How to share common context between classes?

Question

Present Scenario :

I have a set of classes that all take a common argument context in constructor, and all the classes inherit from a common base.

class base:
    def common_method(self):
        pass

class a(base):
    def __init__(self,context, aa):
        pass

class b(base):
    def __init__(self, context, bb):
        pass

# ...

class z(base):
    def __init__(self, context, zz):
        pass

Usage in main:

context = get_context_from_some_method()

A = a(context, 1)
B = b(context, 2)
C = c(context, 3)
D = d(context, 4)
.
.
.
Z = z(context, 26)

Problem:

• Is there a tricky way possible to supply context to all the classes implicitly ?
• We have a common base class, but I don't see a obvious way to let it set the context for all the classes.
• It can be assumed that I want to keep context common to all my class instances.
• Just a random thought - Can meta class help somehow ?

I know it seems silly, but I just want to get rid of the redundancy in some way, so that I can set a global context somehow and concentrate on my objects.

Please suggest some way around ?

** Update to Question **

I can-not set a context in the base class, since this is to be used in a web application. So, many pages with different context will be using the class structure. So, If I set a context in base then it will conflict to the context that will be set by another instance of webpage that will use the same base. Since, in a web-application all the above classes will be in memory common to all pages.

Answer 1

Edit: If you don't want to / can't use a class variable, your best bet is to use a factory function. Either make it a static method on the base class, or a module level function.

def make_instances(context, *instances):
    return [cls(context, *args) for cls, args in instances]

A, B, ..., Z = make_instances(get_context_from_some_method(), 
                 (a, (1,)), (b, (2,)), ..., (z, (26,)))

# or
instances = make_instances(get_context_from_some_method(), 
             zip(list_of_subclasses, ((x,) for x in range(1, 27))))

Alternatively, and I don't know if this works in your situation, just use a module level global variable:

class z(base):
    def __init__(self, zz):
        self.zz = zz
        self.context = context

context = 'abc'
Z = z(26)

---

In addition to the advice to use class variables from the other answer, I advise you to copy the context onto the instances, so that you can later change the context without affecting already created instances.

class base:
    context = None # if you want to be able to create without context.
    # just omit the previous line if you want an error
    # when you haven't set a context and you try to instantiate a subclass

class a(base):
    def __init__(self, aa):
        self.aa = aa
        self.context = self.context # sets an instance variable
        # so the class variable can be changed

class b(base):
    def __init__(self, bb):
        self.bb = bb
        self.context = self.context

base.context = 'context'

A = a(1)
B = b(2)

base.context = 'newcontext'

print A.context # context