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Recursion error in python setattr [duplicate]

I want to define a class containing read and write methods, which can be called as follows:
instance.read
instance.write
instance.device.read
instance.device.write
To not use interlaced classes, my idea was to overwrite the __getattr__ and __setattr__ methods and to check, if the given name is device to redirect the return to self. But I encountered a problem giving infinite recursions. The example code is as follows:
class MyTest(object):
def __init__(self, x):
self.x = x
def __setattr__(self, name, value):
if name=="device":
print "device test"
else:
setattr(self, name, value)
test = MyTest(1)
As in __init__ the code tried to create a new attribute x, it calls __setattr__, which again calls __setattr__ and so on. How do I need to change this code, that, in this case, a new attribute x of self is created, holding the value 1?
Or is there any better way to handle calls like instance.device.read to be 'mapped' to instance.read?
As there are always questions about the why: I need to create abstractions of xmlrpc calls, for which very easy methods like myxmlrpc.instance,device.read and similar can be created. I need to 'mock' this up to mimic such multi-dot-method calls.

You must call the parent class __setattr__ method:
class MyTest(object):
def __init__(self, x):
self.x = x
def __setattr__(self, name, value):
if name=="device":
print "device test"
else:
super(MyTest, self).__setattr__(name, value)
# in python3+ you can omit the arguments to super:
#super().__setattr__(name, value)
Regarding the best-practice, since you plan to use this via xml-rpc I think this is probably better done inside the _dispatch method.
A quick and dirty way is to simply do:
class My(object):
def __init__(self):
self.device = self

Or you can modify self.__dict__ from inside __setattr__():
class SomeClass(object):
def __setattr__(self, name, value):
print(name, value)
self.__dict__[name] = value
def __init__(self, attr1, attr2):
self.attr1 = attr1
self.attr2 = attr2
sc = SomeClass(attr1=1, attr2=2)
sc.attr1 = 3

You can also use object.
class TestClass:
def __init__(self):
self.data = 'data'
def __setattr__(self, name, value):
print("Attempt to edit the attribute %s" %(name))
object.__setattr__(self, name, value)

or you can just use #property:
class MyTest(object):
def __init__(self, x):
self.x = x
#property
def device(self):
return self

If you don't want to specify which attributes can or cannot be set, you can split the class to delay the get/set hooks until after initialization:
class MyTest(object):
def __init__(self, x):
self.x = x
self.__class__ = _MyTestWithHooks
class _MyTestWithHooks(MyTest):
def __setattr__(self, name, value):
...
def __getattr__(self, name):
...
if __name__ == '__main__':
a = MyTest(12)
...
As noted in the code you'll want to instantiate MyTest, since instantiating _MyTestWithHooks will result in the same infinite recursion problem as before.

Overwrite base class attribute with #property of the same name

I am trying to subclass a python class and overwrite a regular attribute with a #property function. The catch is that I can't modify the parent class, and the api for the child class needs to look the same as the parent class (but behave differently). (So my question is different from this one in which the parent class also used a #property method to access the underlying attribute.)
The simplest possible example is
# assume this class can't be overwritten
class Parent(object):
def __init__(self, a):
self.attr = a
# how do I make this work?
class Child(Parent):
def __init__(self, a):
super(Child, self).__init__(a)
# overwrite access to attr with a function
#property
def attr(self):
return super(Child, self).attr**2
c = Child(4)
print c.attr # should be 16
This produces an error when the parent init method is called.
<ipython-input-15-356fb0400868> in __init__(self, a)
2 class Parent(object):
3 def __init__(self, a):
----> 4 self.attr = a
5
6 # how do I make this work?
AttributeError: can't set attribute
Hopefully it is clear what I want to do and why. But I can't figure out how.

This is easily fixed by adding a setter method
class Child(Parent):
def __init__(self, a):
self._attr = None
super(Child, self).__init__(a)
# overwrite access to a with a function
#property
def attr(self):
return self._attr**2
#attr.setter
def attr(self, value):
self._attr = value

Is there a way to make code run when a class is derived from a particular parent class in Python?

