list as an instance attribute

Mon Sep 14 04:36:18 EDT 2009

Daniel Santos a écrit :
> Here goes,
>> I have a base class that is the following :
>> class primitive:

pep08 : class names should be Capitalized.
Also, if you're using Python 2.x, make it:
class Primitive(object):
 #...
>> 	def __init__(self):
> 		self.name = ""
> 		self.transforms = []
>> 	def copyInternalState(self, sourceObj, copyName):

pep08 : methods, attributes and variables names should be all_lower
> 		return null

s/null/None/g
>> 	def copy(self, copyName):
>> 		# copy the source object internal state
> 		primitiveCopy = self.copyInternalState(self, copyName)
>> 		# copy the transformations
> 		primitiveCopy.transforms = []

If copyInternalState is supposed to return an instance of a 
(well-behaved) subclass of 'primitive' - or any object conforming to the 
(implicit) 'primitive' interface, you shouldn't have to do this.
> 		if self.transforms != []:

An empty list has a false value in a boolean test, so the above test 
would be better written as :
 if self.transforms:
But anyway, the whole test is plain useless:
for item in []:
 print "you won't see me"
> 			for transf in self.transforms:
> 				primitiveCopy.transforms.append(transf.copy())
>> 		return primitiveCopy
>> 	# more methods. the ones listed should be enough to get the picture
>> And a derived class,
>> class CircularCilinder(primitive):
>>> 	def __init__(self, name):
>> 		self.name = name
> 		self.baseCenterVertex = [0, 0, 0]
> 		self.heightVertex = [0, 1, 0]
> 		self.radius = 1

You're not calling the __init__ method of primitive. Your code here 
should be:
 def __init__(self, name):
 super(CircularCilinder, self).__init__(name)
 self.baseCenterVertex = [0, 0, 0]
 self.heightVertex = [0, 1, 0]
 self.radius = 1
> 	def copyInternalState(self, sourceObj, copyName):
>> 		copy = CircularCilinder(copyName)
>> 		copy.setHeight(self.heightVertex[1])
> 		copy.setRadius(self.radius)

Python is not Java. We have good support for computed properties, so you 
*don't* need explicit getters/setters - just start with a plain 
attribute access, you can always turn it into a computed one latter if 
and when the need arises.
> 		return copy
>>> I then have a script that creates a CircularCilinder,
>> cilinder = CircularCilinder("cilinder")
> cilinderCopy = cilinder.copy("cilinderCopy")
>>> when I run this script I get the error,
>> Traceback (most recent call last):
> File "./testscript.py", line 26, in <module>
> cilinderCopy = cilinder.copy("cilinderCopy")
> File "/cygdrive/d/cg/brlcad_api/trunk/src/ds_brlcad_modeler/api/primitive.py",
> line 64, in copy
> if self.transforms != []:
> AttributeError: CircularCilinder instance has no attribute 'transforms'

Indeed - you didn't call the superclass's __init__
>> the transforms instance attribute is not known in the derived class. But I
> wanted to avoid adding a self.transforms = [] line to the derived class
> __init__ method.

Should be the same wrt/ the 'name' attribute FWIW.
HTH