Creating a global variable (from a string) from within a class

Question

Context: I'm making a Ren'py game. The value is Character(). Yes, I know this is a dumb idea outside of this context.

I need to create a variable from an input string inside of a class that exists outside of the class' scope:

class Test:
    def __init__(self):
        self.dict = {} # used elsewhere to give the inputs for the function below.

    def create_global_var(self, variable, value):
        # the equivalent of exec("global {0}; {0} = {1}".format(str(variable), str(value)))
        # other functions in the class that require this.

Test().create_global_var("abc", "123") # hence abc = 123

I have tried vars()[], globals()[variable] = value, etc, and they simply do not work (they don't even define anything) Edit: this was my problem.

I know that the following would work equally as well, but I want the variables in the correct scope:

setattr(self.__class__, variable, value) # d.abc = 123, now. but incorrect scope.

How can I create a variable in the global scope from within a class, using a string as the variable name, without using attributes or exec in python?

And yes, i'll be sanity checking.

If you say that this is a "dumb idea", why not think of a better one? — iTayb
– iTayb, Commented Nov 9, 2012 at 15:35
I say it merely because of similar questions I have seen asked that result in 20 answers that do not answer the original question, and merely say how much of a bad idea it is — Amelia
– Amelia, Commented Nov 9, 2012 at 15:36
Perhaps you should take a step back and start by explaining why you think you would need or want to do this. You're more likely to get a helpful answer that way. — Iguananaut
– Iguananaut, Commented Nov 9, 2012 at 15:38
If a group of experienced programmers respond by saying something is a bad idea, then that advice should be taken on board as constructive criticism and not simply dismissed. — Talvalin
– Talvalin, Commented Nov 9, 2012 at 15:40
@Hiroto That's fine, but you can't assume people know that. I've seen Ren'Py before, but I don't know how it's implemented, for example. — Iguananaut
– Iguananaut, Commented Nov 9, 2012 at 16:41

Wouter · Accepted Answer · 2022-06-04 15:44:39Z

7

First things first: what we call the "global" scope in Python is actually the "module" scope (on the good side, it diminishes the "evils" of using global vars).

Then, for creating a global var dynamically, although I still can't see why that would be better than using a module-level dictionary, just do:

globals()[variable] = value

This creates a variable in the current module. If you need to create a module variable on the module from which the method was called, you can peek at the globals dictionary from the caller frame using:

from inspect import currentframe
currentframe(1).f_globals[variable] = name

Now, the this seems especially useless since you may create a variable with a dynamic name, but you can't access it dynamically (unless using the globals dictionary again)

Even in your test example, you create the "abc" variable passing the method a string, but then you have to access it by using a hardcoded "abc" - the language itself is designed to discourage this (hence the difference to Javascript, where array indexes and object attributes are interchangeable, while in Python you have distinct Mapping objects)

My suggestion is that you use a module-level explicit dictionary and create all your dynamic variables as key/value pairs there:

names = {}
class Test(object):
    def __init__(self):
        self.dict = {} # used elsewhere to give the inputs for the function below.

    def create_global_var(self, variable, value):
         names[variable] = value

(on a side note, in Python 2 always inherit your classes from "object")

edited Jun 4, 2022 at 15:44

Wouter

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answered Nov 9, 2012 at 15:41

jsbueno

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5 Comments

Iguananaut Over a year ago

OP claimed this wasn't working for them. But it should: >>> class A(object): ... def foo(self, bar): ... globals()['foo'] = bar ... >>> A().foo('asdf') >>> foo 'asdf'

Amelia Over a year ago

oddly, when i first tried this, it didn't work. I tried again, and it does indeed work. then again, I'm using a custom interpreter running inside a game engine, hence the odd need for this.

jfs Over a year ago

@Iguananaut: It might not work due to the __main__ gotcha. The module can be available via different names. globals() refers to the module the function is defined in i.e., globals() inside mod.func() refer to mod variables.

Amelia Over a year ago

the actual need for the dynamic names is because the Character() variables and options are read from a file, and then globally defined for use in gameplay.

Iguananaut Over a year ago

@J.F.Sebastian Ah good point; that's exactly what was going on with my example.

Wessie · Accepted Answer · 2012-11-09 15:40:13Z

2

You can use setattr(__builtins__, 'abc', '123') for this.

Do mind you that this is most likely a design problem and you should rethink the design.

answered Nov 9, 2012 at 15:40

Wessie

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1 Comment

Roman Bodnarchuk Over a year ago

You shouldn't touch __builtins__, it is CPython implementation detail.

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