Injecting new names into the above frame

Peter Waller peter.waller at gmail.com
Fri Sep 12 14:44:57 CEST 2008


Dear Pythoners,

I know this will probably be perceived as 'evil voodoo', and fair
enough: it probably is. I guess it is unpythonic.

.. but I want to know how to do it anyway - mostly for my own
interest.

Consider the following snippet of code:

---
def Get( *names ):
    if not names: return None

    frame = sys._getframe(1)
    prevFrameLocals = frame.f_locals

    for name in names:
        prevFrameLocals[ name ] = FetchObjectNamed( name )

Get("a", "b", "c")

print a, b, c
---

FetchObjectNamed() is an arbitrary function which takes a string and
returns an object it got from some store somewhere.

This works fine at the module level, because names in the locals/
globals dictionary can be played with in this way. The idea is to save
lots of typing, i.e.

a, b, c = Get("a","b","c")

..gets frustrating after much typing for many objects with long names.
This is just an example, there are other instances I have where it
would be nice to inject names into the frame above.

Of course, we hit a road block when we call 'Get' from a function
rather than a module, because the locals dictionary does not get
copied back into the code object automatically, so we have to add this
snippet before the Get() function returns:

from ctypes import pythonapi, py_object, c_int
pythonapi.PyFrame_LocalsToFast( py_object( frame ), 1 )

This copies back the names into the code object, and works fine.. that
is, if the names already exist within the code object.

def MyFunction():
    a = None
    Get("a")
    print a # Works
    Get("b")
    print b # Name error, b is undefined

Is there any way for Get() to define a new variable within
MyFunction's code object? Or is there any programmatic way to, at
runtime, insert new names into functions?

I don't care how hacky it is and whether it requires making calls to
python's internals with ctypes - maybe the whole code object needs to
be replaced? is it even possible to do that when the Get() function is
about to return to this new code object?

Cheers,

- Peter



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