First, let's consider this working example using get and set methods for the variable x
class Foo:
def __init__(self):
self._x = 0
def set_x(self, x):
self._x = x
def get_x(self):
return self._x
class Bar:
def __init__(self, set_method):
self._set_method = set_method
def set_x(self, x):
self._set_method(x)
f = Foo()
f.set_x(5)
print(f.get_x())
# Prints 5
b = Bar(f.set_x)
b.set_x(10)
print(f.get_x())
# Prints 10
As you can see I pass the possibility to set the variable x of the instance f of class Foo, to the instance b of class Bar.
Now, I would like to do the same, but with property decorators instead, roughly like this
class Foo:
def __init__(self):
self._x = 0
#property
def x(self):
return self._x
#x.setter
def x(self, x):
self._x = x
class Bar:
def __init__(self, x_property):
self._x_property = x_property
def set_x(self, x):
self.x_property = x
f = Foo()
f.x = 5
print(f.x)
# Prints 5
b = Bar(f.x)
b.set_x(10)
print(f.x)
# Prints 5
What happens is that the value 5, instead of the property, gets passed to instance b, meaning that b can't access x in instance f. Is there a nice way to solve this?
I would then also like to do the same thing for the get method. In the first code that requires me to pass both methods, but if there is a way to get the second code to work I would hopefully only have to pass on the property which I then can set and get as a normal variable.
I would really want to use the property decorators or similar as it cleans up my code a lot. I use python 3.5.2.
Thanks,
Andreas
You can accomplish this by accessing the fset attribute of Foo.x. Note the use of class-dot notation rather than instance-dot. fset takes two arguments: the instance to access and the value to write. Here is a working example
class Foo:
#property
def x(self):
return self._x
#x.setter
def x(self, x):
self._x = x
class Bar:
def __init__(self, x_property):
self.x_property = x_property
def set_x(self, foo, value):
self.x_property(foo, value)
f = Foo()
f.x = 5
print(f.x)
b = Bar(Foo.x.fset)
b.set_x(f, 10)
print(f.x)
Notice that we had to pass f to set_x because we need it to invoke the setter. You could eliminate the f param by using partial from the functools module to bind f to the property setter. Pass the partial binding in to the constructor of Bar.
class Bar:
def __init__(self, x_property):
self.x_property = x_property
def set_x(self, value):
self.x_property(value)
f = Foo()
b = Bar(partial(Foo.x.fset, f))
b.set_x(10)
print(f.x)
It might be wise to rename x_property and this point. It is really just a function as far as Bar is concerned. It wouldn't have to be a property.
Related
I have this code:
class A:
def __init__(self, vals: list):
self._vals = vals
def __len__(self) -> int:
# some side effects like logging maybe
return len(self._vals)
def print_len(self) -> None:
# some function that uses the len above
print(len(self))
class B(A):
def __len__(self) -> int:
return 0
The issue is, I want print_len to always call A.__len__. I can do this:
class A:
def __init__(self, vals: list):
self._vals = vals
def __len__(self) -> int:
return len(self._vals)
def print_len(self) -> None:
print(A.__len__(self))
class B(A):
def __len__(self) -> int:
return 0
But it feels wrong. Basically I want B to lie about __len__ to outside callers, but internally use the correct len specified in A.
So
a = A([1, 2, 3])
print(len(a)) # print 3
a.print_len() # print 3 - no surprises there
b = B([1, 2, 3])
print(len(b)) # print 0 - overload the __len__
b.print_len() # want this to be 3 using A's __len__, not 0 using B's __len__
Is there any way to ensure a class always uses its own version of a method rather than a subclass' version? I thought name mangling of dunder methods would help here.
I think your approach is a good one. The zen of Python states that "There should be one-- and preferably only one --obvious way to do it." and I think you've found it.
That being said, you can do this via name mangling. You just need to prefix the method with double underscores (don't add them to the end like magic methods). This will create a private method which won't ever be overwritten by subclasses.
I think this might be self-defeating since you're now putting the computation in a different method.
class A:
def __init__(self, vals: list):
self._vals = vals
def __len__(self) -> int:
return self.__length()
def __length(self) -> int:
return len(self._vals)
def print_len(self) -> None:
print(self.__length())
I am trying to make a class that has a bunch of children that all have their own respective methods but share common methods through the parent. The problem is I need to create an instance of the child class in the parent method but am not sure how to go about it
my code so far looks like this
def filterAttribute(self, attribute, value):
newlist = []
for thing in self._things:
if thing._attributes[attribute] == value:
newlist.append(thing)
return self.__init__(newlist)
the class constructor takes in a list as its sole argument. Does anyone know if there is a standard way of doing this because my code is returning a NoneType object
Here are a few examples of classes I have made
This is the parent class:
class _DataGroup(object):
def __init__(self, things=None):
self._things=things
def __iter__(self):
for x in self._things:
yield x
def __getitem__(self, key):
return self._things[key]
def __len__(self):
return len(self._things)
def extend(self, datagroup):
if(isinstance(datagroup, self.__class__)):
self._things.extend(datagroup._things)
self._things = list(set(self._things))
def filterAttribute(self, attribute, value):
newlist = []
for thing in self._things:
if thing._attributes[attribute] == value:
newlist.append(thing)
#return self.__init__(newlist)
return self.__init__(newlist)
this is one of the child classes
class _AuthorGroup(_DataGroup):
def __init__(self, things=None):
self._things = things
def getIDs(self):
return [x.id for x in self._things]
def getNames(self):
return [x.name for x in self._things]
def getWDs(self):
return [x.wd for x in self._things]
def getUrns(self):
return [x.urn for x in self._things]
def filterNames(self, names, incl_none=False):
newlist = []
for thing in self._things:
if((thing is not None or (thing is None and incl_none)) and thing.name in names):
newlist.append(thing)
return _AuthorGroup(newlist)
The functionality I am looking for is that I can use the parent class's with the child classes and create instances of the child classes instead of the overall DataGroup parent class
So if I correctly understand what you are trying to accomplish:
You want a Base Class 'DataGroup' which has a set of defined attributes and methods;
You want one or mpore child classes with the ability to inherit both methods and attributes from the base class as well as have the ability to over-ride base class methjods if necessary: and
You want to invoke the child class without also having to manually invoke the base class.