For example, if I create the class Foo, then later derive the subclass Bar, I want the myCode() method of Foo to run.
class Foo(object):
x = 0
def __init__(self):
pass
def myCode(self):
if(self.x == 0):
raise Exception("nope")
class Bar(Foo): #This is where I want myCode() to execute
def baz(self):
pass
This should happen any time a class is derived from the base class Foo. Is it possible to do this in Python? I'm using Python 3 if it matters.
Note: In my real code, Foo is actually an abstract base class.
Edit: I also need access to derived class member data and methods in myCode().

Use a metaclass:
class MetaClass:
def __init__(cls, name, bases, dictionary):
if name is not 'Parent':
print('Subclass created with name: %s' % name)
super().__init__(name, bases, dictionary)
class Parent(metaclass=MetaClass):
pass
class Subclass(Parent):
pass
Output:
Subclass created with name: Subclass
Metaclasses control how classes themselves are created. Subclass inherits its metaclass from Parent, and thus that code gets run when it is defined.
Edit: As for your use case with an abstract base class, off the top of my head I think you'd just need to define your metaclass as a subclass of ABCMeta, but I didn't test that.

May this code can help you:
class Foo:
def myCode(self):
print('myCode within Foo')
def __init__(self):
if type(self) != Foo:
self.myCode()
class Bar(Foo):
def __init__(self):
super(Bar, self).__init__()
def baz(self):
pass
Test:
>>>
>>> f = Foo()
>>> b = Bar()
myCode within Foo
>>>

This works:
class MyMeta(type):
def __new__(cls, name, parents, dct):
if name is not 'Foo':
if 'x' not in dct:
raise Exception("Nein!")
elif 'x' in dct and dct['x'] == 0:
raise Exception("Nope!")
return super(MyMeta, cls).__new__(cls, name, parents, dct)
Output:
class Bar(Foo):
x = 0
> Exception: Nope!
This is my specific use case if anyone wants to comment on whether or not this is appropriate:
class MagmaMeta(type):
def __new__(cls, name, parents, dct):
# Check that Magma instances are valid.
if name is not 'Magma':
if 'CAYLEY_TABLE' not in dct:
raise Exception("Cannot create Magma instance without CAYLEY_TABLE")
else:
# Check for square CAYLEY_TABLE
for row in CAYLEY_TABLE:
if not len(row) == len(dct['CAYLEY_TABLE']):
raise Exception("CAYLEY_TABLE must be a square array")
# Create SET and ORDER from CAYLEY_TABLE
dct['SET'] = set([])
for rows in CAYLEY_TABLE:
for x in rows:
dct['SET'].add(x)
dct['ORDER'] = len(dct['SET'])
return super(MyMeta, cls).__new__(cls, name, parents, dct)

dynamically adding callable to class as instance "method"

I implemented a metaclass that tears down the class attributes for classes created with it and builds methods from the data from those arguments, then attaches those dynamically created methods directly to the class object (the class in question allows for easy definition of web form objects for use in a web testing framework). It has been working just fine, but now I have a need to add a more complex type of method, which, to try to keep things clean, I implemented as a callable class. Unfortunately, when I try to call the callable class on an instance, it is treated as a class attribute instead of an instance method, and when called, only receives its own self. I can see why this happens, but I was hoping someone might have a better solution than the ones I've come up with. Simplified illustration of the problem:
class Foo(object):
def __init__(self, name, val):
self.name = name
self.val = val
self.__name__ = name + '_foo'
self.name = name
# This doesn't work as I'd wish
def __call__(self, instance):
return self.name + str(self.val + instance.val)
def get_methods(name, foo_val):
foo = Foo(name, foo_val)
def bar(self):
return name + str(self.val + 2)
bar.__name__ = name + '_bar'
return foo, bar
class Baz(object):
def __init__(self, val):
self.val = val
for method in get_methods('biff', 1):
setattr(Baz, method.__name__, method)
baz = Baz(10)
# baz.val == 10
# baz.biff_foo() == 'biff11'
# baz.biff_bar() == 'biff12'
I've thought of:
Using a descriptor, but that seems way more complex than is necessary here
Using a closure inside of a factory for foo, but nested closures are ugly and messy replacements for objects most of the time, imo
Wrapping the Foo instance in a method that passes its self down to the Foo instance as instance, basically a decorator, that is what I actually add to Baz, but that seems superfluous and basically just a more complicated way of doing the same thing as (2)
Is there a better way then any of these to try to accomplish what I want, or should I just bite the bullet and use some closure factory type pattern?

