Why is __getattribute__ not invoked on an implicit __getitem__-invocation?
While trying to wrap arbitrary objects, I came across a problem with dictionaries and lists. Investigating, I managed to come up with a simple piece of code whose behaviour I simply do not understand. I hope some of you can tell me what is going on:
>>> class Cl(object): # simple class that prints (and suppresses) each attribute lookup
... def __getattribute__(self, name):
... print 'Access:', name
...
>>> i = Cl() # instance of class
>>> i.test # test that __getattribute__ override works
Access: test
>>> i.__getitem__ # test that it works for special functions, too
Access: __getitem__
>>> i['foo'] # but why doesn't this work?
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Cl' object has no attribute '__getitem__'
python magic-methods
edited May 27 '15 at 13:00
CommonGuy
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
add a comment |
While trying to wrap arbitrary objects, I came across a problem with dictionaries and lists. Investigating, I managed to come up with a simple piece of code whose behaviour I simply do not understand. I hope some of you can tell me what is going on:
>>> class Cl(object): # simple class that prints (and suppresses) each attribute lookup
... def __getattribute__(self, name):
... print 'Access:', name
...
>>> i = Cl() # instance of class
>>> i.test # test that __getattribute__ override works
Access: test
>>> i.__getitem__ # test that it works for special functions, too
Access: __getitem__
>>> i['foo'] # but why doesn't this work?
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Cl' object has no attribute '__getitem__'
python magic-methods
edited May 27 '15 at 13:00
CommonGuy
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
add a comment |
While trying to wrap arbitrary objects, I came across a problem with dictionaries and lists. Investigating, I managed to come up with a simple piece of code whose behaviour I simply do not understand. I hope some of you can tell me what is going on:
>>> class Cl(object): # simple class that prints (and suppresses) each attribute lookup
... def __getattribute__(self, name):
... print 'Access:', name
...
>>> i = Cl() # instance of class
>>> i.test # test that __getattribute__ override works
Access: test
>>> i.__getitem__ # test that it works for special functions, too
Access: __getitem__
>>> i['foo'] # but why doesn't this work?
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Cl' object has no attribute '__getitem__'
python magic-methods
edited May 27 '15 at 13:00
CommonGuy
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
While trying to wrap arbitrary objects, I came across a problem with dictionaries and lists. Investigating, I managed to come up with a simple piece of code whose behaviour I simply do not understand. I hope some of you can tell me what is going on:
>>> class Cl(object): # simple class that prints (and suppresses) each attribute lookup
... def __getattribute__(self, name):
... print 'Access:', name
...
>>> i = Cl() # instance of class
>>> i.test # test that __getattribute__ override works
Access: test
>>> i.__getitem__ # test that it works for special functions, too
Access: __getitem__
>>> i['foo'] # but why doesn't this work?
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Cl' object has no attribute '__getitem__'
python magic-methods
python magic-methods
edited May 27 '15 at 13:00
CommonGuy
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
edited May 27 '15 at 13:00
CommonGuy
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
edited May 27 '15 at 13:00
CommonGuy
9,332133859
edited May 27 '15 at 13:00
CommonGuy
9,332133859
edited May 27 '15 at 13:00
CommonGuy
9,332133859
9,332133859
asked Jul 6 '12 at 9:59
holbechholbech
21627
asked Jul 6 '12 at 9:59
holbechholbech
21627
asked Jul 6 '12 at 9:59
holbechholbech
21627
21627
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Magic __methods__() are treated specially: They are internally assigned to "slots" in the type data structure to speed up their look-up, and they are only looked up in these slots. If the slot is empty, you get the error message you got.
See Special method lookup for new-style classes in the documentation for further details. Excerpt:
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the
__getattribute__()method even of the object’s metaclass.
[…]
Bypassing the
__getattribute__()machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Magic __methods__() are treated specially: They are internally assigned to "slots" in the type data structure to speed up their look-up, and they are only looked up in these slots. If the slot is empty, you get the error message you got.
See Special method lookup for new-style classes in the documentation for further details. Excerpt:
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the
__getattribute__()method even of the object’s metaclass.
[…]
Bypassing the
__getattribute__()machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
add a comment |
Magic __methods__() are treated specially: They are internally assigned to "slots" in the type data structure to speed up their look-up, and they are only looked up in these slots. If the slot is empty, you get the error message you got.
See Special method lookup for new-style classes in the documentation for further details. Excerpt:
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the
__getattribute__()method even of the object’s metaclass.
[…]
Bypassing the
__getattribute__()machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
add a comment |
Magic __methods__() are treated specially: They are internally assigned to "slots" in the type data structure to speed up their look-up, and they are only looked up in these slots. If the slot is empty, you get the error message you got.
See Special method lookup for new-style classes in the documentation for further details. Excerpt:
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the
__getattribute__()method even of the object’s metaclass.
[…]
Bypassing the
__getattribute__()machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
Magic __methods__() are treated specially: They are internally assigned to "slots" in the type data structure to speed up their look-up, and they are only looked up in these slots. If the slot is empty, you get the error message you got.
See Special method lookup for new-style classes in the documentation for further details. Excerpt:
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the
__getattribute__()method even of the object’s metaclass.
[…]
Bypassing the
__getattribute__()machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
edited Jul 6 '12 at 10:08
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
answered Jul 6 '12 at 10:02
Sven MarnachSven Marnach
345k77743695
345k77743695
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
add a comment |
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
The link in the answer is not valid any more. Here is a newer one with a version binding: docs.python.org/3.7/reference/datamodel.html#special-lookup.
– Alex
Nov 15 '18 at 17:36
add a comment |
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