how to create local dynamic variables

Question

this is my code, I want to use eval() to get the rule status but eval() needs local variables, there is many classes that inherits the base class, so I need to rewrite get_stat() in every class.

I want to avoid this, an idea is to create dynamic variables in get_stat(),eg. class b dynamically creates variables a and b in func get_stat()

How should I create dynamic varables in function? or any other way to avoid this stupid idea. I use python 3.2.3, locals() does not work

class base(object):
    def check(self):
        stat = get_stat()
    def get_stat(self):
        pass

class b(base):
    rule = 'a > 5 and b < 3'
    a = 0
    b = 0

    def update_data(self, a, b):
        self.a = a
        self.b = b

    def get_stat(self):
        a = self.a
        b = self.b
        return eval(rule)

class b(base):
    rule = 'd > 5 and e < 3'
    d = 0
    e = 0

    def update_data(self, d, e):
        self.d = d
        self.e = e

    def get_stat(self):
        d = self.d
        e = self.e
        return eval(rule)

i don't see what's dynamic here! You're still re-writing rule for every class, you might as well just re-write the function — vikki
– vikki, Commented Nov 20, 2012 at 11:15
@vikki: I suppose it's dynamic in the sense that the rule attribute of each subclass can be changed on-the-fly and get_stat() will return results based on it's current value. You're right about the fact that rule could just as well be made a method, although the syntax needed to change it might not be as convenient or clean looking as would be this way. — martineau
– martineau, Commented Nov 20, 2012 at 15:09

codeape · Accepted Answer · 2012-11-20 11:14:19Z

2

You can pass a dictionary to the eval() function containing the variables to evaluate the expression against:

>>> eval("a + b", {"a": 2, "b": 3})
5

Pass self.__dict__ to give access to an object's attributes:

>>> class MyClass(object):
...     def __init__(self):
...             self.a = 2
...             self.b = 3
... 
>>> obj = MyClass()
>>> obj.__dict__
{'a': 2, 'b': 3}
>>> eval("a + b", obj.__dict__)
5

Links:

answered Nov 20, 2012 at 11:14

codeape

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2 Comments

martineau Over a year ago

I think to be crystal clear you should state that they could use return eval(rule, self.__dict__) or return eval(rule, vars(self)).

tripleee Over a year ago

Probably store these in an attribute, like self.variables = {"a": 2, "b": 3} and then the code can simply be eval(rule, self.variables)

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