How is the __getattribute__ method used?
It is called before the normal dotted lookup. If it raises AttributeError, then we call __getattr__.
Use of this method is rather rare. There are only two definitions in the standard library:
$ grep -Erl "def __getattribute__\(self" cpython/Lib | grep -v "/test/"
cpython/Lib/_threading_local.py
cpython/Lib/importlib/util.py
Best Practice
The proper way to programmatically control access to a single attribute is with property. Class D should be written as follows (with the setter and deleter optionally to replicate apparent intended behavior):
class D(object):
def __init__(self):
self.test2=21
@property
def test(self):
return 0.
@test.setter
def test(self, value):
'''dummy function to avoid AttributeError on setting property'''
@test.deleter
def test(self):
'''dummy function to avoid AttributeError on deleting property'''
And usage:
>>> o = D()
>>> o.test
0.0
>>> o.test = 'foo'
>>> o.test
0.0
>>> del o.test
>>> o.test
0.0
A property is a data descriptor, thus it is the first thing looked for in the normal dotted lookup algorithm.
Options for __getattribute__
You several options if you absolutely need to implement lookup for every attribute via __getattribute__.
- raise
AttributeError, causing __getattr__ to be called (if implemented)
- return something from it by
- using
super to call the parent (probably object's) implementation
- calling
__getattr__
- implementing your own dotted lookup algorithm somehow
For example:
class NoisyAttributes(object):
def __init__(self):
self.test=20
self.test2=21
def __getattribute__(self, name):
print('getting: ' + name)
try:
return super(NoisyAttributes, self).__getattribute__(name)
except AttributeError:
print('oh no, AttributeError caught and reraising')
raise
def __getattr__(self, name):
"""Called if __getattribute__ raises AttributeError"""
return 'close but no ' + name
>>> n = NoisyAttributes()
>>> nfoo = n.foo
getting: foo
oh no, AttributeError caught and reraising
>>> nfoo
'close but no foo'
>>> n.test
getting: test
20
What you originally wanted.
And this example shows how you might do what you originally wanted:
class D(object):
def __init__(self):
self.test=20
self.test2=21
def __getattribute__(self,name):
if name=='test':
return 0.
else:
return super(D, self).__getattribute__(name)
And will behave like this:
>>> o = D()
>>> o.test = 'foo'
>>> o.test
0.0
>>> del o.test
>>> o.test
0.0
>>> del o.test
Traceback (most recent call last):
File "<pyshell#216>", line 1, in <module>
del o.test
AttributeError: test
Code review
Your code with comments. You have a dotted lookup on self in __getattribute__.
This is why you get a recursion error. You could check if name is "__dict__" and use super to workaround, but that doesn't cover __slots__. I'll leave that as an exercise to the reader.
class D(object):
def __init__(self):
self.test=20
self.test2=21
def __getattribute__(self,name):
if name=='test':
return 0.
else: # v--- Dotted lookup on self in __getattribute__
return self.__dict__[name]
>>> print D().test
0.0
>>> print D().test2
...
RuntimeError: maximum recursion depth exceeded in cmp
