Before this is flagged as a duplicate, I know this question has been answered before, but the solutions provided there don't seem to apply to my case. I'm trying to programmatically set class properties. I know I can use property for that, so I thought about doing this:
class Foo:
def __init__(self, x):
self._x = x
def getx(): return self._x
def setx(y): self._x = y
self.x = property(fget=getx, fset=setx)
However, when I run this interactively, I get:
>>> f = Foo(42)
>>> f.x
<property object at 0x0000000>
>>> f._x
42
>>> f.x = 1
>>> f.x
1
Is there any way to solve this?
Edit:
I feel I may have left out too much, so here's what I am actually trying to reach. I have a class with a class variable called config, which contains configuration values to set as properties. The class should be subclassed to implement the config variable:
class _Base:
config = ()
def __init__(self, obj, **kwargs):
self._obj = obj()
for kwarg in kwargs:
# Whatever magic happens here to make these properties
# Sample implementation
class Bar(_Base):
config = (
"x",
"y"
)
def __init__(self, obj, x, y):
super().__init__(obj, x=x, y=y)
Which now allows for manipulation:
>>> b = Bar(x=3, y=4)
>>> b.x
3
>>> # Etc.
I'm trying to keep this as DRY as possible because I have to subclass _Base a lot.
property objects are descriptors, and descriptors are only invoked when defined on the class or metaclass. You can't put them directly on an instance; the __getattribute__ implementation for classes simply don't invoke the binding behaviour needed.
You need to put the property on the class, not on each instance:
class Foo:
def __init__(self, x):
self._x = x
#property
def x(self): return self._x
#x.setter
def x(self, y): self._x = y
If you have to have a property that only works on some instances, you'll have to alter your getter and setter methods to vary behaviour (like raise an AttributeError for when the state of the instance is such that the attribute should 'not exist').
class Bar:
def __init__(self, has_x_attribute=False):
self._has_x_attribute = has_x_attribute
self._x = None
#property
def x(self):
if not self._has_x_attribute:
raise AttributeError('x')
return self._x
#x.setter
def x(self, y):
if not self._has_x_attribute:
raise AttributeError('x')
self._x = y
The property object still exists and is bound, but behaves as if the attribute does not exist when a flag is set to false.
Related
Where I am now looks like this:
class A(object):
def __init__(self, val):
self.x=val
self.y=42
# other fields
class B(object):
def __init__(self):
self.a=22
# other fields
class C(A,B):
def __init__(self, val):
super(C,self).__init__(val)
#property
def x(self):
# if A.x is None return a value that I can compute from A.y and B.a
# if A.x is not None return it
#x.setter
def x(self, val):
# set the field value
Sometimes I just want to set an assumed value for x by hand, in which case I would just use an A. In other cases I want to use a more complicated approach that involves computing A.x's value on the basis of information that is organized into a B. The idea in this code is to make a C class that can look like an A (in terms of the x field) but doesn't need that field value to be set by hand, instead it just gets derived.
What I can't figure out is how to have the C.x property shadow the A.x field in a sensible way.
The line self.x = val in the A.__init__ method will simply invoke your C.x setter. You already have everything handled here. You are handling per instance attributes here, not attributes on a class that are inherited by subclasses.
All you need to do is to set a different attribute in the setter to represent the x value. You could name it _x, for example:
class C(A, B):
_x = None
#property
def x(self):
if self._x is not None:
return self._x
return self.a + self.y
#x.setter
def x(self, val):
self._x = val
Note that if all C.__init__ does is call super().__init__, you don't need it at all. However, you do need to make sure at least A.__init__() plays along in the inheritance structure; add in more calls to super().__init__():
class A(object):
def __init__(self, val, *args, **kwargs):
super(A, self).__init__(*args, **kwargs)
self.x = val
self.y = 42
class B(object):
def __init__(self, *args, **kwargs):
super(B, self).__init__(*args, **kwargs)
self.a = 22
Using *args and **kwargs allows these methods to pass on any extra arguments to other classes in the hierarchy.
Demo, using the above classes:
>>> c = C(None)
>>> c.x
64
>>> c.x = 15
>>> c.x
15
I want to be able to create a concrete instance of a class that inherits from another concrete class, which in turn inherits from an abstract class.
The basic pattern is:
from abc import ABCMeta, abstractproperty
class Foo(object):
__metaclass__ = ABCMeta
#abstractproperty
def x(self):
pass
#abstractproperty
def y(self):
pass
class Bar(Foo):
x = None
y = None
def __init__(self, x, y):
self.x = x
self.y = y
#property
def x(self):
return self.x
#x.setter
def x(self, value):
self.x = value
#property
def y(self):
return self.y
#y.setter
def y(self, value):
self.y = value
class Baz(Bar):
def __init__(self):
super().__init__(x=2, y=6)
a = Baz()
When I try to create the instance of Baz I get a RecursionError: maximum recursion depth exceeded error. (As well as a pylint warning telling me that the signatures of the setter methods don't match the signatures of the base class)
However, if I remove the setters, I get an error self.x = x AttributeError: can't set attribute
What's the correct pattern to do this?
