Is there a way to check if an object has an attribute, that doesn't rely on __getattr__ or object implementation specifics?
Consider the code below. I want Proxy to delegate to Wrapped what it can't handle. The code works, but I'd like to avoid the test attr in self.__dict__ (I'd prefer a stable interface to do this, without using implementation quirks). The function hasattr doesn't help here, because it gets routed to the wrapped object via __getattr__.
class Wrapped:
def __init__(self):
self.a = 0
self.b = 1
class Proxy:
def __init__(self, wrapped):
object.__setattr__(self, 'wrapped', wrapped)
object.__setattr__(self, 'a', 2)
def __getattr__(self, attr):
return getattr(self.wrapped, attr)
def __setattr__(self, attr, value):
if attr in self.__dict__: # <-- Don't like this, hasattr doesn't work
object.__setattr__(self, attr, value)
elif hasattr(self.wrapped, attr):
setattr(self.wrapped, attr, value)
else: object.__setattr__(self, attr, value)
Testdrive:
>>> w = Wrapped()
>>> p = Proxy(w)
>>> p.a
2
>>> p.b
1
>>> p.a = 3
>>> p.a
3
>>> w.a
0
>>> p.b = 4
>>> p.b
4
>>> w.b
4
>>> p.c = 5
>>> p.c
5
>>> w.c
AttributeError: 'Wrapped' object has no attribute 'c'
The built-in hasattr() function
is implemented by calling getattr(object, name) and seeing whether it raises an AttributeError or not.
The inspect module has the getattr_static() method, which can be used to
Retrieve attributes without triggering dynamic lookup via the descriptor protocol, __getattr__() or __getattribute__()".
Using this method, you could define a hasattr_static() method similarly to hasattr(), but calling inspect.getattr_static(object, name) instead of getattr(object, name):
import inspect
def hasattr_static(object, name):
try:
inspect.getattr_static(object, name)
return True
except AttributeError:
return False
And then use this in the __setattr__() method of your Proxy class as a check:
def __setattr__(self, attr, value):
if hasattr_static(self, attr):
object.__setattr__(self, attr, value)
elif hasattr(self.wrapped, attr):
setattr(self.wrapped, attr, value)
else:
object.__setattr__(self, attr, value)
Alternatively, you coud use the dir() function instead of __dict__ to retrieve a list of attributes of your object, or use inspect.getmembers().
Related
I created this class to test some of the __getattr__ features:
class Trace:
def __init__(self, val):
self.val = val
def __setattr__(self, attr, value):
print('set ' + attr)
def __getattr__(self, attr):
print('get ' + attr)
I then created an instance
a = Trace(10)
print(a.val)
a.val = 5
print(a.val)
But, even if I fetched only existing attributes, this was the output:
set val
get val
None
set val
get val
None
I'm using Python 3.7.
__getattr__ is only called when an attribute isn’t found normally, but __setattr__ has no such restriction (that’s why there’s no equivalent of __getattribute__ for it). You overrode __setattr__ to not actually set an instance attribute, so the attribute is always missing and __getattr__ is called anyway. (Yes, it applies in __init__ too.)
Remove __setattr__ to see your expected behaviour quickly, or add its default behaviour back:
def __setattr__(self, attr, value):
print('set ' + attr)
super().__setattr__(attr, value)
There mustn't be an error (according to the docs of Python 2.7):
class C(object):
def __init__(self):
self.d = {}
def set_key(self, key, val):
self.d[key] = val
def get_key(self, key):
return self.d[key]
c = C()
delattr(c, 'set_key')
However:
AttributeError: set_key
I can do delattr on a class object. Can I remove a bound function from an instance?
set_key is an attribute of the class, not the instance. delattr(C, 'set_key') works as expected.
If you want the function not to be available from only the one instance, you can try c.set_key = None.
I am not quite sure why Python (2.7 at least) isn't allowing bound methods to be deleted in this manner.
