JeromeJ, le 22/11/2013 à 04:51
Quelques trucs avancés Python.

Savoir ça pourrait bien vous sauvez quelques heures de votre vie.

"For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary." → You cannot modify magic method directly from an instance, for instance, doing that wont work:

>>> class C:
...     pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()

"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:" → When a magic method is used and not called explicitely, then all special methods like __getattribute__ are also bypassed.

For instance:

>>> class Meta(type):
...    def __getattribute__(*args):
...       print("Metaclass getattribute invoked")
...       return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
...     def __len__(self):
...         return 10
...     def __getattribute__(*args):
...         print("Class getattribute invoked")
...         return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__()                 # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c)          # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c)                      # Implicit lookup
10
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