You need to change names for your x() / y() methods or for your x / y properties, for example rename
class Bar(Foo):
x = None
y = None
To:
class Bar(Foo):
x_val = None
y_val = None
And rename the references to x / y as well.
What you did is basically:
def x():
return x()
It happened because your def x overridden the x = None, so x is a function(property) that is calling itself. Avoid this by using another attribute(named differently) for storing the actual value of x.
Example from python docs (https://docs.python.org/3.5/library/functions.html#property):
class C:
def __init__(self):
self._x = None
#property
def x(self):
return self._x
#x.setter
def x(self, value):
self._x = value
Note: attribute names starting with underscore should be considered "private" and should not be directly accessed outside of the class. But it's only a convention for programmers, technically they are just normal attributes and you can do whatever you want, but it's nice to follow some convention, isn't it?
I have a class A made by someone else, that I cannot edit:
class A:
def __init__(self, x):
self.x = x
Now I'm trying to inherit my own class B from A, and have x as a property instead of an attribute.
Is this possible?
I already tried:
class B(A):
def __init__(self, x):
super().__init__(x)
#property
def x(self):
return super().x
#x.setter
def x(self, x):
super().x = x
print(x) # my stuff goes here
But as I expected, it's not possible: AttributeError: 'super' object has no attribute 'x'
Is there any other method, some workaroud maybe?
No, you cannot use super() for anything other than class attributes; x is an instance attribute, and there is no inheritance mechanism for attributes.
Instance attributes live in a single namespace; there is no 'parent instance' attribute namespace.
You can still reach the attribute in the instance __dict__ object:
class B(A):
#property
def x(self):
return self.__dict__['x']
#x.setter
def x(self, x):
self.__dict__['x'] = x
print(x) # my stuff goes here
A property is a data descriptor, meaning that it is looked up before the instance attributes are consulted (see the descriptor howto), but you can still access it directly.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
What is the benefit to using a ‘get function’ for a python class?
I just started to read Python, but I wonder why does Python need setter and getter at all? it already have object variables which act like property
Consider
class C(object):
def _init_(self):
self._x = None
def get_x(self):
return self._x
def set_x(self, value):
self._x = valu
x = property(get_x, set_x)
Can we just use C.x = "value" to do what we want to do here? what is the benefit of property?
BTW, creating property/setter/getter in this way is cumbersome to me, is there any way to simplify this? like
class C()
has_attributes("x", "y", "z")
You can use a property to obtain what you want:
class C(object):
def _init_(self):
self._x = None
#property
def x(self):
return self._x
#x.setter
def x(self, value):
self._x = value
Then you can access the attribute with the usual attribute syntax:
c = C()
c.x = 10 #calls the setter
print(c.x) #calls the getter
There are some reasons to use a property instead of a plain data attribute:
You can document the attribute
You can control the access to the attribute, either by making it read-only or by checking the type/value being set
You do not break backwards compatibility: if something was a plain instance attribute and then you decide to transform it into a property the code that worked with the attribute will still work. If you used get/set explicit methods all the code that used the old API would have to change
It's more readable then using explicit get/set methods.
Use plain attributes:
class C(object):
def __init__(self):
self.x = None
Later, when and if it would be necessary, x can be changed to a property. The beauty of "#property" it that is allows developer not to use getters, setters and "#property".
class C(object):
def _init_(self):
self._x = None
def get_x(self):
return self._x
def set_x(self, value):
self._x = value
x = property(get_x, set_x)
now you can use c.x = "foo" with the set and gets, transparently.
The purpose of a set and getter is don't expose the class internals.
Imagine that in the future
self._x
changes to
sql.save(id(self), value)
and get_x to:
value= sql.sql(id(self))
return convertFromDbFormatToExpectedApiFormat(value)
You will only have to change the code of getter and setters in only that class, not change
all the classes that communicates with it.
class C(object):
def _init_(self):
self.sql = DDBB()
def get_x(self):
dbregistry = self.sql.fetch(id(self))
return convertFromDbFormatToExpectedApiFormat(dbregistry)
def set_x(self, value):
self.sql.save(id(self), value)
x = property(get_x, set_x)
I have a class like this, in which I have declared a property x, and overridden __delattr__:
class B(object):
def __init__(self, x):
self._x = x
def _get_x(self):
return self._x
def _set_x(self, x):
self._x = x
def _del_x(self):
print '_del_x'
x = property(_get_x, _set_x, _del_x)
def __delattr__(self, name):
print '__del_attr__'
Now when I run
b = B(1)
del b.x
Only __del_attr__ get invoked, anybody knows why and how to solve this problem?
You have to call the __delattr__ of your ancestor to achieve this correctly.
class B(object):
.....
def __delattr__(self, name):
print '__del_attr__'
super(B, self).__delattr__(name) # explicit call to ancestor (not automatic in python)
And then running :
b = B(1)
del b.x
Will output:
__del_attr__
_del_x
_del_x is called from the default __del_attr__. But since you have overridden __del_attr__, the onus is on you to call _del_x from inside your __del_attr__.