Is it possible to redefine which object the brackets [] use?
I can subclass the list object, but how to I make the interpreter use my subclass in place of the buildin list object? Is it possible?
(I'm pretty sure I'm using the wrong terms for the question- feel free to edit)
>>> class mlist(list):
... def __init__(self):
... list.__init__(self)
... def __getitem__(self, item):
... return list.__getitem__(self, item) * 2
...
>>> testlist = mlist()
>>> testlist.append(21)
>>> testlist[0]
42
>>> list = mlist() # maybe setting the 'list' type will do it?
>>> testlist = []
>>> testlist.append(21)
>>> testlist[0]
21 # Nope
>>>
I don't have a practical use for this- just curious.
Try running the code after you've run the code you posted
>>> testlist = list()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'mlist' object is not callable
Now, determine the type using the code I've posted
>>> type([])
<type 'list'>
>>> type(list)
<class '__main__.mlist'>
>>> type(testlist)
<type 'list'>
it seems that [] creates list, instead of mlist, it looks strange :S
Update
I checked the bytecode generated using dis, and the code below was generated
>>> import dis # python's disassembler
>>> def code1():
... return []
...
>>> dis.dis(code1)
2 0 BUILD_LIST 0
3 RETURN_VALUE
>>> def code2():
... return list()
...
>>> dis.dis(code2)
2 0 LOAD_GLOBAL 0 (list)
3 CALL_FUNCTION 0
6 RETURN_VALUE
It appears that list will invoke whatever is assigned to it, while [] will be converted to BUILD_LIST bytecode. It appears that [] is not translated to list, hence []'s behavior is stucked to creating list.
Update 2
Python class can be updated
>>> class NewList(list):
... pass
...
>>> a = NewList()
>>> a.append(23)
>>> a[0]
23
>>> def double_getitem(self, key):
... return list.__getitem__(self, key) * 2
...
>>> NewList.__getitem__ = double_getitem
>>> a[0]
46
Well, except for builtin classes, like list
>>> list.__getitem__ = double_getitem
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'list'