Question

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 1

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.