That said, here is one way to simulate your desired behaviour:
def delete_method(obj, method_name):
def stub(*args, **kwargs):
raise AttributeError("'%s' object has no attribute '%s'" % (
obj.__class__.__name__, method_name))
setattr(obj, method_name, stub)
c = C()
delete_method(c, 'set_key')
c.set_key('the_answer', 42)
When run, this gives
AttributeError: 'C' object has no attribute 'set_key'
Note that this technique isn't universal: for example, you won't be able to use it to remove a method from an instance of the dict class.
There is also a method to hide member methods (not deleting them) to pass restricted interface to another place:
def safe_interface(obj, enabled):
"Returns an interface object blocking functions which are not in enabled."
def assert_name(name):
if name not in enabled:
raise KeyError('No attribute %s.' % name)
class Caller(object):
def __getattribute__(self, name):
assert_name(name)
return getattr(obj, name)
def __setattr__(self, name, val):
assert_name(name)
setattr(obj, name, val)
c = Caller()
return c
if __name__ == '__main__':
class C(object):
def __init__(self):
self.d = {}
def set_key(self, key, val):
self.d[key] = val
def get_key(self, key):
return self.d[key]
c = C()
d = safe_interface(c, ['set_key'])
key = 'c'
val = 'm'
d.set_key(key, val)
print c.get_key(key)
print d.get_key(key) # error, this method is not allowed
I have some question about encapsulation nested attributes in python. Let's assume few classes:
Here we have a main class (DataWrapper) that includes two more classes: InnerWrapper1 and InnerWrapper2. Both inner wrappers includes two attributes.
class DataWrapper(object):
#property
def inner_wrapper1(self):
return self.__inner_wrapper1
#inner_wrapper1.setter
def inner_wrapper1(self, value):
self.__inner_wrapper1 = value
#property
def inner_wrapper2(self):
return self.__inner_wrapper2
#inner_wrapper2.setter
def inner_wrapper2(self, value):
self.__inner_wrapper2 = value
class InnerWrapper1(object):
#property
def property1(self):
return self.__property1
#property1.setter
def property1(self, value):
self.__property1 = value
#property
def property2(self):
return self.__property2
#property2.setter
def property2(self, value):
self.__property2 = value
class InnerWrapper2(object):
#property
def property3(self):
return self.__property3
#property3.setter
def property3(self, value):
self.__property3 = value
#property
def property4(self):
return self.__property4
#property4.setter
def property4(self, value):
self.__property4 = value
Is it possible to override somehow getattr and setattr methods to make possible below encapsulation? What I want to achieve is to have an access to those nested attributes from the top class- DataWrapper.
data_wrapper = DataWrapper()
data_wrapper.property1 = "abc"
...
var = data_wrapper.property2
...
The first thing that came to my mind was to execute hasattr in getattr, but that gave a maximum recursion depth...
Here's a complete code:
class DataWrapper(object):
def __init__(self):
self.inner_wrapper1 = InnerWrapper1()
self.inner_wrapper2 = InnerWrapper2()
#property
def inner_wrapper1(self):
return self.__inner_wrapper1
#inner_wrapper1.setter
def inner_wrapper1(self, value):
self.__inner_wrapper1 = value
#property
def inner_wrapper2(self):
return self.__inner_wrapper2
#inner_wrapper2.setter
def inner_wrapper2(self, value):
self.__inner_wrapper2 = value
def __setattr__(self, attribute, value):
#if attribute in {'innerwrapper1', 'innerwrapper2'}:
if attribute in ['inner_wrapper1', 'inner_wrapper2']:
return super(DataWrapper, self).__setattr__(attribute, value)
if hasattr(self.inner_wrapper1, attribute):
return setattr(self.inner_wrapper1, attribute, value)
elif hasattr(self.inner_wrapper2, attribute):
return setattr(self.inner_wrapper2, attribute, value)
def __getattr__(self, attribute):
try:
return getattr(self.inner_wrapper1, attribute)
except AttributeError: pass
try:
return getattr(self.inner_wrapper2, attribute)
except AttributeError: pass
class InnerWrapper1(object):
#property
def property1(self):
return self.__property1
#property1.setter
def property1(self, value):
self.__property1 = value
#property
def property2(self):
return self.__property2
#property2.setter
def property2(self, value):
self.__property2 = value
class InnerWrapper2(object):
#property
def property3(self):
return self.__property3
#property3.setter
def property3(self, value):
self.__property3 = value
#property
def property4(self):
return self.__property4
#property4.setter
def property4(self, value):
self.__property4 = value
def main():
data_wrapper = DataWrapper()
data_wrapper.property1 = "abc"
if __name__ == "__main__":
main()
You get an infinite recursion error because you forgot to take into account setting the inner_wrapper1 and inner_wrapper2 attributes in your __init__ method.
