There are three classes :
A, B and C
The __init__ of B creates an object of A. Using the mutators, I will be able to change the attributes of A from B for the instance created.
However, I am not unable to find any way to use that instance of A created by B to be used in C without passing the Object explicitly to the __init__ method [ not C.__init(self, object: A) ]
Is there any way to implicitly allow C to use that instance of A ?
I am new to python and not sure if this a valid question. I have looked at other sources where it explicitly passes the object to class C
class A:
def __init__(self):
x = []
y = []
class C :
def __init__(self):
#[get obj1 without passing the instance in init]
self.value = None
def method1():
self.value = len([]) #len(obj1 of A.x)
class B:
def __init__(self):
obj1 = A()
obj1.x = [1,2,3,4]
obj1.y = [1,2,3]
obj2 = B()
print(obj2.value) #this should be the length of x in the instance A created above
Here is a simple example:
class A:
def __init__(self, i = ""):
self.item = i
class B:
def __init__(self):
self.a = A("hello")
class C:
def __init__(self):
b = B()
print(b.a.item)
c = C()
Output:
hello
Let's say we have classes A and B:
class A:
def hello_world(self):
print("hello world")
class B:
def __init__(self):
self.a = A()
def hello_world(self):
self.a.hello_world()
You create an instance of class B (which will create an instance of class A inside):
b = B()
You can then pass a reference to either b or b.a to any function of an instance of class C (either a constructor or not)
class C:
def hello_world(self, a):
a.hello_world()
c = C()
c.hello_world(b.a)
You can also use global variables:
class C:
def hello_world(self):
b.a.hello_world()
c = C()
c.hello_world()
Here the instances of class C will rely on variable b to be in place and just use its a attribute.
Using global variables in classes is generally considered to be hard to maintain and a bad practice. If your class depends on a value or an instance of some class you should pass the reference in the constructor (__init__ function) or in the function that's using it.
If these classes are in different different python files then you can also use these classes by importing the class name and creating an object of that class:
eg:
file1.py
class A:
def __init__(self):
x = []
y = []
file2.py
from file1 import A
class C :
def __init__(self):
[get obj1 without passing the instance in init]
self.value = None
self.obj_a = A()
def xyz(self):
print "in class c"
file3.py
from file2 import C
from file1 import A
Class B:
def __init__(self):
self.obj_a = A()
self.obj_c = C()
def another_func(self):
print self.obj_c.xyz()# it will print "in class c"
Related
I thought that this code would work
class A:
def __init__(self):
self.x = 1
B = self.create_b()
print(B.y)
def create_b(self):
class B:
def __init__(self):
self.y = self.x
return B
A = A()
but I receive the following error
AttributeError: type object 'B' has no attribute 'y'
What am I doing wrong?
You're confusing classes with class instances (not Python modules). In Python class statements are executable and create a callable object that you must then be called to create instance objects of the class that was defined.
Regular methods of a class automatically receive a first argument that's the instance they belong to, and by convention, this argument is usually called self.
Here's what I mean:
class A:
def __init__(self):
self.x = 1
B = self.create_b() # Create B class.
b = B(self) # Create instance of B class passing this instance of A.
print(b.y)
def create_b(self):
class B:
def __init__(self, a_inst):
self.y = a_inst.x
return B
a = A() # -> 1
There are three problems with this code. The first is that since create-b returns a class object, not an instance of the class, B's __init__ was never run. You could solve this with
class A:
def __init__(self):
self.x = 1
B = self.create_b()
b = B()
print(b.y)
def create_b(self):
class B:
def __init__(self):
self.y = self.x
return B
A = A()
The second is that nested classes do not have access to the wrapping method's local namespace like a nested function (closure) would. When attempting self.y = self.x, instances of class B have no special relationship with the instance of A that created them. You could solve this with
class A:
def __init__(self):
self.x = 1
B = self.create_b(self)
b = B()
print(b.y)
def create_b(self):
class B:
def __init__(self, a):
self.y = a.x
return B
A = A()
The third is that python creates a weakref to classes when they are defined that never goes away. Each time you call create_b, you create a small memory leak. You could solve this with
class A:
def __init__(self):
self.x = 1
b = B(self)
print(b.y)
class B:
def __init__(self, a):
self.y = a.x
A = A()
I'm trying to call for value from class B that is nested in class A and use it in class C.
I'm getting AttributeError:
class A():
class B():
a = 1
class C():
b = 2
c = B.a + b
AttributeError: class B has no attribute 'a'
I also tried to call From 'A', Pycharm recognize it, but python still get AttributeError:
class A(object):
class B(object):
a = 1
class C(object):
b = 2
c = A.B.a + b
AttributeError: class A has no attribute 'B'
Does someone have an idea of how to use it?