One way to do this is to attach the callable objects to the class as unbound methods. The method constructor will work with arbitrary callables (i.e. instances of classes with a __call__() method)—not just functions.
from types import MethodType
class Foo(object):
def __init__(self, name, val):
self.name = name
self.val = val
self.__name__ = name + '_foo'
self.name = name
def __call__(self, instance):
return self.name + str(self.val + instance.val)
class Baz(object):
def __init__(self, val):
self.val = val
Baz.biff = MethodType(Foo("biff", 42), None, Baz)
b = Baz(13)
print b.biff()
>>> biff55
In Python 3, there's no such thing as an unbound instance method (classes just have regular functions attached) so you might instead make your Foo class a descriptor that returns a bound instance method by giving it a __get__() method. (Actually, that approach will work in Python 2.x as well, but the above will perform a little better.)
from types import MethodType
class Foo(object):
def __init__(self, name, val):
self.name = name
self.val = val
self.__name__ = name + '_foo'
self.name = name
def __call__(self, instance):
return self.name + str(self.val + instance.val)
def __get__(self, instance, owner):
return MethodType(self, instance) if instance else self
# Python 2: MethodType(self, instance, owner)
class Baz(object):
def __init__(self, val):
self.val = val
Baz.biff = Foo("biff", 42)
b = Baz(13)
print b.biff()
>>> biff55

The trouble you're running into is that your object is not being bound as a method of the Baz class you're putting it in. This is because it is not a descriptor, which regular functions are!
You can fix this by adding a simple __get__ method to your Foo class that makes it into a method when it's accessed as a descriptor:
import types
class Foo(object):
# your other stuff here
def __get__(self, obj, objtype=None):
if obj is None:
return self # unbound
else:
return types.MethodType(self, obj) # bound to obj

How to make a class property? [duplicate]

This question already has answers here:
Using property() on classmethods
(19 answers)
Closed 3 years ago.
In python I can add a method to a class with the #classmethod decorator. Is there a similar decorator to add a property to a class? I can better show what I'm talking about.
class Example(object):
the_I = 10
def __init__( self ):
self.an_i = 20
#property
def i( self ):
return self.an_i
def inc_i( self ):
self.an_i += 1
# is this even possible?
#classproperty
def I( cls ):
return cls.the_I
#classmethod
def inc_I( cls ):
cls.the_I += 1
e = Example()
assert e.i == 20
e.inc_i()
assert e.i == 21
assert Example.I == 10
Example.inc_I()
assert Example.I == 11
Is the syntax I've used above possible or would it require something more?
The reason I want class properties is so I can lazy load class attributes, which seems reasonable enough.

Here's how I would do this:
class ClassPropertyDescriptor(object):
def __init__(self, fget, fset=None):
self.fget = fget
self.fset = fset
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
return self.fget.__get__(obj, klass)()
def __set__(self, obj, value):
if not self.fset:
raise AttributeError("can't set attribute")
type_ = type(obj)
return self.fset.__get__(obj, type_)(value)
def setter(self, func):
if not isinstance(func, (classmethod, staticmethod)):
func = classmethod(func)
self.fset = func
return self
def classproperty(func):
if not isinstance(func, (classmethod, staticmethod)):
func = classmethod(func)
return ClassPropertyDescriptor(func)
class Bar(object):
_bar = 1
#classproperty
def bar(cls):
return cls._bar
#bar.setter
def bar(cls, value):
cls._bar = value
# test instance instantiation
foo = Bar()
assert foo.bar == 1
baz = Bar()
assert baz.bar == 1
# test static variable
baz.bar = 5
assert foo.bar == 5
# test setting variable on the class
Bar.bar = 50
assert baz.bar == 50
assert foo.bar == 50
The setter didn't work at the time we call Bar.bar, because we are calling
TypeOfBar.bar.__set__, which is not Bar.bar.__set__.
Adding a metaclass definition solves this:
class ClassPropertyMetaClass(type):
def __setattr__(self, key, value):
if key in self.__dict__:
obj = self.__dict__.get(key)
if obj and type(obj) is ClassPropertyDescriptor:
return obj.__set__(self, value)
return super(ClassPropertyMetaClass, self).__setattr__(key, value)
# and update class define:
# class Bar(object):
# __metaclass__ = ClassPropertyMetaClass
# _bar = 1
# and update ClassPropertyDescriptor.__set__
# def __set__(self, obj, value):
# if not self.fset:
# raise AttributeError("can't set attribute")
# if inspect.isclass(obj):
# type_ = obj
# obj = None
# else:
# type_ = type(obj)
# return self.fset.__get__(obj, type_)(value)
Now all will be fine.