When you do this:
self.inner_wrapper1 = InnerWrapper()
Python will also use your __setattr__ method. This then tries to use self.inner_wrapper1 which doesn't yet exist so __getattr__ is called, which tries to use self.inner_wrapper1 which doesn't yet exist, and you enter into an infinite recursion loop.
In __setattr__ delegate attribute setting to the superclass:
def __setattr__(self, attribute, value):
if attribute in {'innerwrapper1', 'innerwrapper2'}:
return super(DataWrapper, self).__setattr__(attribute, value)
if hasattr(self.inner_wrapper1, attribute):
return setattr(self.inner_wrapper1, attribute, value)
elif hasattr(self.inner_wrapper2, attribute):
return setattr(self.inner_wrapper2, attribute, value)
If you used a single leading underscore for 'private' attributes (so _innerwrapper1 and _innerwrapper2) you could just test for that:
def __setattr__(self, attribute, value):
if attribute[0] == '_': # private attribute
return super(DataWrapper, self).__setattr__(attribute, value)
so you don't have to hardcode a whole set of names.
Since your updated full script uses __inner_wrapper1 and __inner_wrapper2 as the actual attribute names, and you are using properties, you'll have to adjust your __setattr__ test to look for those names. Because you are using double-underscore names you need to adjust for the name mangling of such attributes:
def __setattr__(self, attribute, value):
if attribute in {
'inner_wrapper1', 'inner_wrapper2',
'_DataWrapper__inner_wrapper1', '_DataWrapper__inner_wrapper2'}:
return super(DataWrapper, self).__setattr__(attribute, value)
Unless you are going to subclass DataWrapper and must protect your attributes from accidental overriding, I'd avoid using double-underscored names altogether, however. In Pythonic code, you don't worry about other code accessing attributes, there is no concept of truly private attributes.
Using properties is also overkill here; properties don't buy you encapsulation, in Python you'd only use those to simplify the API (replacing a method call with attribute access).
Note that the hasattr() tests for the InnerWrapper* property* attributes will fail because you don't have default values:
>>> inner = InnerWrapper1()
>>> hasattr(inner, 'property1')
False
hasattr() doesn't test for properties, it simply tries to access an attribute and if any exception is raised it returns False:
>>> inner = InnerWrapper1()
>>> hasattr(inner, 'property1')
False
>>> inner.property1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 43, in property1
AttributeError: 'InnerWrapper1' object has no attribute '_InnerWrapper1__property1'
>>> inner.property1 = 'foo'
>>> inner.property1
'foo'
>>> hasattr(inner, 'property1')
True
By removing all the #property objects you can simplify this greatly:
class DataWrapper(object):
def __init__(self):
self._inner_wrapper1 = InnerWrapper1()
self._inner_wrapper2 = InnerWrapper2()
def __setattr__(self, attribute, value):
if attribute[0] == '_':
return super(DataWrapper, self).__setattr__(attribute, value)
if hasattr(self._inner_wrapper1, attribute):
return setattr(self._inner_wrapper1, attribute, value)
elif hasattr(self._inner_wrapper2, attribute):
return setattr(self._inner_wrapper2, attribute, value)
def __getattr__(self, attribute):
try:
return getattr(self._inner_wrapper1, attribute)
except AttributeError: pass
return getattr(self._inner_wrapper2, attribute)
class InnerWrapper1(object):
property1 = None
property2 = None
class InnerWrapper2(object):
property3 = None
property4 = None
I'm writing a python class that uses __setattr__ and __getattr__ to provide custom attribute access.
However, some attributes can't be handled in a generic way, so I was hoping to use descriptors for those.