Thanks
The problem is that the class template (A) is not constructed while you're calling A.B.a. That is, A is not bound yet to a class.
Try this workaround:
class A():
class B():
a = 1
Now create C separately (A is already defined):
class C():
b = 2
c = A.B.a + b
And reference C from A:
A.C = C
This can possibly be done via meta-classes, but could be an over-kill here.
At compile time, the class definition for class A is not complete hence you can not access the classes, variables and methods defined in a parent class inside a nested class.
You can try separating the class definitions though as suggested by #Reut Sharabani.
You can not access the class by its name, while the class definition statement is still executed.
class A(object):
class B(object):
a = 1
class C(object):
b = 2
c = A.B.a + b # here class A statement is still executed, there is no A class yet
To solve the problem you must defer the execution of those statements :
move the all those statements to a classmethod
call them after the classes was defined.
class A(object):
class B(object):
#classmethod
def init(cls):
cls.a = 1
class C(object):
#classmethod
def init(cls):
cls.b = 2
cls.c = A.B.a + cls.b
#classmethod
def init(cls):
cls.B.init()
cls.C.init()
A.init()
Suppose I have two Python classes, A and B, and that B is an attribute of A. Can a method of B modify a property of A? for example, I would like to be able to call
A.B.setXinA(1)
A.x
>>> 1
One way around it would be embed a reference to A in B:
A.B.reftoA = A
But that's rather ugly... Is there a way to access the higher-level class directly? Below is a working example using the second method:
class A:
def __init__(self, b):
b.parent = self
setattr(self, b.name, b)
class B:
def __init__(self, name):
self.name = name
b = B('abc')
a = A(b) # b is now a.abc
abc.parent.x = 1
a.x
>>> 1
What about a method in B like this:
class B:
def __init__(self, name):
self.name = name
def setXinA(self, x):
self.parent.x = x
Then:
>>> b = B('abc')
>>> a = A(b)
>>> b.setXinA(19)
>>> print(A.x)
19
This way requires that setXinA is called by an instance of B rather than just B.setXinA(42) for example. Also, it sets x as an attribue of the class A, rather than any particular instance of A.
I have class A class B and class C.
class A and B can affect class C. so they need to refer to the same instance of the class.
#a.py
from C import C
Cinstance = C()
Cinstance.add()
#b.py
class b(object)
#i need to refer to 'cinstance' here to control the same instance of the class
#C.py
class C(object)
def __init__(self):
self.a=1
def add(self):
self.a += 1
print a
How do i need to import and instanciate the classes for it to work this way? I am new to programming and still learning so things that are obvious are still a little difficult for me right now.
class A:
def __init__(self,cInst):
self.c = cInst
class B:
def __init__(self,cInst):
self.c = cInst
cInst = C()
a = A(cInst)
b = B(cInst)
something like that maybe
based on what you have there I think the easiest thing would be to import Cinstance from module a.
from a import Cinstance
You can pass an instance of A and B to your C.__init__ method and save them as attributes of C.
I'm on my phone, so the code below isn't tested
class C(object):
def __init__(self, a, b):
self.a = a
self.b = b
>>> c = C(A(), B())
Assume that we have an object k of type class A. We defined a second class B(A). What is the best practice to "convert" object k to class B and preserve all data in k?
This does the "class conversion" but it is subject to collateral damage. Creating another object and replacing its __dict__ as BrainCore posted would be safer - but this code does what you asked, with no new object being created.
class A(object):
pass
class B(A):
def __add__(self, other):
return self.value + other
a = A()
a.value = 5
a.__class__ = B
print a + 10
a = A() # parent class
b = B() # subclass
b.value = 3 # random setting of values
a.__dict__ = b.__dict__ # give object a b's values
# now proceed to use object a
Would this satisfy your use case? Note: Only the instance variables of b will be accessible from object a, not class B's class variables. Also, modifying variables in a will modify the variable in b, unless you do a deepcopy:
import copy
a.__dict__ = copy.deepcopy(b.__dict__)
class A:
def __init__(self, a, b):
self.a = a
self.b = b
class B(A):
def __init__(self, parent_instance, c):
# initiate the parent class with all the arguments coming from
# parent class __dict__
super().__init__(*tuple(parent_instance.__dict__.values()))
self.c = c
a_instance = A(1, 2)
b_instance = B(a_instance, 7)
print(b_instance.a + b_instance.b + b_instance.c)
>> 10
Or you could have a sperate function for this:
def class_converter(convert_to, parent_instance):
return convert_to(*tuple(parent_instance.__dict__.values()))
class B(A):
def __init__(self, *args):
super().__init__(*args)
self.c = 5
But using the 2nd method, I wasn't able to figure out how to pass additional values