Cannot change global variables in a function through an exec() statement?

python exec global
linkmaster03 picture linkmaster03 · Jan 18, 2010 · Viewed 12.5k times · Source

Why can I not change global variables from inside a function, using exec()? It works fine when the assignment statement is outside of exec(). Here is an example of my problem:

>>> myvar = 'test'
>>> def myfunc():
...     global myvar
...     exec('myvar = "changed!"')
...     print(myvar)
... 
>>> myfunc()
test
>>> print(myvar)
test

Answer

Alex Martelli picture Alex Martelli · Jan 18, 2010

Per the docs, the exec statement takes two optional expressions, defaulting to globals() and locals(), and always performs changes (if any) in the locals() one.

So, just be more explicit/specific/precise...:

>>> def myfunc():
...   exec('myvar="boooh!"', globals())
... 
>>> myfunc()
>>> myvar
'boooh!'

...and you'll be able to clobber global variables to your heart's contents.

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