preventing python object from adding variable

Question

How to prevent python object from adding variables

class baseClass1:
    count=0;
    def displayCount(self):
        print "Total Employee %d" % baseClass1.count;


base = baseClass1();
base.type = "class"; #  i want to throw an error here

It is (sort of) possible, but why do you feel you need to do it? — BrenBarn
– BrenBarn, Commented Dec 17, 2014 at 7:13
Unrelated to your problem, but you don't need to use semicolons at the end of the line. — viraptor
– viraptor, Commented Dec 17, 2014 at 7:17
@viraptor Semi-colons are, however, an optional part of the language. — Brōtsyorfuzthrāx
– Brōtsyorfuzthrāx, Commented Dec 17, 2014 at 8:10
@Shule I was just pointing out to the obvious python beginner that it's not the common style. Parentheses are also valid and optional, but nobody writes if ((((((((expression)))))))): — viraptor
– viraptor, Commented Dec 17, 2014 at 9:27

viraptor · Accepted Answer · 2014-12-17 07:14:57Z

5

You can use __slots__ - have a look at the documentation.

class baseClass1(object):
    __slots__ = ['count']

The exception thrown on unknown attribute will be AttributeError.

You have to make sure you use the new-style classes for this to work (explicitly inherit from object)

answered Dec 17, 2014 at 7:14

viraptor

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Comments

Not_a_Golfer · Accepted Answer · 2014-12-17 08:06:33Z

1

You can override the class' __setattr__ and do whatever checks you want. In this example I'm not allowing and setting of members that were not defined in the constructor. It will save you from manually maintaining that list.

class baseClass1:
        # allowed field names
        _allowed = set()

        def __init__(self): 
            # init stuff
            self.count=0
            self.bar = 3

            # now we "freeze the object" - no more setattrs allowed
            self._frozen = True

        def displayCount(self):

            print "Total Employee %d" % baseClass1.count;



        def __setattr__(self, name, value):

            # after the object is "frozen" we only allow setting on allowed field
            if  getattr(self, '_frozen', False) and name not in self.__class__._allowed:
                raise RuntimeError("Value %s not allowed" % name)
            else:
                # we add the field name to the allowed fields
                self.__class__._allowed.add(name)
                self.__dict__[name] = value



    base = baseClass1();
    base.count = 3 #won't raise
    base.bar = 2 #won't raise
    base.type = "class"; # throws

edited Dec 17, 2014 at 8:06

answered Dec 17, 2014 at 7:21

Not_a_Golfer

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6 Comments

pradeep_kv Over a year ago

your solution will throw exception when we are trying to change the count value. base.count =2 # also throws

Not_a_Golfer Over a year ago

@beginner of course it will. you can simply add a check that we're not changing count. BTW count should be a member and not a class member. I'll modify my code.

pradeep_kv Over a year ago

if we have more that ten variables are we going to put excuse for all those variables. Is there any other solution . Any one clarification will it work for if any static variables present

Not_a_Golfer Over a year ago

@beginner let's suppose that you want it to support what's set by the constructor and nothing else - I'll modify my answer here. Re static members - what exactly do you want to do? setattr is for object members, not class members.

pradeep_kv Over a year ago

static members that is clear for me now. Thanks for reply. We are including this if name !='count' but if we are having more than ten variables we cant use this right ? i am asking is there any solution to take of that also ?

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