If you define classproperty as follows, then your example works exactly as you requested.
class classproperty(object):
def __init__(self, f):
self.f = f
def __get__(self, obj, owner):
return self.f(owner)
The caveat is that you can't use this for writable properties. While e.I = 20 will raise an AttributeError, Example.I = 20 will overwrite the property object itself.

[answer written based on python 3.4; the metaclass syntax differs in 2 but I think the technique will still work]
You can do this with a metaclass...mostly. Dappawit's almost works, but I think it has a flaw:
class MetaFoo(type):
#property
def thingy(cls):
return cls._thingy
class Foo(object, metaclass=MetaFoo):
_thingy = 23
This gets you a classproperty on Foo, but there's a problem...
print("Foo.thingy is {}".format(Foo.thingy))
# Foo.thingy is 23
# Yay, the classmethod-property is working as intended!
foo = Foo()
if hasattr(foo, "thingy"):
print("Foo().thingy is {}".format(foo.thingy))
else:
print("Foo instance has no attribute 'thingy'")
# Foo instance has no attribute 'thingy'
# Wha....?
What the hell is going on here? Why can't I reach the class property from an instance?
I was beating my head on this for quite a while before finding what I believe is the answer. Python #properties are a subset of descriptors, and, from the descriptor documentation (emphasis mine):
The default behavior for attribute access is to get, set, or delete the
attribute from an object’s dictionary. For instance, a.x has a lookup chain
starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing
through the base classes of type(a) excluding metaclasses.
So the method resolution order doesn't include our class properties (or anything else defined in the metaclass). It is possible to make a subclass of the built-in property decorator that behaves differently, but (citation needed) I've gotten the impression googling that the developers had a good reason (which I do not understand) for doing it that way.
That doesn't mean we're out of luck; we can access the properties on the class itself just fine...and we can get the class from type(self) within the instance, which we can use to make #property dispatchers:
class Foo(object, metaclass=MetaFoo):
_thingy = 23
#property
def thingy(self):
return type(self).thingy
Now Foo().thingy works as intended for both the class and the instances! It will also continue to do the right thing if a derived class replaces its underlying _thingy (which is the use case that got me on this hunt originally).
This isn't 100% satisfying to me -- having to do setup in both the metaclass and object class feels like it violates the DRY principle. But the latter is just a one-line dispatcher; I'm mostly okay with it existing, and you could probably compact it down to a lambda or something if you really wanted.

If you use Django, it has a built in #classproperty decorator.
from django.utils.decorators import classproperty
For Django 4, use:
from django.utils.functional import classproperty

I think you may be able to do this with the metaclass. Since the metaclass can be like a class for the class (if that makes sense). I know you can assign a __call__() method to the metaclass to override calling the class, MyClass(). I wonder if using the property decorator on the metaclass operates similarly.
Wow, it works:
class MetaClass(type):
def getfoo(self):
return self._foo
foo = property(getfoo)
#property
def bar(self):
return self._bar
class MyClass(object):
__metaclass__ = MetaClass
_foo = 'abc'
_bar = 'def'
print MyClass.foo
print MyClass.bar
Note: This is in Python 2.7. Python 3+ uses a different technique to declare a metaclass. Use: class MyClass(metaclass=MetaClass):, remove __metaclass__, and the rest is the same.

