Python exec() in other context

Question

I would like to exec() code in the context of another function. Just doing it naively like

def f1():
 exec("b = 5")
def f2():
 f1()
 print(b)
f2()

won't work because b is not defined in f2:

Traceback (most recent call last):
 File "/tmp/s.py", line 10, in <module>
 f2()
 File "/tmp/s.py", line 7, in f2
 print(b)
NameError: name 'b' is not defined

I do not want to pass "b = 5" to f2.

Is it perhaps possible to pass the context of f2 to exec? Any hints?

• Sayse: You've completely changed your code sample since I posted that comment

Answer 1

You can give exec contexts through the two other parameters it supports:

exec(object[, globals[, locals]])

So to marshal the context from the previous function, include those as well. For example to marshal the local context only:

>>> def f1(code, ctx):
... exec(code, {}, ctx)
>>> def f2():
... a = 42
... f1("print(a + 1)", locals())
>>> f2()
43

To work with the modified context in future calls, own the context you're sending in yourself:

>>> def f2():
... ctx = {'a': 42}
... f1("a += 1", ctx)
... print(ctx)
...
>>> f2()
{'a': 43}

• Nico Schlömer: Thanks Mats. Thats already great for getting context into exec! How would you get something out of exec?
• MatsLindh: You own the context instead, i.e. you give a dict that you keep a reference to, and that dict will be modified as the context. I've added an example to my answer.

Answer 2

You could do something like this

class Context:
 pass
def f1(code, c):
 exec(code)
def f2():
 c = Context()
 c.a = 42
 f1("print(c.a + 1)", c)
f2()

where you add the variable to a context class and pass that along.