A problem arises in that for a descriptor, the descriptor's __get__ will be invoked in favour of the instances __getattr__, but when assigning to an attribute, __setattr__ will be invoked in favour of the descriptors __set__.
An example:
class MyDesc(object):
def __init__(self):
self.val = None
def __get__(self, instance, owner):
print "MyDesc.__get__"
return self.val
def __set__(self, instance, value):
print "MyDesc.__set__"
self.val = value
class MyObj(object):
foo = MyDesc()
def __init__(self, bar):
object.__setattr__(self, 'names', dict(
bar=bar,
))
object.__setattr__(self, 'new_names', dict())
def __setattr__(self, name, value):
print "MyObj.__setattr__ for %s" % name
self.new_names[name] = value
def __getattr__(self, name):
print "MyObj.__getattr__ for %s" % name
if name in self.new_names:
return self.new_names[name]
if name in self.names:
return self.names[name]
raise AttributeError(name)
if __name__ == "__main__":
o = MyObj('bar-init')
o.bar = 'baz'
print o.bar
o.foo = 'quux'
print o.foo
prints:
MyObj.__setattr__ for bar
MyObj.__getattr__ for bar
baz
MyObj.__setattr__ for foo
MyDesc.__get__
None
The descriptor's __set__ is never called.
Since the __setattr__ definition isn't just overriding behaviour for a limited set of names, there's no clear place that it can defer to object.__setattr__
Is there a recommended way to have assigning to attributes use the descriptor, if available, and __setattr__ otherwise?
I think I'd approach this by having a mechanism to automatically mark which are the
descriptors in each class, and wrap the __setattr__ in a way that it'd call
object's normal behavior for those names.
This can be easily achieved with a metaclass (and a decorator for __setattr__
def setattr_deco(setattr_func):
def setattr_wrapper(self, attr, value):
if attr in self._descriptors:
return object.__setattr__(self, attr, value)
return setattr_func(self, attr, value)
return setattr_wrapper
class MiscSetattr(type):
def __new__(metacls, name, bases, dct):
descriptors = set()
for key, obj in dct.items():
if key == "__setattr__":
dct[key] = setattr_deco(obj)
elif hasattr(obj, "__get__"):
descriptors.add(key)
dct["_descriptors"] = descriptors
return type.__new__(metacls, name, bases, dct)
# and use MiscSetattr as metaclass for your classes
One of possible ways:
def __setattr__(self, name, value):
print "MyObj.__setattr__ for %s" % name
for cls in self.__class__.__mro__ + (self, ):
if name in cls.__dict__:
return object.__setattr__(self, name, value)
print 'New name', name, value
self.new_names[name] = value
It checks if name already defined in class, base classes or instance and then it calls object.__setattr__ which will execute descriptor __set__.
Another way:
def __setattr__(self, name, value):
print "MyObj.__setattr__ for %s" % name
try:
object.__getattribute__(self, name)
except AttributeError:
print 'New name', name, value
self.new_names[name] = value
else:
object.__setattr__(self, name, value)
But it will call descriptor's __get__.
P.S.
I'm not sure about need to check all __mro__ members since MyObj will contain inherited class members in __dict__.
Maybe for cls in (self.__class__, self):... will be enough.
I'm solving the python koans.
I haven't got any real problem until the 34th.
this is the problem:
Project: Create a Proxy Class
In this assignment, create a proxy class (one is started for you
below). You should be able to initialize the proxy object with any
object. Any attributes called on the proxy object should be forwarded
to the target object. As each attribute call is sent, the proxy
should record the name of the attribute sent.
The proxy class is started for you. You will need to add a method
missing handler and any other supporting methods. The specification
of the Proxy class is given in the AboutProxyObjectProject koan.
