I am translating some code from lisp to Python.
In lisp, you can have a let construct with the variables introduced declared as special and thus having dynamic scope. (See http://en.wikipedia.org/wiki/Dynamic_scope#Dynamic_scoping)
How can I do likewise in Python? It seems the language does not support this directly, if true, what would be a good way to emulate it?
I feel Justice is plain right in his reasoning here.
On the other hand -- I can't resist implementing proof of concept for still another programing paradigm "unnatural" to Python -- I simply love doing this. :-)
So, I created a class whose objects'attributes are scopped just like you require (and can be created dynamically). As I said, it is just in a proof of concept state - but I think most usual errors, (like trying to access a variable ina scope it is not defined at all) should have errors raised, even if not the proper ones (IndexError due to a stack underflow instead of AttributeError, for example)
import inspect
class DynamicVars(object):
def __init__(self):
object.__setattr__(self, "variables", {})
def normalize(self, stackframe):
return [hash(tpl[0]) for tpl in stackframe[1:]]
def __setattr__(self, attr, value):
stack = self.normalize(inspect.stack())
d = {"value": value, "stack": stack}
if not attr in self.variables:
self.variables[attr] = []
self.variables[attr].append(d)
else:
our_value = self.variables[attr]
if our_value[-1]["stack"] == stack:
our_value[-1]["value"] = value
elif len(stack) <= len(our_value):
while our_value and stack != our_value["stack"]:
our_value.pop()
our_value.append(d)
else: #len(stack) > len(our_value):
our_value.append(d)
def __getattr__(self, attr):
if not attr in self.variables:
raise AttributeError
stack = self.normalize(inspect.stack())
while self.variables[attr]:
our_stack = self.variables[attr][-1]["stack"]
if our_stack == stack[-len(our_stack):]:
break
self.variables[attr].pop()
else:
raise AttributeError
return self.variables[attr][-1]["value"]
# for testing:
def c():
D = DynamicVars()
D.c = "old"
print D.c
def a():
print D.c
a()
def b():
D.c = "new"
a()
b()
a()
def c():
D.c = "newest"
a()
b()
a()
c()
a()
c()
2020 update - Another similar question showed up, and I crafted a hack that needs no special namespace objects (but which resorts to using inner things from cPython, like updating the locals() to actual variables: https://stackoverflow.com/a/61015579/108205 (works with Python 3.8)