Using globals in Python exec

Question

I'm trying to create a custom python interpreter type application. I'm using exec statement (in Python 2.7.6) to execute given code but globals aren't working quite as expected. Could someone explain why this does not work:

def print_x():
    print(x)


g = {'x': 10, 'print_x': print_x}
l = {}

exec('print_x()', g, l)

The result (whether the print_x function is in g or l), is an error:

NameError: global name 'x' is not defined

So, do the globals passed to exec not carry over to called functions?

But I could use exec('print(x)', g, l) just fine. It just doesn't work with the print_x function. — jordanwh
– jordanwh, Commented Jun 18, 2016 at 4:41
stackoverflow.com/questions/2904274/… - seems similar to your case — dmitryro
– dmitryro, Commented Jun 18, 2016 at 4:43
Thank you for that link. It is similar but not quite the same as my program. I have now tried printing out globals() and locals() in the print_x() function and sure enough, 'x' is not there. I'm just not quite sure why that is. — jordanwh
– jordanwh, Commented Jun 18, 2016 at 4:50

mgilson · Accepted Answer · 2016-06-18 04:54:37Z

3

The x inside the function is picked up from the globals of the namespace where the function is defined. However, you're calling it in a different namespace. This isn't too different from having multiple modules:

# foo.py
def print_x():
    print(x)

and then trying to use it in a different module:

# bar.py
from foo import print_x

x = 10
print_x()  # NameError

Ultimately, g is the globals in the execution context of exec. After it calls the print_x function (which was defined in a different global context), you can't expect print_x to know anything about the globals in exec's execution context -- print_x only knows the globals in it's module's context.

edited Jun 18, 2016 at 4:54

answered Jun 18, 2016 at 4:52

mgilson

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4 Comments

jordanwh Over a year ago

Ok, thank you. So, I will need to define the print_x() function using exec? Is there any other way to change the namespace of a function?

Tadhg McDonald-Jensen Over a year ago

@jordanwh if you really want to change the globals you could create a copy of the function with your own scope passed for the new functions globals.

mgilson Over a year ago

@jordanwh -- I think I've seen some (maybe cpython specific) solutions that created a new function from and old one using types.FunctionType or something similar. I don't know how to do it off the top of my head so you'll probably need to do some sleuthing if you really want to try to make it work ...

jordanwh Over a year ago

@Tadhg McDonald-Jensen Perfect. Thank you.

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