Python equivalent of JavaScript's `Object.defineProperty()`

Question

As I transition from JavaScript to Python, I noticed I haven't figured out a way to add properties to the data type classes. For example, in JavaScript, if I wanted to be able to type arr.last and have it return the last element in the array arr, or type arr.last = 'foo' and to set the last element to 'foo', I would use:

Object.defineProperty(Array.prototype,'last',{ get:function(){ return this[this.length-1]; }, set:function(val){ this[this.length-1] = val; } }); var list = ['a','b','c']; console.log(list.last); // "c" list.last = 'd'; console.log(list); // ["a","b","d"]

However, in Python, I'm not sure how to do the equivalent of Object.defineProperty(X.prototype,'propname',{get:function(){},set:function(){}});

Note: I am not asking for how to do the specific example function, I am trying to be able to define a property with a get and set onto the primitive data types (str, int, float, list, dict, set, etc.)

Perhaps you want to read about the standard types? You can do this stuff directly in Python, with no messy bits getting in the way. In other words, in trying to define properties and getters/setters, you probably have an XY Problem. — TigerhawkT3
– TigerhawkT3, Commented Sep 4, 2015 at 0:51

Community · Accepted Answer · 2017-05-23 11:52:17Z

In Python 2¹, adding new attributes (aka member objects, including methods) to a new-style class (one that derives from object) is as easy as simply defining them:

class Foo(object): def __init__(self): self._value = "Bar" def get_value(self): return self._value def set_value(self, val): self._value = val def del_value(self): del self._value Foo.value = property(get_value, set_value, del_value) f = Foo() print f.value f.value = "Foo" print f.value

I use the property builtin that Dan D. mentioned in his answer, but this actually assigns the attribute after the class is created, like the question asks.

Online demo

^{1: in Python 3, it's even simpler, since all classes are new-style classes}

Dan D. · Accepted Answer · 2015-09-04 00:54:47Z

See the documentation of the property function. It has examples. The following is the result of print property.__doc__ under Python 2.7.3:

property(fget=None, fset=None, fdel=None, doc=None) -> property attribute fget is a function to be used for getting an attribute value, and likewise fset is a function for setting, and fdel a function for del'ing, an attribute. Typical use is to define a managed attribute x: class C(object): def getx(self): return self._x def setx(self, value): self._x = value def delx(self): del self._x x = property(getx, setx, delx, "I'm the 'x' property.") Decorators make defining new properties or modifying existing ones easy: class C(object): @property def x(self): return self._x @x.setter def x(self, value): self._x = value @x.deleter def x(self): del self._x

This does not add existing functionality at runtime, like OP asked.

Community · Accepted Answer · 2017-05-23 11:59:09Z

0

If I understood you correctly, you want to edit existing classes (add method) Check out this thread Python: changing methods and attributes at runtime

edited May 23, 2017 at 11:59

CommunityBot

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answered Sep 4, 2015 at 0:50

Anthony N.

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2 Comments

Cyoce Over a year ago

I'm not sure if I understand that post correctly, but it appears that that is making a new class that can have its attributes modified, as opposed to what I want (being able to modify the attributes of a class that was defined with Python (e.g. int)

user2864740 Over a year ago

@Cyoce Not all classes are equal in Python, largely due to performance/implementation considerations.

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