As far as I can tell, there is no way to write a setter for a class property without creating a new metaclass.
I have found that the following method works. Define a metaclass with all of the class properties and setters you want. IE, I wanted a class with a title property with a setter. Here's what I wrote:
class TitleMeta(type):
#property
def title(self):
return getattr(self, '_title', 'Default Title')
#title.setter
def title(self, title):
self._title = title
# Do whatever else you want when the title is set...
Now make the actual class you want as normal, except have it use the metaclass you created above.
# Python 2 style:
class ClassWithTitle(object):
__metaclass__ = TitleMeta
# The rest of your class definition...
# Python 3 style:
class ClassWithTitle(object, metaclass = TitleMeta):
# Your class definition...
It's a bit weird to define this metaclass as we did above if we'll only ever use it on the single class. In that case, if you're using the Python 2 style, you can actually define the metaclass inside the class body. That way it's not defined in the module scope.

def _create_type(meta, name, attrs):
type_name = f'{name}Type'
type_attrs = {}
for k, v in attrs.items():
if type(v) is _ClassPropertyDescriptor:
type_attrs[k] = v
return type(type_name, (meta,), type_attrs)
class ClassPropertyType(type):
def __new__(meta, name, bases, attrs):
Type = _create_type(meta, name, attrs)
cls = super().__new__(meta, name, bases, attrs)
cls.__class__ = Type
return cls
class _ClassPropertyDescriptor(object):
def __init__(self, fget, fset=None):
self.fget = fget
self.fset = fset
def __get__(self, obj, owner):
if self in obj.__dict__.values():
return self.fget(obj)
return self.fget(owner)
def __set__(self, obj, value):
if not self.fset:
raise AttributeError("can't set attribute")
return self.fset(obj, value)
def setter(self, func):
self.fset = func
return self
def classproperty(func):
return _ClassPropertyDescriptor(func)
class Bar(metaclass=ClassPropertyType):
__bar = 1
#classproperty
def bar(cls):
return cls.__bar
#bar.setter
def bar(cls, value):
cls.__bar = value
bar = Bar()
assert Bar.bar==1
Bar.bar=2
assert bar.bar==2
nbar = Bar()
assert nbar.bar==2

I happened to come up with a solution very similar to #Andrew, only DRY
class MetaFoo(type):
def __new__(mc1, name, bases, nmspc):
nmspc.update({'thingy': MetaFoo.thingy})
return super(MetaFoo, mc1).__new__(mc1, name, bases, nmspc)
#property
def thingy(cls):
if not inspect.isclass(cls):
cls = type(cls)
return cls._thingy
#thingy.setter
def thingy(cls, value):
if not inspect.isclass(cls):
cls = type(cls)
cls._thingy = value
class Foo(metaclass=MetaFoo):
_thingy = 23
class Bar(Foo)
_thingy = 12
This has the best of all answers:
The "metaproperty" is added to the class, so that it will still be a property of the instance
Don't need to redefine thingy in any of the classes
The property works as a "class property" in for both instance and class
You have the flexibility to customize how _thingy is inherited
In my case, I actually customized _thingy to be different for every child, without defining it in each class (and without a default value) by:
def __new__(mc1, name, bases, nmspc):
nmspc.update({'thingy': MetaFoo.services, '_thingy': None})
return super(MetaFoo, mc1).__new__(mc1, name, bases, nmspc)

If you only need lazy loading, then you could just have a class initialisation method.
EXAMPLE_SET = False
class Example(object):
#classmethod
def initclass(cls):
global EXAMPLE_SET
if EXAMPLE_SET: return
cls.the_I = 'ok'
EXAMPLE_SET = True
def __init__( self ):
Example.initclass()
self.an_i = 20
try:
print Example.the_I
except AttributeError:
print 'ok class not "loaded"'
foo = Example()
print foo.the_I
print Example.the_I
But the metaclass approach seems cleaner, and with more predictable behavior.
Perhaps what you're looking for is the Singleton design pattern. There's a nice SO QA about implementing shared state in Python.