does python 2.5 have an equivalent to Tcl's uplevel command?

Question

Does python have an equivalent to Tcl's uplevel command? For those who don't know, the "uplevel" command lets you run code in the context of the caller. Here's how it might look in python:

def foo():
    answer = 0
    print "answer is", answer # should print 0
    bar()
    print "answer is", answer # should print 42


def bar():
    uplevel("answer = 42")

It's more than just setting variables, however, so I'm not looking for a solution that merely alters a dictionary. I want to be able to execute any code.

Answer 1

In general, what you ask is not possible (with the results you no doubt expect). E.g., imagine the "any code" is x = 23. Will this add a new variable x to your caller's set of local variables, assuming you do find a black-magical way to execute this code "in the caller"? No it won't -- the crucial optimization performed by the Python compiler is to define once and for all, when def executes, the exact set of local variables (all the barenames that get assigned, or otherwise bound, in the function's body), and turn every access and setting to those barenames into very fast indexing into the stackframe. (You could systematically defeat that crucial optimization e.g. by having an exec '' at the start of every possible caller -- and see your system's performance crash through the floor in consequence).

Except for assigning to the caller's local barenames, exec thecode in thelocals, theglobals may do roughly what you want, and the inspect module lets you get the locals and globals of the caller in a semi-reasonable way (in as far as deep black magic -- which would make me go postal on any coworker suggesting it be perpetrated in production code -- can ever be honored with the undeserved praise of calling it "semi-reasonable", that is;-).

But you do specify "I want to be able to execute any code." and the only solution to that unambiguous specification (and thanks for being so precise, as it makes answering easier!) is: then, use a different programming language.

Answer 2

Is the third party library written in Python? If yes, you could rewrite and rebind the function "foo" at runtime with your own implementation. Like so:

import third_party

original_foo = third_party.foo
def my_foo(*args, **kwds):
    # do your magic...
    original_foo(*args, **kwds)
third_party.foo = my_foo

I guess monkey-patching is slighly better than rewriting frame locals. ;)

Answer 3

After some experimentation I have something that somewhat works. It won't let me set variables that are local, but it does let me set instance variables and call methods that belong to the caller which -- I think -- is all I need for my current problem.

Can anyone improve on this?

def uplevel(script):
    '''emulate Tcl's 'uplevel' command'''
    import sys
    f = sys._getframe(2)
    locals = f.f_locals
    globals = f.f_globals
    exec script in globals, locals