How to create module-wide variables in Python?

Question

Is there a way to set up a global variable inside of a module? When I tried to do it the most obvious way as appears below, the Python interpreter said the variable __DBNAME__ did not exist.

...
__DBNAME__ = None

def initDB(name):
    if not __DBNAME__:
        __DBNAME__ = name
    else:
        raise RuntimeError("Database name has already been set.")
...

And after importing the module in a different file

...
import mymodule
mymodule.initDB('mydb.sqlite')
...

And the traceback was: UnboundLocalError: local variable '__DBNAME__' referenced before assignment

Any ideas? I'm trying to set up a singleton by using a module, as per this fellow's recommendation.

Answer 1

This is a very basic question, you should do your homework a bit. Please see: http://docs.python.org/reference/simple%5Fstmts.html#the-global-statement

Answer 2

For this, you need to declare the variable as global. However, a global variable is also accessible from outside the module by using module_name.var_name. Add this as the first line of your module:

global __DBNAME__

Answer 3

Here is what is going on.

First, the only global variables Python really has are module-scoped variables. You cannot make a variable that is truly global; all you can do is make a variable in a particular scope. (If you make a variable inside the Python interpreter, and then import other modules, your variable is in the outermost scope and thus global within your Python session.)

All you have to do to make a module-global variable is just assign to a name.

Imagine a file called foo.py, containing this single line:

X = 1

Now imagine you import it.

import foo
print(foo.X)  # prints 1

However, let's suppose you want to use one of your module-scope variables as a global inside a function, as in your example. Python's default is to assume that function variables are local. You simply add a global declaration in your function, before you try to use the global.

def initDB(name):
    global __DBNAME__  # add this line!
    if __DBNAME__ is None: # see notes below; explicit test for None
        __DBNAME__ = name
    else:
        raise RuntimeError("Database name has already been set.")

By the way, for this example, the simple if not __DBNAME__ test is adequate, because any string value other than an empty string will evaluate true, so any actual database name will evaluate true. But for variables that might contain a number value that might be 0, you can't just say if not variablename; in that case, you should explicitly test for None using the is operator. I modified the example to add an explicit None test. The explicit test for None is never wrong, so I default to using it.

Finally, as others have noted on this page, two leading underscores signals to Python that you want the variable to be "private" to the module. If you ever do an import * from mymodule, Python will not import names with two leading underscores into your name space. But if you just do a simple import mymodule and then say dir(mymodule) you will see the "private" variables in the list, and if you explicitly refer to mymodule.__DBNAME__ Python won't care, it will just let you refer to it. The double leading underscores are a major clue to users of your module that you don't want them rebinding that name to some value of their own.

It is considered best practice in Python not to do import *, but to minimize the coupling and maximize explicitness by either using mymodule.something or by explicitly doing an import like from mymodule import something.

Answer 4

You are falling for a subtle quirk. You cannot re-assign module-level variables inside a python function. I think this is there to stop people re-assigning stuff inside a function by accident.

You can access the module namespace, you just shouldn't try to re-assign. If your function assigns something, it automatically becomes a function variable - and python won't look in the module namespace.

You can do:

__DB_NAME__ = None

def func():
    if __DB_NAME__:
        connect(__DB_NAME__)
    else:
        connect(Default_value)

but you cannot re-assign __DB_NAME__ inside a function.

One workaround:

__DB_NAME__ = [None]

def func():
    if __DB_NAME__[0]:
        connect(__DB_NAME__[0])
    else:
        __DB_NAME__[0] = Default_value

Note, I'm not re-assigning __DB_NAME__, I'm just modifying it's contents.