Peter Waller wrote:
> 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.
Well, if you're really just asking out of curiosity, it should be
sufficient to tell you that this is not possible.
> [...] 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
The answer to why this doesn't work lies in the disassembly of that
function:
0 LOAD_CONST 0 (0)
3 STORE_FAST 0 (0)
6 LOAD_GLOBAL 1 (1)
9 LOAD_CONST 1 (1)
12 CALL_FUNCTION 1
15 POP_TOP
16 LOAD_FAST 0 (0) <- This is a
19 PRINT_ITEM
20 PRINT_NEWLINE
21 LOAD_GLOBAL 1 (1)
24 LOAD_CONST 2 (2)
27 CALL_FUNCTION 1
30 POP_TOP
31 LOAD_GLOBAL 2 (2) <- This is b
34 PRINT_ITEM
35 PRINT_NEWLINE
36 LOAD_CONST 0 (0)
39 RETURN_VALUE
Since you have an assignment to the name a, a is recognized as a local
name at compile time. b is not recognized as a local name at compile
time, so even if you inject a value for b into the locals dictionary,
the byte code still looks up the value as a global name.
> 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?
Not without making fundamental changes to Python itself. The fact that
this isn't possible is a feature, in my opinion. I like the fact that I
can call a function and be *absolutely certain* that it's not going to
pollute my local namespace.
Hope this helps,
--
Carsten Haese
http://informixdb.sourceforge.net
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