Override list's builtins dynamically in class

Question

Purely curiosity question:

class Li(list): pass
m, n= Li([1]), Li([2])
def r(*args, **kwargs): raise Exception('hop')
setattr(m, '__iadd__', r)
m += n
print m # [1, 2]
setattr(Li, '__iadd__', r)
m += n

Output:

[1, 2]
Traceback (most recent call last):
  File "C:\...\test_override.py", line 8, in <module>
    m+=n
  File "C:\...\test_override.py", line 3, in r
    def r(*args, **kwargs): raise Exception('hop')
Exception: hop

If I use setattr(m, 'append', r) then m.append(2) will fail. So is __iadd__ called on the class object ?

Also can I use settattr from class scope ? A naive attempt as in:

def r(*args, **kwargs): raise Exception('hop')
class Li(list):
    for s in {'__iadd__','append'}: setattr(Li, s, r)

fails with NameError: name 'Li' is not defined

Answer 1

In new style classes, the instance __getattr__ method no longer intercepts calls made by built-in operations. When the built-in operation is used, the search begins at the class level instead of at the instance level. Nevertheless, an explicit call to the method name will work:

>>> class Li(list): pass
>>> m, n= Li([1]), Li([2])
>>> def r(*args, **kwargs):
        raise Exception('hop')

>>> setattr(m, '__iadd__', r)
>>> m += n           # no exception raised

>>> m.__iadd__(n)    # explicitly calling the method searches the instance first
Traceback (most recent call last):
  File "<pyshell#76>", line 1, in <module>
    m.__iadd__(n)
  File "<pyshell#73>", line 1, in r
    def r(*args, **kwargs): raise Exception('hop')
Exception: hop

One way to accomplish your second goal is to use a metaclass, especially if you only want to create the attributes once (as per your comments):

>>> class Meta(type):
    def __new__(meta, name, bases, attrdict):
        print 'adding class attributes!'
        for s in {'__iadd__','append'}:
            attrdict[s] = r
        return super(Meta, meta).__new__(meta, name, bases, attrdict)
    def __init__(cls, name, bases, attrdict):
        super(Meta, cls).__init__(name, bases, attrdict)


>>> class Li(list):
    __metaclass__ = Meta


adding class attributes!          # attributes added here
>>> t = Li()                      # but not on instance creation
>>> Li.__dict__
dict_proxy({'__module__': '__main__', '__metaclass__': <class '__main__.Meta'>, '__iadd__': <function r at 0x02AAA270>, '__dict__': <attribute '__dict__' of 'Li' objects>, '__weakref__': <attribute '__weakref__' of 'Li' objects>, '__doc__': None, 'append': <function r at 0x02AAA270>})