Python: How to make a cross-module variable?

Question

The __debug__ variable is handy in part because it affects every module. If I want to create another variable that works the same way, how would I do it?

The variable (let's be original and call it 'foo') doesn't have to be truly global, in the sense that if I change foo in one module, it is updated in others. I'd be fine if I could set foo before importing other modules and then they would see the same value for it.

Answer 1

Okay, turning this on it's head, why do you want a global variable? ..or in other words, there are probably better ways of achieving the solution you're after if you can explain exactly what you want?

Answer 2

This sounds like modifying the __builtin__ name space. To do it:

import __builtin__
__builtin__.foo = 'some-value'

Do not use the __builtins__ directly (notice the extra "s") - apparently this can be a dictionary or a module. Thanks to ΤΖΩΤΖΙΟΥ for pointing this out, more can be found here.

Now foo is available for use everywhere.

I don't recommend doing this generally, but the use of this is up to the programmer.

Assigning to it must be done as above, just setting foo = 'some-other-value' will only set it in the current namespace.

Answer 3

I don't endorse this solution in any way, shape or form. But if you add a variable to the __builtin__ module, it will be accessible as if a global from any other module that includes __builtin__ -- which is all of them, by default.

a.py contains

print foo

b.py contains

import __builtin__
__builtin__.foo = 1
import a

The result is that "1" is printed.

Edit: The __builtin__ module is available as the local symbol __builtins__ -- that's the reason for the discrepancy between two of these answers.

Answer 4

Define a module ( call it "globalbaz" ) and have the variables defined inside it. All the modules using this "pseudoglobal" should import the "globalbaz" module, and refer to it using "globalbaz.var_name"

This works regardless of the place of the change, you can change the variable before or after the import. The imported module will use the latest value. (I tested this in a toy example)

For clarification, globalbaz.py looks just like this:

var_name = "my_useful_string"

Answer 5

If you need a global cross-module variable maybe just simple global module-level variable will suffice.

a.py:

var = 1

b.py:

import a
print a.var
import c
print a.var

c.py:

import a
a.var = 2

Test:

$ python b.py
# -> 1 2

Real-world example: Django's settings.py (though in Django apps settings are used by importing the object django.conf.settings).

Answer 6

Global variables are usually a bad idea, but you can do this by assigning to __builtins__:

__builtins__.foo = 'something'
print foo

Also, modules themselves are variables that you can access from any module. So if you define a module called my_globals.py:

# my_globals.py
foo = 'something'

Then you can use that from anywhere as well:

import my_globals
print my_globals.foo

Using modules rather than modifying __builtins__ is generally a cleaner way to do globals of this sort.

Answer 7

You can pass the globals of one module to onother:

In Module A:

import module_b
my_var=2
module_b.do_something_with_my_globals(globals())
print my_var

In Module B:

def do_something_with_my_globals(glob): # glob is simply a dict.
    glob["my_var"]=3

Answer 8

You can already do this with module-level variables. Modules are the same no matter what module they're being imported from. So you can make the variable a module-level variable in whatever module it makes sense to put it in, and access it or assign to it from other modules. It would be better to call a function to set the variable's value, or to make it a property of some singleton object. That way if you end up needing to run some code when the variable's changed, you can do so without breaking your module's external interface.

It's not usually a great way to do things — using globals seldom is — but I think this is the cleanest way to do it.