If this in fact is your problem, this is how I would proceed:
Note: I have modified several functions, since I think you have several other issues with your code, for example in the base class self._things is set up as a list, but in the functions get_item and filterAttribute you are assuming self._things is a dictionary structure. I have modified the functions so all assume a dict structure for self._things
class _DataGroup:
def __init__(self, things=None):
if things == None:
self._things = dict() #Sets up default empty dict
else:
self._things=things
def __iter__(self):
for x in self._things.keys():
yield x
def __len__(self):
return len(self._things)
def extend(self, datagroup):
for k, v in datagroup.items():
nv = self._things.pop(k, [])
nv.append(v)
self._things[k] = nv
# This class utilizes the methods and attributes of DataGroup
# and adds new methods, unique to the child class
class AttributeGroup(_DataGroup):
def __init__(self, things=None):
super.__init__(things)
def getIDs(self):
return [x for x in self._things]
def getNames(self):
return [x.name for x in self._things]
def getWDs(self):
return [x.wd for x in self._things]
def getUrns(self):
return [x.urn for x in self._things]
# This class over-rides a DataGroup method and adds new attribute
class NewChild(_DataGroup):
def __init__(self, newAttrib, things = None):
self._newattrib = newAttrib
super.__init__(self, things)
def __len__(self):
return max(len(self._newattrib), len(self._things))
These examples are simplified, since I am not absolutely sure of what you really want.
Consider the two classes below.
class Alpha:
def __init__(self):
pass
def Bar(self, x):
def Foo(mult):
return x * mult
self._Foo = Foo
def Foo(self, mult):
return self._Foo(mult)
class Beta:
def __init__(self):
pass
def Bar(self, x):
self._x = x
def Foo(self, mult):
return self._x * mult
For Alpha with a deferred function _Foo, I believe it is more efficient memory-wise since it only evaluates x when the function is called. For Beta on the other hand, x is stored explicitly as a class attribute.
The question is, where exactly is x stored in Alpha? How efficient is it compared to Beta?
x is not stored in alpha as a class attribute, it is only created when you call the function, Foo. This eliminates the need unnecessary data storage.
I have a class which caches some values to avoid computing them many times, for instance
class A(object):
def __init__(self, a, b):
self.a = a
self.b = b
self._value = None
#property
def value(self):
if self._value is None:
self._value = # <complex code that produces value>
return self._value
In this way, self._value is computed only once and all the other times the precomputed value is returned. So far so good.
Now, let's suppose I want to subclass A with class B. In our case class B will have its own method of computing self._value but it sometimes will need A's value, like in this example:
class B(A):
def __init__(self, a, b):
super().__init__(a, b)
#property
def value(self):
if self._value is not None:
self._value = # <complex code that produces B's version of value>
return self._value
def get_old_value(self):
return super().value # here comes the trouble
Now, clearly the trouble is that if get_old_value() is called before value() it will cache A's value forever. If value() is called before get_old_value() in the same way, get_old_value() will actually always return value().
Of course, one could simply use A's <complex code that produces value>, in the implementation of get_old_value() but that would duplicate code (which would pretty much make subclassing useless) or even wrap <complex code that produces value> inside another method in A and call that method in get_old_value() but this would not use caching at all.
Another way could be the following:
def get_old_value(self):
result = super().value
self._c = None
return result
but that would anyway remove caching for A's version of value and does not look clean at all. Is there any better way to accomplish this?
One thing I want to add is that in my code A and B make really sense as superclass and subclass, otherwise I would consider composition.
What you need to do is use name-mangling -- this will allow each class/subclass to maintain a private version of the variable so they don't clobber each other:
class A(object):
def __init__(self, a, b):
self.a = a
self.b = b
self.__value = None
#property
def value(self):
if self.__value is None:
self.__value = 7
return self.__value
class B(A):
def __init__(self, a, b):
super().__init__(a, b)
self.__value = None
#property
def value(self):
if self.__value is None:
self.__value = 17
return self.__value
def get_old_value(self):
return super().value # no more trouble here
And in use:
>>> b = B(1, 2)
>>> print(b.value)
17
>>> print(b.get_old_value())
7
Please note you now need to set __value in B's __init__ as well.
See also this answer for a couple more tidbits about name-mangling.
There is an interesting question on python 3.5. For example I've smth like:
class A:
__x = 0
def __init__(self, x):
self.__x = x
def set_x(self,x): __x=x
def get_x(self): return x
class B(A):
__y = 0
def __init(self, x, y)
self.__y = y
super(B, self).__init__(x)
def set_y(self,y): __y=y
def get_y(self): return y
def toString(self): return "x = {} and y = {}".format(self.__x,
self.__y);
test = B(7,3)
test.toString()
Why do I have an error here: "B object has no attribute _B__x", if the method super() let me to use all methods of parante class?
Sure, if I write, like:
def toString(self): return "x = {} and y = {}".format(self.get_x(),
self.__y);
It works well!
You do not get access to private variables/members through super().
So in order to access B.__x, you will have to do
def toString(self): return "x = {} and y = {}".format(self._B__x,
self.__y);