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
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>})
