Frame hacking

[George Sakkis]

I wonder if the following is possible:

def inject_n_call(f unc, **kwds):
'''Call func by first updating its locals with kwds.'''

def f():
return x*y

>>inject_n_call (f, x=3, y=4)

12

I've been playing with sys.settrace, updating frame.f_locals in the
trace function, but it doesn't seem to work. Any other ideas ?

George

[Gabriel Genellina]

Re: Frame hacking

On 12 dic, 17:46, "George Sakkis" <george.sak...@ gmail.comwrote:

I wonder if the following is possible:
>
def inject_n_call(f unc, **kwds):
'''Call func by first updating its locals with kwds.'''
>
def f():
return x*y
>

>>eval(f.func_c ode, dict(x=3,y=4))

12

[George Sakkis]

Re: Frame hacking

Gabriel Genellina wrote:

On 12 dic, 17:46, "George Sakkis" <george.sak...@ gmail.comwrote:
>

I wonder if the following is possible:

def inject_n_call(f unc, **kwds):
'''Call func by first updating its locals with kwds.'''

def f():
return x*y

>

>eval(f.func_co de, dict(x=3,y=4))

12

Sweet! I think I just reinvented what eval does in this case by
fiddling with sys.settrace and frame.f_globals . Glad to trash my
20-line function for an one-liner :)

Regards,
George

[George Sakkis]

Re: Frame hacking

George Sakkis wrote:

Gabriel Genellina wrote:

On 12 dic, 17:46, "George Sakkis" <george.sak...@ gmail.comwrote:

I wonder if the following is possible:
>
def inject_n_call(f unc, **kwds):
'''Call func by first updating its locals with kwds.'''
>
def f():
return x*y
>

>>eval(f.func_c ode, dict(x=3,y=4))

12

>
Sweet! I think I just reinvented what eval does in this case by
fiddling with sys.settrace and frame.f_globals . Glad to trash my
20-line function for an one-liner :)

Actually I thought about this and it would be more convenient in my
case if I could change the "signature" of f to "def f(x,y)" so that I
can pass positional arguments instead of a keywords (don't ask why).
I've tried creating a new code object by tweaking co_varnames,
co_argcount, co_nlocals and making a new function out of it but it
doesn't work.. does co_code have to be changed as well, and if so, how?

George

[fumanchu]

Re: Frame hacking

George Sakkis wrote:

Actually I thought about this and it would be more convenient in my
case if I could change the "signature" of f to "def f(x,y)" so that I
can pass positional arguments instead of a keywords (don't ask why).
I've tried creating a new code object by tweaking co_varnames,
co_argcount, co_nlocals and making a new function out of it but it
doesn't work.. does co_code have to be changed as well, and if so, how?

Yes; since x and y then become locals and are looked up using LOAD_FAST
instead of LOAD_GLOBAL. Easiest approach: write the new function
yourself and pass it to dis.dis() and expreiment with the differences
in bytecode.

Here are some helpers I use for bytecode hacking (from the top of

http://projects.amor.org/dejavu/browser/trunk/codewalk.py):

from opcode import cmp_op, opname, opmap, HAVE_ARGUMENT
from types import CodeType, FunctionType, MethodType

from compiler.consts import *
CO_NOFREE = 0x0040


def named_opcodes(b its):
"""Change initial numeric opcode bits to their named
equivalents."""
bitnums = []
bits = iter(bits)
for x in bits:
bitnums.append( opname[x])
if x >= HAVE_ARGUMENT:
try:
bitnums.append( bits.next())
bitnums.append( bits.next())
except StopIteration:
break
return bitnums

def numeric_opcodes (bits):
"""Change named opcode bits to their numeric equivalents."""
bitnums = []
for x in bits:
if isinstance(x, basestring):
x = opmap[x]
bitnums.append( x)
return bitnums

_deref_bytecode = numeric_opcodes (['LOAD_DEREF', 0, 0, 'RETURN_VALUE'])
# CodeType(argcou nt, nlocals, stacksize, flags, codestring, constants,
# names, varnames, filename, name, firstlineno,
# lnotab[, freevars[, cellvars]])
_derefblock = CodeType(0, 0, 1, 3, ''.join(map(chr , _deref_bytecode )),
(None,), ('cell',), (), '', '', 2, '',
('cell',))
def deref_cell(cell ):
"""Return the value of 'cell' (an object from a func_closure)." ""
# FunctionType(co de, globals[, name[, argdefs[, closure]]])
return FunctionType(_d erefblock, {}, "", (), (cell,))()


Robert Brewer
System Architect
Amor Ministries
fumanchu@amor.o rg