Here is the format of my code:
class A(object):
def __init__(self, x, other):
self.other = other
self.x = x
class B(A):
def __init__(self):
# place code here
def something_else(self):
return self.x["foo"]
x is an object which I would like to call, with a subscript later on (in something_else.
I would like only x to be inherited from the parent class.
It is important that other is not inherited, so super().__init__ is not suitable.
I have attempted a workaround by creating a function within class A:
def x(self):
return self.x
so I could call super().x() in class B, but this doesn't work either.
I have attempted calling directly super.x["foo"], and this doesn't work.
How can I achieve what I want in my case?
Thanks!
Variables don't always have to be registered in the __init__ function, if you want x from class A, have a method in A:
def set_x(self, x):
self.x = x
# other stuff
you'll still be able to call set_x from class B as all functions are inherited, from there you can instantiate property x without calling __init__ from A.
Related
Was wondering if there was a way to set a class attribute to a specific instance from within the class definition. For example,
class Value:
def __init__(self, x):
self.x = x
# Something like
# half = Value(0.5)
>>> Value.half.x
0.5
>>> Value.half.half.x
0.5
I'm also aware I can easily set it outside the class that seems a bit more bulky and error prone, like this
class Value:
def __init__(self, x):
self.x = x
Value.half = Value(0.5)
>>> Value.half.x
0.5
>>> Value.half.half.x
0.5
No. At the time the body of the class is being evaluated, the class doesn't yet exist. A class statement is a declarative syntax for calling a metaclass:
class Value:
def __init__(self, x):
self.x = x
is roughly equivalent to
def init(self, x):
self.x = x
Value = type('Value', (object,), {'__init__': init})
Your class attribute would have to be a member of the dict passed as the third argument, which has to be fully defined before type is called.
not quite, but you can make a class method that return a new instance of your class in whatever way you want with the classmethod decorator
>>> class Value:
def __init__(self, x):
self.x=x
def __repr__(self):
return f"{type(self).__name__}({self.x})"
#classmethod
def half(cls):
return cls(0.5)
>>> Value(10)
Value(10)
>>> Value.half()
Value(0.5)
>>>
look like in py3.9 you can combine it with the property decorator to accomplish just that, see linked documentation above (but I don't have it at the moment)
Simply, you can't because the class hasn't yet existed. But you can use either metaclass or class decorator to achieve the same goal as the following shows:
#Metaclass
class Meta(type):
def __init__(cls, clsname, clsbases, clsdict):
cls.half = cls(0.5)
class Value(metaclass=Meta):
def __init__(self, x):
self.x = x
#Decorator
def decorator(cls):
cls.half = cls(0.5)
return cls
#decorator
class Value2:
def __init__(self, x):
self.x = x
print(Value.half.half.x)
print(Value.half.x)
print(Value2.half.half.x)
print(Value2.half.x)
I have a situation where I'm using #classmethod to create a constructor for a class. Within this constructor, a function gets called, which then in turn calls another function. But either this doesn't work or (more probably) I'm doing something to make it not work. Here's an example in miniature:
class testclass:
def __init__(self, x):
self.x = x
#classmethod
def constructor(cls, x):
adj_x = cls.outer_adjust(cls, x)
return testclass(adj_x)
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
return x + 1
test_instance = testclass.constructor(4)
This produces an error message:
inner_adjust() missing 1 required positional argument: 'x'
I can make it work by explicitly passing self to inner_adjust, eg
def outer_adjust(self, x):
return self.inner_adjust(self, x)
But this then means that the outer_adjust method can't be used outside of the constructor, which is not what I want.
Any assistance gratefully received.
Here's a more detailed example, with two constructors shown. I'm trying to follow the approach to constructors described in
What is a clean, pythonic way to have multiple constructors in Python?
Which is essentially that the constructors do some processing to figure out what variables they should pass to init when instantiating the class.
Both constructors give the same error:
if_char_is_z_make_it_a() missing 1 required positional argument: 'char_input'
As before, I need to be able to use the if_char_is_make_it_a function outside of the constructor (ie, when using the class normally).
class testclass:
def __init__(self, char):
self.char = char
#classmethod
def constructor_from_int(cls, int_input):
as_char = chr(int_input)
char = cls.process_char(cls, as_char)
return testclass(char)
#classmethod
def constructor_from_char(cls, char_input):
char = cls.process_char(cls, char_input)
return testclass(char)
def process_char(self, char_input):
processed_char = '(' + char_input + ')'
output_char = self.if_char_is_z_make_it_a(processed_char)
return output_char
def if_char_is_z_make_it_a(self, char_input):
if char_input == '(z)':
return '(a)'
return char_input
test_instance = testclass.constructor_from_char('a')
When you call cls.outer_adjust from constructor you are calling the unbound outer_adjust method.
