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I have a class as follows:
class A:
def __init__(self):
pass
def add_attr(self, name):
setattr(self, name, 'something')
How do I define custom setter, getter for self.name? I cannot use __setattr__, __getattribute__ because that will change the behaviour of add_attr too.
EDIT: the users of this class will add arbitrary number of attributes with arbitrary names:
a = A()
a.add_attr('attr1')
a.add_attr('attr2')
I want custom behavior for only these user added attributes.
Building off #Devesh Kumar Singh’s answer, I would implement it in some way like this:
class A:
def __init__(self):
self.attrs = {}
def __setattr__(self, key, value):
if key in self.attrs:
self.set_attr(key, value)
else:
object.__setattr__(self, key, value)
def __getattribute__(self, key):
if key in self.__dict__.get(attrs, {}):
return self.__dict__['get_attr'](self, key)
return object.__getattribute__(self, key)
def get_attr(self, key):
r = self.attrs[key]
# logic
return r
def set_attr(self, key, value):
# logic
self.attrs[key] = value
def add_attr(self, key, value=None):
self.attrs[key] = value
add_attr is only used to initialise the variable the first time. You could also edit __setattr__ to set all new attributes in the self.attrs rather than self.__dict__
Custom getter and setter logic? That's what a property is made for. Usually these are used to magically mask function calls and make them look like attribute access
class MyDoubler(object):
def __init__(self, x):
self._x = x
#property
def x(self):
return x * 2
#x.setter
def x(self, value):
self._x = value
>>> md = MyDoubler(10)
>>> md.x
20
>>> md.x = 20
>>> md.x
40
>>> md._x
20
But there's no rule saying you can't abuse that power to add custom behavior to your getters and setters.
class A(object):
def __init__(self):
pass
#staticmethod
def default_getter_factory(name):
def default_getter(self):
return self.name
return default_getter
#staticmethod
def default_setter_factory(name):
def default_setter(self, value):
setattr(self, name, value)
return default_setter
def add_attr(self, name, getterfactory=None, setterfactory=None):
private_name = f"_{name}"
if getterfactory is None:
getterfactory = self.__class__.default_getter_factory
if setterfactory is None:
setterfactory = self.__class__.default_setter_factory
getter, setter = getterfactory(private_name), setterfactory(private_name)
getter = property(getter)
setattr(self.__class__, name, getter)
setattr(self.__class__, name, getter.setter(setter))
That said this is all a bit silly, and chances are that whatever it is you're trying to do is a thing that shouldn't be done. Dynamic programming is all well and good, but if I were to review code that did this, I would think very long and hard about alternative solutions before approving it. This reeks of technical debt to me.
One possibility I could think of is to have a dictionary of dynamic attributes, and set and get the dynamic attributes using the dictionary
class A:
def __init__(self):
#Dictionary of attributes
self.attrs = {}
#Set attribute
def set_attr(self, name):
self.attrs[name] = 'something'
#Get attribute
def get_attr(self, name):
return self.attrs.get(name)
a = A()
a.set_attr('var')
print(a.get_attr('var'))
The output will be something
Or an alternate is to use property decorator to add arguments explicitly outside the class, as described here
class A:
def __init__(self):
pass
a = A()
#Add attributes via property decorator
a.attr_1 = property(lambda self: self.attr_1)
a.attr_2 = property(lambda self: self.attr_2)
#Assign them values and print them
a.attr_1 = 4
a.attr_2 = 6
print(a.attr_1, a.attr_2)
The output will be 4 6
I am gonna answer my own question just for reference. This is based on others' answers here. The idea is to use default __setattr__ and __getattribute__ on attributes not added through add_attr.
