Why is Python's 'exec' behaving oddly?

Question

Why does the following code work while the code after it breaks?

I'm not sure how to articulate my question in english, so I attached the smallest code I could come up with to highlight my problem.

(Context: I'm trying to create a terminal environment for python, but for some reason the namespaces seem to be messed up, and the below code seems to be the essence of my problem)

No errors:

d={}
exec('def a():b',d)
exec('b=None',d)
exec('a()',d)

Errors:

d={}
exec('def a():b',d)
d=d.copy()
exec('b=None',d)
d=d.copy()
exec('a()',d)

• coldspeed95: That's your hint to not use exec and eval ;-)
• PM 2Ring: Tricky. Note that if you do the exec that defines b before the one that defines a then the 2nd version works. Or if you get rid of the 1st copy (but I guess you already know that). I'm still not quite sure why the copying messes stuff up, but I guess it's has something to do with the fact that b is a global

Answer 1

It is because the d does not use the globals provided by exec; it uses the mapping to which it stored the reference in the first exec. While you set 'b' in the new dictionary, you never set b in the globals of that function.

>>> d={}
>>> exec('def a():b',d)
>>> exec('b=None',d)
>>> d['a'].__globals__ is d
True
>>> 'b' in d['a'].__globals__
True

vs

>>> d={}
>>> exec('def a():b',d)
>>> d = d.copy()
>>> exec('b=None',d)
>>> d['a'].__globals__ is d
False
>>> 'b' in d['a'].__globals__
False

---

If exec didn't work this way, then this too would fail:

mod.py

b = None
def d():
 b

main.py

from mod import d
d()

A function will remember the environment where it was first created.

---

It is not possible to change the dictionary that an existing function points to. You can either modify its globals explicitly, or you can make another function object altogether:

from types import FunctionType
def rebind_globals(func, new_globals):
 f = FunctionType(
 code=func.__code__,
 globals=new_globals,
 name=func.__name__,
 argdefs=func.__defaults__,
 closure=func.__closure__
 )
 f.__kwdefaults__ = func.__kwdefaults__
 return f
def foo(a, b=1, *, c=2):
 print(a, b, c, d)
# add __builtins__ so that `print` is found... 
new_globals = {'d': 3, '__builtins__': __builtins__}
new_foo = rebind_globals(foo, new_globals)
new_foo(a=0)

• Ryan Burgert: Thank you - this does help! Is there any way to go about injecting globals into a function? That way I could change the value of b in main.py in a way that b() would respond to
• Antti Haapala: @RyanBurgert added