Assign Global Variable Inside Class From Class Variable

Question

I have a question about classes in Python. I have create a class that looks something like this:

class Model(Object):

    __table__ = 'table_name'

    def func():

    def func2():

The table name is a global variable that the functions feed off of. I am trying to add old data so I would like to change the table name depending on if I am filling in from old information, so this is what I tried.

class Model(Object):

    def __init__(self, backfill = False):
         self.backfill = backfill

    if self.backfill:
         __table__ = 'table_name'
    else:
         __table__ = 'table_name2'

    def func():

    def func2():

The final call would be something like this:

if backfill:
     model = Model(True).func()
else:
     model = Model.func()

Then I realized this wouldn't work because I cannot call self.backfill from outside the class definitions. I even tried creating a definition and calling that inside the class but that did not work either.

My question is:

Is there a way to initialize the global variable inside the class from the class variables? Or is there a better way to do this in general?

The table name is a global variable that the functions feed off of - can you clarify what you mean by 'feed off of'? — Andrew Li
– Andrew Li, Commented Oct 1, 2016 at 0:09
__table__ is not a global variable. It is an attribute of class Model. — zvone
– zvone, Commented Oct 1, 2016 at 0:17
if self.backfill is wrong because there is not self there. What are you actually trying to do? There is no theoretical way in which a class attribute (or a global variable for that matter) can depend on an instance (i.e. self) because there can be multiple instances, but there is only one class. — zvone
– zvone, Commented Oct 1, 2016 at 0:20

Frodon · Accepted Answer · 2016-10-01 00:43:54Z

__table__ is a class variabe, which means you can access it from an instance or from the class itself. You can update any instance value according to the value of backfill in the __init__ method:

class Model(Object):
    __table__ = 'table_name'
    def __init__(self, backfill = False):
        self.backfill = backfill
        if self.backfill:
            self.__table__ = 'table_name2'

Then to create an instance, just give the backfill parameter to the constructor, without any if/else statement:

print(Model.__table__)
# table_name
backfill = True
model = Model(backfill)
print(model.__table__)
# table_name2

But I don't see the point of using a class variable in your case. You can just define it in the __init__ method:

class Model(Object):
    def __init__(self, backfill = False):
        self.backfill = backfill
        self.__table__ = 'table_name'
        if self.backfill:
            self.__table__ = 'table_name2'

Thank you Frodon! This is exactly what I was looking for - I am new to object oriented programming so couldn't articulate what exactly it was that I wanted but you were able to get around that! Thanks!

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