Thus, you pass the class itself as self and not an instance to a method that expects to receive an instance as argument.
Although, there is no real reason to have a constructor method. This is exactly what __init__ is for.
class testclass:
def __init__(self, x):
self.x = self.outer_adjust(x)
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
return x + 1
test_instance = testclass(4)
If you absolutely need the transformation on x to be done before the instantiation, then use __new__ instead. Although, this is generally not necessary.
Multiple constructors
If for some reason you still need to have a constructor method, by example if you want multiple constructors. Then keep in mind that outer_adjust and inner_adjust are instance methods, this means they must be called after you have created an instance.
class testclass:
def __init__(self, x):
self.x = x
#classmethod
def constructor1(cls, x):
instance = cls(x)
instance.outer_adjust()
return instance
#classmethod
def constructor2(cls, x):
instance = cls(x)
instance.inner_adjust()
return instance
def outer_adjust(self):
print('Do something else')
return self.inner_adjust()
def inner_adjust(self):
self.x += 1
As a sidenote, notice how I did not need to call testclass, but simply called cls in the constructor methods. Since this is a class method, we do not need to explicitly name the class. This is better, especially if you are to use inheritance.
Basically what you are doing here shall be done via the __new__ which serve as constructor.
class testclass:
def __init__(self, x):
self.x = x
def __new__(cls, *args, **kwargs):
instance = super(testclass, cls).__new__(cls, *args, **kwargs)
instance.outer_adjust(args[0])
return instance
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
self.x = x + 1
test_instance = testclass(4)
You are abusing self. The point of the class method is to use the cls argument as constructor, instead of explicitly naming the class by testclass(adj_x). Also, during the cls.outer_adjust(cls, x) call, you are passing the class instead of the instance, which happens to work because you are not using any instance attributes.
As to your questions, there's no way to avoid the x argument. inner_adjust increases some value by 1, so you must give it something to increase. The idea would be to have
def constructor(cls, x):
return cls(x)
def inner_adjust(self):
return self.x += 1
and then do something like
object= testclass.constructor(12)
object.inner_adjust()
I have a class A:
class A(object):
def pprint(x):
print(x)
Then I have a class B:
class B(object):
def pprint(x):
x += 1
# find a way to call A.pprint(x)
Then I have a child class:
class Child(B, A):
pass
Which should be used:
child = Child()
child.pprint(1)
>>> 2
I can make changes to B but not to A. I cannot refer to A directly in B. B will never be instantiated directly, always via children class.
After the explanation - what you need is not super() you need something like sibling_super() to find the next class in the multiple inheritance chain. You can poll Python's MRO for that, for example:
class A(object):
def pprint(self, x): # just to make it valid, assuming it is valid in the real code
print(x)
class B(object):
#staticmethod
def sibling_super(cls, instance):
mro = instance.__class__.mro()
return mro[mro.index(cls) + 1]
def pprint(self, x):
x += 1
self.sibling_super(B, self).pprint(self, x)
class Child(B, A):
pass
child = Child()
child.pprint(1) # 2
You have a couple of options for accessing the A method from the B class without having B inherit from A.
First, you could create a staticmethod and call it from B.
class A(object):
#staticmethod
def pprint(x):
print(x)
class B(object):
def pprint(self, x):
print(x + 1)
A.pprint(x)
Or you could inherit A in B like this:
class A(object):
def pprint(self, x):
print(x)
class B(A):
def pprint(self, x):
print(x + 1)
super(B, self).pprint(x)
Then for your Child class only inherit from B:
class Child(B):
pass
>>> c = Child()
>>> c.pprint(1)
2
1
OK, newest solution.
import inspect
class C(B, A):
def pprint(self, x):
a_class = inspect.getmro(Child)[-2]
a_class.pprint(self, x)
Since object will be the last result in inspect.getmro(Child) we skip that one to get the one before the last one, which is A. We then call that class's pprint method. You could also, to be more sure, if you know the __name__ of the class you want to call, iterate over the results from inspect.getmro(Child) and find the one that you want.
I have a class (named "A") with some instance variables. I want to add the dir() of this variables to the dir() of instances of class A.