class A:
def __init__(self):
self.attrs = {}
def add_attr(self, name):
self.attrs[name] = 'something'
def __getattribute__(self, name):
try:
object.__getattribute__(self, 'attrs')[name] # valid only if added by user
# custom logic and return
except (KeyError, AttributeError):
return object.__getattribute__(self, name)
def __setattr__(self, name, val):
# similar to __getattribute__
Lets take the following example of class decorators (origin http://www.informit.com/articles/article.aspx?p=1309289&seqNum=4):
class GenericDescriptor:
def __init__(self, getter, setter):
self.getter = getter
self.setter = setter
def __get__(self, instance, owner=None):
if instance is None:
return self
return self.getter(instance)
def __set__(self, instance, value):
return self.setter(instance, value)
def valid_string(attr_name, empty_allowed=True, regex=None,
acceptable=None):
def decorator(cls):
name = "__" + attr_name
def getter(self):
return getattr(self, name)
def setter(self, value):
assert isinstance(value, str), (attr_name +
" must be a string")
if not empty_allowed and not value:
raise ValueError("{0} may not be empty".format(
attr_name))
if ((acceptable is not None and value not in acceptable) or
(regex is not None and not regex.match(value))):
raise ValueError("{attr_name} cannot be set to "
"{value}".format(**locals()))
setattr(self, name, value)
setattr(cls, attr_name, GenericDescriptor(getter, setter))
return cls
return decorator
#valid_string("name", empty_allowed=False)
class StockItem:
name = None
def __init__(self, **kwargs):
if kwargs.get('second_call'):
pass
# proceed normally without calling #valid_string
self.name = kwargs.get('name', None)
self.price = kwargs.get('price', None)
self.quantity = kwargs.get('quantity', None)
if __name__ == "__main__":
import doctest
doctest.testmod()
# valid value for name
cameras1 = StockItem(name="Camera", price=45.99, quatity=2)
# invalid value for name according to #valid_string but I need this to be also valid if 'second_call'
cameras2 = StockItem(name=67, price=45.99, quatity=2, second_call=True)
The StockItem class constructor is invoked twice and on the second turn I want the #valid_string decorator to be somehow canceled (I don't want name attribute's value to be altered anymore).
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.
import inspect
import functools
def for_all_test_methods(decorator):
def decorate(cls):
for name, value in inspect.getmembers(cls, inspect.isroutine):
if name.startswith('test'):
setattr(cls, name, test_decorator(getattr(cls, name)))
return cls
return decorate
def test_decorator(func):
#functools.wraps(func)
def wrapper(*args, **kwargs):
print(func.__name__, args, kwargs)
res = func(*args, **kwargs)
return res
return wrapper
#for_all_test_methods(test_decorator)
class Potato(object):
def test_method(self):
print('in method')
class Spud(Potato):
def test_derived(self):
print('in derived')
Now if I create a spud instance the test_method which it has inherited remains decorated, but it has an undecorated method test_derived. Unfortunately, if I add the class decorator onto Spud aswell, then his test_method gets decorated twice!
How do I correctly propagate decorators from the parent class onto the children?
You cannot avoid decorating derived classes; you can find subclasses of a class after subclasses have been decorated, but not auto-decorate them. Use a metaclass instead of you need that sort of behaviour.
You can do one of two things:
Detect already-decorated methods; if there is a __wrapped__ attribute you have a wrapper:
def for_all_test_methods(decorator):
def decorate(cls):
for name, value in inspect.getmembers(cls, inspect.isroutine):
if name.startswith('test') and not hasattr(value, '__wrapped__'):
setattr(cls, name, test_decorator(getattr(cls, name)))
return cls
return decorate
Limit the class decorator to direct methods only:
def for_all_test_methods(decorator):
def decorate(cls):
for name, value in cls.__dict__.iteritems():
if name.startswith('test') and inspect.isroutine(value)):
setattr(cls, name, test_decorator(getattr(cls, name)))
return cls
return decorate
Here is how you can accomplish this by using a metaclass instead of decorating the class:
import inspect
import functools
def test_decorator(func):
#functools.wraps(func)
def wrapper(*args, **kwargs):
print(func.__name__, args, kwargs)
res = func(*args, **kwargs)
return res
return wrapper
def make_test_deco_type(decorator):
class TestDecoType(type):
def __new__(cls, clsname, bases, dct):
for name, value in dct.items():
if name.startswith('test') and inspect.isroutine(value):
dct[name] = decorator(value)
return super().__new__(cls, clsname, bases, dct)
return TestDecoType
class Potato(object, metaclass=make_test_deco_type(test_decorator)):
def test_method(self):
print('in method')
class Spud(Potato):
def test_derived(self):
print('in derived')
On Python 2.x you would use __metaclass__ = make_test_deco_type(test_decorator) as the first line of the class body instead of having the metaclass=... portion of the class statement. You would also need to replace super() with super(TestDecoType, cls).
