I have found some vaguely related questions to this question, but not any clean and specific solution for CPython. And I assume that a "valid" solution is interpreter specific.
First the things I think I understand:
locals()
gives a non-modifiable dictionary.frame.f_locals
gives a locals()
like dictionary, but less prone to hackish things through exec
. Or at least I have been less able to do hackish undocumented things like the locals()['var'] = value ; exec ""
exec
is capable to do weird things to the local variables, but it is not reliable --e.g. I read somewhere that it doesn't work in Python 3. Haven't tested.So I understand that, given those limitations, it will never be safe to add extra variables to the locals, because it breaks the interpreter structure.
However, it should be possible to change a variable already existing, isn't it?
Things that I considered
f
, one can access the f.func_code.co_nlocals
and f.func_code.co_varnames
.frame.f_locals
. This is in the use case of setting a tracer through sys.settrace
.The variables should be somewhere, preferably writeable... but I am not capable of finding it. Even if it is an array (for interpreter efficient access), or I need some extra C-specific wiring, I am ready to commit to it.
How can I achieve that modification of variables from a tracer function or from a decorated wrapped function or something like that?
A full solution will be of course appreciated, but even some pointers will help me greatly, because I'm stuck here with lots of non writeable dictionaries :-/
Edit: Hackish exec
is doing things like this or this
Based on the notes from MariusSiuram, I wrote a recipe that show the behavior.
The conclusions are:
So, here is the code:
import inspect
import ctypes
def parent():
a = 1
z = 'foo'
print('- Trying to add a new variable ---------------')
hack(case=0) # just try to add a new variable 'b'
print(a)
print(z)
assert a == 1
assert z == 'foo'
try:
print (b)
assert False # never is going to reach this point
except NameError, why:
print("ok, global name 'b' is not defined")
print('- Trying to remove an existing variable ------')
hack(case=1)
print(a)
assert a == 2
try:
print (z)
except NameError, why:
print("ok, we've removed the 'z' var")
print('- Trying to update an existing variable ------')
hack(case=2)
print(a)
assert a == 3
def hack(case=0):
frame = inspect.stack()[1][0]
if case == 0:
frame.f_locals['b'] = "don't work"
elif case == 1:
frame.f_locals.pop('z')
frame.f_locals['a'] += 1
else:
frame.f_locals['a'] += 1
# passing c_int(1) will remove and update variables as well
# passing c_int(0) will only update
ctypes.pythonapi.PyFrame_LocalsToFast(
ctypes.py_object(frame),
ctypes.c_int(1))
if __name__ == '__main__':
parent()
The output would be like:
- Trying to add a new variable ---------------
1
foo
ok, global name 'b' is not defined
- Trying to remove an existing variable ------
2
foo
- Trying to update an existing variable ------
3
It exists an undocumented C-API call for doing things like that:
PyFrame_LocalsToFast
There is some more discussion in this PyDev blog post. The basic idea seems to be:
import ctypes
...
frame.f_locals.update({
'a': 'newvalue',
'b': other_local_value,
})
ctypes.pythonapi.PyFrame_LocalsToFast(
ctypes.py_object(frame), ctypes.c_int(0))
I have yet to test if this works as expected.
Note that there might be some way to access the Fast
directly, to avoid an indirection if the requirements is only modification of existing variable. But, as this seems to be mostly non-documented API, source code is the documentation resource.
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