question about class methods

[Darren Dale]

I've run across some code in a class method that I don't understand:

def example(self, val=0)
if val and not self:
if self._exp < 0 and self._exp >= -6:

I am unfamiliar with some concepts here:

1) Under what circumstances would "if not self" be True?

2) If "not self" is True, how can self have attributes?

(This is slightly simplified code from the decimal.Decimal.__str__ method,
line 826 in python-2.4.4)

Thanks,
Darren

[Marc 'BlackJack' Rintsch]

In <et**********@ruby.cit.cornell.edu>, Darren Dale wrote:

I've run across some code in a class method that I don't understand:

def example(self, val=0)
if val and not self:
if self._exp < 0 and self._exp >= -6:

I am unfamiliar with some concepts here:

1) Under what circumstances would "if not self" be True?

If an object is "true" is determined either by a `__nonzero__()` method
that returns `True` or `False` or a `__len__()` method where anything
except 0 means "true".

2) If "not self" is True, how can self have attributes?

Attributes are independent from the "truth" value of an object.

Ciao,
Marc 'BlackJack' Rintsch

[Gabriel Genellina]

En Wed, 14 Mar 2007 18:04:00 -0300, Darren Dale <dd**@cornell.edu>
escribió:

I've run across some code in a class method that I don't understand:

def example(self, val=0)
if val and not self:
if self._exp < 0 and self._exp >= -6:

0) "Normal" methods are not class methods, but instance methods. A class
method is a method who operates on the class itself; its first argument is
the class (maybe a derived one).

I am unfamiliar with some concepts here:

1) Under what circumstances would "if not self" be True?

"if not self" would be "if self evaluates to False as a boolean": 0, 0.0,
0j, (), [], {}... Other objects usually evaluate always to True, except if
they define some special methods: __nonzero__ and __len__. Look those on
the Python Reference Manual.

2) If "not self" is True, how can self have attributes?

Perhaps you think of self being None - that should never occur unless you
called the method in some convoluted way.
The class migh inherit from list, by example, and you want to compute the
average:

class StatList(list):
def avg(self):
if self: return sum(self)/len(self)
else: raise ValueError("Can't compute average on empty list")

(This is slightly simplified code from the decimal.Decimal.__str__
method,
line 826 in python-2.4.4)

A Decimal number is False when 0. `if not self` is a faster way to say `if
self==0`

--
Gabriel Genellina