Note: This is a bit trickier that it's Ruby Koans counterpart, but you
can do it!
and this is my solution until now:
class Proxy(object):
def __init__(self, target_object):
self._count = {}
#initialize '_obj' attribute last. Trust me on this!
self._obj = target_object
def __setattr__(self, name, value):pass
def __getattr__(self, attr):
if attr in self._count:
self._count[attr]+=1
else:
self._count[attr]=1
return getattr(self._obj, attr)
def messages(self):
return self._count.keys()
def was_called(self, attr):
if attr in self._count:
return True
else: False
def number_of_times_called(self, attr):
if attr in self._count:
return self._count[attr]
else: return False
It works until this test:
def test_proxy_records_messages_sent_to_tv(self):
tv = Proxy(Television())
tv.power()
tv.channel = 10
self.assertEqual(['power', 'channel='], tv.messages())
where tv.messages() is ['power'] because tv.channel=10 is taken by the proxy object and not the television object.
I've tried to manipulate the __setattr__ method, but I always end in a unlimited loop.
edit 1:
I'm trying this:
def __setattr__(self, name, value):
if hasattr(self, name):
object.__setattr__(self,name,value)
else:
object.__setattr__(self._obj, name, value)
But then I get this error in a loop on the last entry:
RuntimeError: maximum recursion depth exceeded while calling a Python object
File "/home/kurojishi/programmi/python_koans/python 2/koans/about_proxy_object_project.py", line 60, in test_proxy_method_returns_wrapped_object
tv = Proxy(Television())
File "/home/kurojishi/programmi/python_koans/python 2/koans/about_proxy_object_project.py", line 25, in __init__
self._count = {}
File "/home/kurojishi/programmi/python_koans/python 2/koans/about_proxy_object_project.py", line 33, in __setattr__
object.__setattr__(self._obj, name, value)
File "/home/kurojishi/programmi/python_koans/python 2/koans/about_proxy_object_project.py", line 36, in __getattr__
if attr in self._count:
The loop is in __getattr__.
You are using hasattr in __setattr__ to decide whether you should write to the local or proxied object. This works well for all but one case.
In your __init__ you have the following line:
self._count = {}
This calls __setattr__ with '_count' which does not exist at that point and therefore (hence hasattr returns False) is forwarded to the proxied object.
If you want to use your approach you have to write your __init__ like this:
def __init__(self, target_object):
object.__setattr__(self, '_count', {})
#initialize '_obj' attribute last. Trust me on this!
object.__setattr__(self, '_obj', target_object)
As I understand maybe your problem is related with the recursive call when you set and attribute value. From docs:
If __setattr__() wants to assign to an instance attribute, it should not simply execute "self.name = value" -- this would cause a recursive call to itself. Instead, it should insert the value in the dictionary of instance attributes, e.g., "self.__dict__[name] = value". For new-style classes, rather than accessing the instance dictionary, it should call the base class method with the same name, for example, "object.__setattr__(self, name, value)".
setattr is called on all assignments. It's more like getattribute than getattr. This also affects code in the __init__ method.
This means that the first branch of this code will almost always fail, only attributes inherited from object will pass the test:
def __setattr__(self, name, value):
if hasattr(self, name):
object.__setattr__(self,name,value)
else:
object.__setattr__(self._obj, name, value)
Instead we
can assume that assignments are meant for the Proxy unless it has an _obj attribute. Hence the comment in __init__. We set up our proxy's attributes, then add the target object and all future assignments get sent to it.
def __setattr__(self, name, value):
if hasattr(self, '_obj'):
object.__setattr__(self._obj, name, value)
else:
object.__setattr__(self, name, value)
But by using hasattr we would also need to alter __getattr__ to check for _obj to prevent recursion:
def __getattr__(self, name):
if '_obj' == name:
raise AttributeError
if attr in self._count:
self._count[attr]+=1
else:
self._count[attr]=1
return getattr(self._obj, attr)
An alternative would be to inspect the proxy's __dict__ attribute directly in the __setattr__ method:
def __setattr__(self, name, value):
if '_obj' in self.__dict__:
...
