False, 0/0L/0.0/0j, empty >> sequence or container, and on objects that defines .__len__() or >> ..__nonzero__() that returns 0 or False. >> >>> ''' >>> object.__bool__(self) >>> If a class defines neither __len__() nor __bool__(), all its instances >>> are considered true. >>> ''' >>> >>> This makes it so all objects except False, None, 0, and empty >>> containers are true by default. I am not convinced that 'if <a >>> generic object>' should have any meaning; it should probably throw an >>> exception. Is it part of Python's look and feel or its mentality? Is >>> it part of the Zen? Certainly other ideal types can't be cast from >>> generic objects, so why booleans? Is it an ineffable component of the >>> author's vision for the language? I think that giving arbitrary >>> syntactic constructs meaning is just space-filler. It's worse than >>> syntactic sugar, it's semantic sugar. Why not assign meanings willy- >>> nilly to other random juxtapositions of tokens? >> >> It's part of the design decision. In almost all cases (in any language), a >> so-called "Design Decision" is rather random and prone to subjective >> judgment, just as the decision to make bool(int) returns False only on 0, >> -1, or for all negative values; whether to make bool(100) and exception or >> True; or round() rounds down or up or even-odd; or the use of brackets vs. >> indentation; or whether to treat empty list as True or False. > > I'm puzzled by the last sentence: > > *** Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit > (Intel)] on win32. *** >>>> >>>> bool(0) > > False >>>> >>>> bool(-1) > > True >>>> >>>> bool(-100) > > True >>>> > > Colin W. > -- > http://mail.python.org/mailman/listinfo/python-list >
bool has been defined thus in Python: False are: * False * None * empty containers, lists etc. * The number zero Everything else is true. -100 is not equal to zero, so its boolean value is True. -- André Engels, andreeng...@gmail.com -- http://mail.python.org/mailman/listinfo/python-list