For example:
class A(object):
def __init__(self, x, y):
self.x = x
self.y = y
class X(object):
def f_x(self):
pass
class Y(object):
def f_y(self):
pass
x = X(); y = Y()
a = A(x,y)
I want f_x and f_y to appear in
dir(a)
Is there a better way, or a more 'correct' one, than just iterating X.dict and Y.dict and for each element, use something like:
setattr(A, str(element), element)
Thanks.
A should really be a subclass of X and Y in this case. (Just be sure to read Michele Simionato's article on super and diamond inheritence before you get too deep into it.)
class X(object):
def f_x(self):
pass
class Y(object):
def f_y(self):
pass
class A(X, Y):
def __init__(self, *args, **kwargs): # splats optional
# do what you need to here
dir(A(X(),Y())) # Ah! Lisp!
However, if you really need things to be magic, then just override __getattr__ for X to look in self.x and self.y before throwing an error. But seriously, don't do this.
Why don't you simply inherit from both classes?
class B(A, X):
pass
a = B()
dir(a)
In Python, consider I have the following code:
class SuperClass(object):
def __init__(self, x):
self.x = x
class SubClass(SuperClass):
def __init__(self, y):
self.y = y
# how do I initialize the SuperClass __init__ here?
How do I initialize the SuperClass __init__ in the subclass? I am following the Python tutorial and it doesn't cover that. When I searched on Google, I found more than one way of doing. What is the standard way of handling this?
Python (until version 3) supports "old-style" and new-style classes. New-style classes are derived from object and are what you are using, and invoke their base class through super(), e.g.
class X(object):
def __init__(self, x):
pass
def doit(self, bar):
pass
class Y(X):
def __init__(self):
super(Y, self).__init__(123)
def doit(self, foo):
return super(Y, self).doit(foo)
Because python knows about old- and new-style classes, there are different ways to invoke a base method, which is why you've found multiple ways of doing so.
For completeness sake, old-style classes call base methods explicitly using the base class, i.e.
def doit(self, foo):
return X.doit(self, foo)
But since you shouldn't be using old-style anymore, I wouldn't care about this too much.
Python 3 only knows about new-style classes (no matter if you derive from object or not).
As of python 3.5.2, you can use:
class C(B):
def method(self, arg):
super().method(arg) # This does the same thing as:
# super(C, self).method(arg)
https://docs.python.org/3/library/functions.html#super
Both
SuperClass.__init__(self, x)
or
super(SubClass,self).__init__( x )
will work (I prefer the 2nd one, as it adheres more to the DRY principle).
See here: http://docs.python.org/reference/datamodel.html#basic-customization
How do I initialize the base (super) class?
class SuperClass(object):
def __init__(self, x):
self.x = x
class SubClass(SuperClass):
def __init__(self, y):
self.y = y
Use a super object to ensure you get the next method (as a bound method) in the method resolution order. In Python 2, you need to pass the class name and self to super to lookup the bound __init__ method:
class SubClass(SuperClass):
def __init__(self, y):
super(SubClass, self).__init__('x')
self.y = y
In Python 3, there's a little magic that makes the arguments to super unnecessary - and as a side benefit it works a little faster:
class SubClass(SuperClass):
def __init__(self, y):
super().__init__('x')
self.y = y
Hardcoding the parent like this below prevents you from using cooperative multiple inheritance:
class SubClass(SuperClass):
def __init__(self, y):
SuperClass.__init__(self, 'x') # don't do this
self.y = y
Note that __init__ may only return None - it is intended to modify the object in-place.
Something __new__
There's another way to initialize instances - and it's the only way for subclasses of immutable types in Python. So it's required if you want to subclass str or tuple or another immutable object.
You might think it's a classmethod because it gets an implicit class argument. But it's actually a staticmethod. So you need to call __new__ with cls explicitly.
We usually return the instance from __new__, so if you do, you also need to call your base's __new__ via super as well in your base class. So if you use both methods:
class SuperClass(object):
def __new__(cls, x):
return super(SuperClass, cls).__new__(cls)
def __init__(self, x):
self.x = x
class SubClass(object):
def __new__(cls, y):
return super(SubClass, cls).__new__(cls)
def __init__(self, y):
self.y = y
super(SubClass, self).__init__('x')
Python 3 sidesteps a little of the weirdness of the super calls caused by __new__ being a static method, but you still need to pass cls to the non-bound __new__ method:
class SuperClass(object):
def __new__(cls, x):
return super().__new__(cls)
def __init__(self, x):
self.x = x
class SubClass(object):
def __new__(cls, y):
return super().__new__(cls)
def __init__(self, y):
self.y = y
super().__init__('x')