How do I get the return value when using Python exec on the code object of a function?

Question

For testing purposes I want to directly execute a function defined inside of another function.

I can get to the code object of the child function, through the code (func_code) of the parent function, but when I exec it, i get no return value.

Is there a way to get the return value from the exec'ed code?

Answer 1

Yes, you need to have the assignment within the exec statement:

>>> def foo():
...     return 5
...
>>> exec("a = foo()")
>>> a
5

This probably isn't relevant for your case since its being used in controlled testing, but be careful with using exec with user defined input.

Answer 2

A few years later, but the following snippet helped me:

the_code = '''
a = 1
b = 2
return_me = a + b
'''

loc = {}
exec(the_code, globals(), loc)
return_workaround = loc['return_me']
print(return_workaround)  # 3

exec() doesn't return anything itself, but you can pass a dict which has all the local variables stored in it after execution. By accessing it you have a something like a return.

I hope it helps someone.

Answer 3

While this is the ugliest beast ever seen by mankind, this is how you can do it by using a global variable inside your exec call:

def my_exec(code):
    exec('global i; i = %s' % code)
    global i
    return i

This is misusing global variables to get your data across the border.

>>> my_exec('1 + 2')
3

Needless to say that you should never allow any user inputs for the input of this function in there, as it poses an extreme security risk.