I am trying to override the __setattr__ method of a Python class, since I want to call another function each time an instance attribute changes its value. However, I don't want this behaviour in the __init__ method, because during this initialization I set some attributes which are going to be used later:
So far I have this solution, without overriding __setattr__ at runtime:
class Foo(object):
def __init__(self, a, host):
object.__setattr__(self, 'a', a)
object.__setattr__(self, 'b', b)
result = self.process(a)
for key, value in result.items():
object.__setattr__(self, key, value)
def __setattr__(self, name, value):
print(self.b) # Call to a function using self.b
object.__setattr__(self, name, value)
However, I would like to avoid these object.__setattr__(...) and override __setattr__ at the end of the __init__ method:
class Foo(object):
def __init__(self, a, b):
self.a = a
self.b = b
result = self.process(a)
for key, value in result.items():
setattr(self, key, value)
# override self.__setattr__ here
def aux(self, name, value):
print(self.b)
object.__setattr__(self, name, value)
I have tried with self.__dict__['__setitem__'] = self.aux and object.__setitem__['__setitem__'] = self.aux, but none of these attemps has effect. I have read this section of the data model reference, but it looks like the assignment of the own __setattr__ is a bit tricky.
How could be possible to override __setattr__ at the end of __init__, or at least have a pythonic solution where __setattr__ is called in the normal way only in the constructor?
Unfortunately, there's no way to "override, after init" python special methods; as a side effect of how that lookup works. The crux of the problem is that python doesn't actually look at the instance; except to get its class; before it starts looking up the special method; so there's no way to get the object's state to affect which method is looked up.
If you don't like the special behavior in __init__, you could refactor your code to put the special knowledge in __setattr__ instead. Something like:
class Foo(object):
__initialized = False
def __init__(self, a, b):
try:
self.a = a
self.b = b
# ...
finally:
self.__initialized = True
def __setattr__(self, attr, value):
if self.__initialzed:
print(self.b)
super(Foo, self).__setattr__(attr, value)
Edit: Actually, there is a way to change which special method is looked up, so long as you change its class after it has been initialized. This approach will send you far into the weeds of metaclasses, so without further explanation, here's how that looks:
class AssignableSetattr(type):
def __new__(mcls, name, bases, attrs):
def __setattr__(self, attr, value):
object.__setattr__(self, attr, value)
init_attrs = dict(attrs)
init_attrs['__setattr__'] = __setattr__
init_cls = super(AssignableSetattr, mcls).__new__(mcls, name, bases, init_attrs)
real_cls = super(AssignableSetattr, mcls).__new__(mcls, name, (init_cls,), attrs)
init_cls.__real_cls = real_cls
return init_cls
def __call__(cls, *args, **kwargs):
self = super(AssignableSetattr, cls).__call__(*args, **kwargs)
print "Created", self
real_cls = cls.__real_cls
self.__class__ = real_cls
return self
class Foo(object):
__metaclass__ = AssignableSetattr
def __init__(self, a, b):
self.a = a
self.b = b
for key, value in process(a).items():
setattr(self, key, value)
def __setattr__(self, attr, value):
frob(self.b)
super(Foo, self).__setattr__(attr, value)
def process(a):
print "processing"
return {'c': 3 * a}
def frob(x):
print "frobbing", x
myfoo = Foo(1, 2)
myfoo.d = myfoo.c + 1
#SingleNegationElimination's answer is great, but it cannot work with inheritence, since the child class's __mro__ store's the original class of super class. Inspired by his answer, with little change,
The idea is simple, switch __setattr__ before __init__, and restore it back after __init__ completed.
class CleanSetAttrMeta(type):
def __call__(cls, *args, **kwargs):
real_setattr = cls.__setattr__
cls.__setattr__ = object.__setattr__
self = super(CleanSetAttrMeta, cls).__call__(*args, **kwargs)
cls.__setattr__ = real_setattr
return self
class Foo(object):
__metaclass__ = CleanSetAttrMeta
def __init__(self):
super(Foo, self).__init__()
self.a = 1
self.b = 2
def __setattr__(self, key, value):
print 'after __init__', self.b
super(Foo, self).__setattr__(key, value)
class Bar(Foo):
def __init__(self):
super(Bar, self).__init__()
self.c = 3
>>> f = Foo()
>>> f.a = 10
after __init__ 2
>>>
>>> b = Bar()
>>> b.c = 30
after __init__ 2