Python: Is it possible to create a global variable with the name given as string?

Question

I can create a global variable in a function:

def f():
    global x
    x=1

f()
print x

But can I do the same if I only have the name of that variable in f()?

def f(varname):
    exec('global ' + varname)
    exec(varname + '=1')

f('x')
print x

This doesn't work - "name 'x' is not defined". Don't tell me that this is a terrible style of programming - I know that. The question is if this is possible to do in Python.

Thanks for the answers, but the question of why exec didn't work remains. Is is somehow executed in a different context? Is it a part of a different module, so that it creates a global variable in a different module?

Almost anything is possible, but things like this are never a good idea (obviously, there are exceptions, but the people who can decide that don't have to ask how it's done -- not necessarily vice versa though). — user395760
– user395760, Commented May 3, 2012 at 16:58

NPE · Accepted Answer · 2012-05-03 15:22:06Z

4

In [1]: globals()['x'] = 42

In [2]: x
Out[2]: 42

It works the same way if done from within a function.

answered May 3, 2012 at 15:22

NPE

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2 Comments

user1084871 Over a year ago

Thanks for the answer! Can you explain why my version didn't work? I suspect that there is something wrong with the exec's context.

NPE Over a year ago

@user1084871: So do I, but I can't quite put my finger on it.

mata · Accepted Answer · 2012-05-03 15:23:05Z

4

yes, you can use globals() for that:

globals()['whatever'] = 123

(and it's good that you realise yourself that this isn't a very good style)

answered May 3, 2012 at 15:23

mata

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Comments

David Heffernan · Accepted Answer · 2012-05-03 15:24:25Z

2

Use the globals function

globals()[varname] = value

The globals function returns a dict containing the module's global variables.

answered May 3, 2012 at 15:24

David Heffernan

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Comments

ch3ka · Accepted Answer · 2012-05-03 15:31:18Z

1

You could, see the other answers.

But you really should not.

It's not that global variables are automatically a bad idea; the bigger problem is that looking at a plain call to f() or f(x), you have no idea you just modified your local (that is, in this case, global) scope.

def f():
    return 1

x = f()
print x

does exactly the same, but documents what's going on in the scope inside that scope.

answered May 3, 2012 at 15:31

ch3ka

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