parameters.py

# a sketch of how Scheme fluids/parameters would work in python

obarray = {}

class Manager(object):
    def __init__(self, parameter, value):
        self.parameter = parameter
        self.value = value

    def __enter__(self):
        self.old_value = self.parameter.get()
        obarray[self.parameter] = self.value

    def __exit__(self, exc_type, exc_value, traceback):
        ## A real pro would handle exceptions
        obarray[self.parameter] = self.old_value

class Parameter(object):
    def __init__(self, value):
        obarray[self] = value

    def get(self):
        return obarray[self]

    def __call__(self, new_value):
        return Manager(self, new_value)

current_foo = Parameter(10)

print "1", current_foo.get()

with current_foo(20):
    print "2", current_foo.get()

    with current_foo(30):
        print "3", current_foo.get()

    print "2", current_foo.get()

print "1", current_foo.get()