from the test, it is a requirement for proxy to log all the attribute calls via proxy. And the proxy has only few built-in methods which are exceptionally used for logging, so my answer was:
class Proxy(object):
def __init__(self, target_object):
self.logs=[]
self._obj = target_object
def __getattribute__(self, attrname):
if attrname in ['_obj','logs','messages','was_called','number_of_times_called'] :
return object.__getattribute__(self, attrname)
else:
self.logs.append(attrname)
return object.__getattribute__((object.__getattribute__(self, '_obj')), attrname)
def __setattr__(self, name, value):
if hasattr(self, '_obj'):
self.logs.append(name)
object.__setattr__(object.__getattribute__(self,'_obj'), name, value)
else :
object.__setattr__(self, name, value)
After this it is quite easy to implement other methods ('messages', 'was_called', ... )
Sorry for necro'ing old question.
and I found out that getattribute can be changed : just check whether the attribute is in the target object.
def __getattribute__(self, attrname):
if attrname not in dir(object.__getattribute__(self, '_obj')):
return object.__getattribute__(self, attrname)
else:
self.logs.append(attrname)
return object.__getattribute__((object.__getattribute__(self, '_obj')), attrname)
class Proxy(object):
"""Proxy class wraps any other class, and adds functionality to remember and report all messages called.
Limitations include that proxy blocks all direct subclass calls to:
messages, number_of_times_called, was_called, _obj, and _message_counts.
These calls must be made directly like my_proxy_instance._obj.messages.
"""
def __init__(self, target_object):
print 'initializing a proxy for ' + target_object.__class__.__name__
# WRITE CODE HERE
self._message_counts = Counter();
#initialize '_obj' attribute last. Trust me on this!
self._obj = target_object
# WRITE CODE HERE
def __getattr__(self, attr_name):
print 'getting an attribute: "' + attr_name + '" from "' + self._obj.__class__.__name__ + '"'
self._message_counts[attr_name] += 1
print self._message_counts
return object.__getattribute__(self._obj, attr_name)
#def __getattribute__(self, attr_name):
# print "intercepted!~ " + attr_name
# object.__getattribute__(self, attr_name)
def __setattr__(self, attr_name, value):
if((attr_name == '_obj') | (attr_name == '_message_counts')): # special proxy attributes.
print 'setting the PROXY attribute: "' + attr_name + '"'
object.__setattr__(self, attr_name, value)
else:
print 'setting the REAL attribute: "' + attr_name + '"'
self._message_counts[attr_name+"="] += 1
object.__setattr__(self._obj, attr_name, value)
def messages(self):
return self._message_counts.keys()
def number_of_times_called(self, attr_name):
return self._message_counts[attr_name]
def was_called(self, attr_name):
return attr_name in self._message_counts
What I did was take all the calls to attributes in the proxy and call them via object.__getattribute__ to avoid recursion.
That did not work for methods so I wrapped the method calls in a try..except AttributeError to try them first in the proxy. and then if they raise an error try them in the child object.
If anyone has a more elegant solution would love to see it.
from runner.koan import *
from collections import Counter
class Proxy(object):
def __init__(self, target_object):
self._messages=[]
self._obj = target_object
def messages(self):
return self._messages
def was_called(self, message):
return message in self._messages
def number_of_times_called(self, message):
_count = Counter(self._messages).get(message)
if _count:
return _count
else: # catch None
return 0
def __getattribute__(self, attr_name):
try: # call on self
retval = object.__getattribute__(self, attr_name)
except AttributeError: # call on child object
retval = self._obj.__getattribute__(attr_name)
object.__getattribute__(self, '_messages').append(attr_name)
return retval
def __setattr__(self, attr_name, attr_value):
if hasattr(self, '_obj'): # call child object and log message
self._obj.__setattr__(attr_name, attr_value)
attr_name += "="
object.__getattribute__(self, '_messages').append(attr_name)
else: # use this before_obj is set in __init__
object.__setattr__(self, attr_name, attr_value)
def messages(self):
return self._messages
why not use method_missing?
my answer:
class Proxy
def initialize(target_object)
#object = target_object
# ADD MORE CODE HERE
#messages = []
end
# WRITE CODE HERE
def method_missing(method_name, *args, &block)
#messages.push method_name unless method_name == :messages
#object.send method_name, *args, &block
end
def messages
#messages
end
def called? target
#messages.include? target
end
def number_of_times_called target
result = 0
#messages.each do |t|
result += 1 if t == target
end
result
end
end