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Dynamic object attribute creation

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  Dynamic object attribute creation "ast" <nomail@invalid.com> - 2016-02-29 16:36 +0100
    Re: Dynamic object attribute creation Random832 <random832@fastmail.com> - 2016-02-29 11:06 -0500
    Re: Dynamic object attribute creation Ian Kelly <ian.g.kelly@gmail.com> - 2016-02-29 09:33 -0700

#103721 — Dynamic object attribute creation

Hello

Object's attributes can be created dynamically, ie

class MyClass:
    pass

obj = MyClass()
obj.test = 'foo'

but why doesn't it work with built-in classes int, float, list.... ?

L = [1, 8, 0]
L.test = 'its a list !'

(however lists are mutable, int, float ... are not)

Traceback (most recent call last):
  File "<pyshell#29>", line 1, in <module>
    L.test = 'its a list !'
AttributeError: 'list' object has no attribute 'test'

but it works on functions:

def funct(a,b):
....print(a+b)

funct.test = 'this is a function'

funct.test
'this is a function'

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#103726

On Mon, Feb 29, 2016, at 10:36, ast wrote:
> but why doesn't it work with built-in classes int, float, list.... ?
> 
> L = [1, 8, 0]
> L.test = 'its a list !'
> 
> (however lists are mutable, int, float ... are not)

Because those classes do not have attribute dictionaries, in order to
save space.

You can make a class without an attribute dictionary, by using
__slots__.

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#103731

On Mon, Feb 29, 2016 at 9:06 AM, Random832 <random832@fastmail.com> wrote:
> On Mon, Feb 29, 2016, at 10:36, ast wrote:
>> but why doesn't it work with built-in classes int, float, list.... ?
>>
>> L = [1, 8, 0]
>> L.test = 'its a list !'
>>
>> (however lists are mutable, int, float ... are not)
>
> Because those classes do not have attribute dictionaries, in order to
> save space.
>
> You can make a class without an attribute dictionary, by using
> __slots__.

You can also make a list that does have an attribute dictionary by
subclassing it.

>>> class MyList(list): pass
...
>>> obj = MyList()
>>> obj.test = 'foo'

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