Question

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.

Answer 1

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'