[ python-Bugs-871378 ] __getattr__ and __add__ collides

[SourceForge.net]

Bugs item #871378, was opened at 2004-01-05 21:12
Message generated for change (Comment added) made by rhettinger
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=871378&group_id=5470

Category: Python Interpreter Core
Group: Python 2.3
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: Huang Guang Shen (shhgs)
Assigned to: Nobody/Anonymous (nobody)
Summary: __getattr__ and __add__ collides

Initial Comment:
My English is not good enough, so I'd rather use code :

class A :
	def __init__(self) :
		self.__content = [] 
	def __add__(self, x) :
		self.__content.append(x)
	def __getattr__(self, attrname) :
		if string.upper(attrname) == "FIRST" :
			return self.__content[0]
		elif string.upper(attrname) == "LAST" :
			return self.__content[-1]

>>> a = A()
>>> a + 12

And it complains.

but when I substitute the __add__ with add(), it seems
right.

class A :
	def __init__(self) :
		self.__content = []
	def add(self, x) :
		self.__content.append(x)
	def __getattr__(self, attrname) :
		if string.upper(attrname) == "FIRST" :
			return self.__content[0]
		elif string.upper(attrname) == "LAST" :
			return self.__content[-1]

>>> a = A()
>>> a.add(12)
>>> a.add(16)
>>> a.first
12
>>> a.last
16

----------------------------------------------------------------------

>Comment By: Raymond Hettinger (rhettinger)
Date: 2004-01-07 14:46

Message:
Logged In: YES 
user_id=80475

It will work for you if you either use new style classes or
define __coerce__:
   class A(Object):
     . . .

or

   def __coerce__(self, other):
       return (self, other)

Old style classes handle operator overloading a bit
differently.  When the lhs is an instance and the rhs is a
number, it will first try to see if __coerce__ is defined. 
Unfortunately, in your example, the __getattr__ definition
intercepts but does not handle the lookup for coerce.

On a separate note, there is no need to write
string.upper(attrname); the preferred style is attrname.upper().




----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=871378&group_id=5470