Question

I have a function P(). A call to load_variables() should give P the variable x.
load_variablesshould be able to accept defaults as keyword arguments.

How can this be done?

I have tried the following:

import inspect
def P():
    x = 1
    load_variables(x = 2)
    return x

def load_variables(**kargs):
    stack = inspect.stack()
    try:
        locals_ = stack[1][0].f_locals
    finally:
        del stack
    for __k, __v in kargs.iteritems():
        locals_[__k] = __v

print P() # => should print 2

The x = 1 line shouldn’t actually be there, as I want load_variables() to just bleed the x into P‘s scope.

Is there another, perhaps better, way to do this? What I want is:

1. Variables have a default value, e.g. x = 2 in the above call to load_variables().
2. I can overwrite these in load_variables, for instance, load_varibales() has access to a dictionary of variables, and if x is already here, we overwrite it, and spill this x instead of the one given as default argument.

Answer 1

The Python compiler and bytecode interpreter handles, where possible, references to local variables as slots into a defined-size array. This means that if a local variable is not initialised (assigned to) in a scope then the language will not know that a slot for the variable exists in the scope, and will instead look for the variable in an enclosing scope or the global scope. Looking at a disassembly of the function P:

def P():
    load_variables(x=2)
    return x

dis.dis(P)
  2           0 LOAD_GLOBAL              0 (load_variables)
              3 LOAD_CONST               1 ('x')
              6 LOAD_CONST               2 (2)
              9 CALL_FUNCTION          256
             12 POP_TOP             

  3          13 LOAD_GLOBAL              1 (x)
             16 RETURN_VALUE        

You can see that without an assignment to x in local scope P will look for it in global scope.

The right way to do this is to explicitly state which variables you expect the load_variables function to return, using unpacking:

x, y, z = load_variables(...)