Why can't I directly add attributes to any python object?

Question

I have this code:

>>> class G: ... def __init__(self): ... self.x = 20 ... >>> gg = G() >>> gg.x 20 >>> gg.y = 2000

And this code:

>>> from datetime import datetime >>> my_obj = datetime.now() >>> my_obj.interesting = 1 *** AttributeError: 'datetime.datetime' object has no attribute 'interesting'

From my Python knowledge, I would say that datetime overrides setattr/getattr, but I am not sure. Could you shed some light here?

EDIT: I'm not specifically interested in datetime. I was wondering about objects in general.

"I was wondering about objects in general." What? You show a general example of a general class which generally has attributes added. What does your edit mean? — S.Lott
– S.Lott, Commented Mar 8, 2009 at 19:13
It means that I was curious about all the classes, not just datetime. I posted this question because I saw that to some classes I could add attributes, while to others I couldn't. — Geo
– Geo, Commented Mar 8, 2009 at 20:54
@Geo: some classes are different -- there's no "general" rule. As your question notes -- some classes can and some classes can't. Since your question shows that there's no general rule, what are you asking? — S.Lott
– S.Lott, Commented Mar 8, 2009 at 22:30

Blair Conrad · Accepted Answer · 2009-03-08 13:36:44Z

34

My guess, is that the implementation of datetime uses __slots__ for better performance.

When using __slots__, the interpreter reserves storage for just the attributes listed, nothing else. This gives better performance and uses less storage, but it also means you can't add new attributes at will.

Read more here: http://docs.python.org/reference/datamodel.html

edited Mar 8, 2009 at 13:36

Blair Conrad

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answered Mar 8, 2009 at 12:54

Epcylon

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2 Comments

Jarret Hardie Over a year ago

Datetime is actually written in C, which gives behaviour very similar to writing a python object that uses slots. Slots are a way of writing objects in python that are almost as efficient as the C versions, without resorting to c

Jarret Hardie Over a year ago

So you could see the behaviour because the object is written in c, because it uses setattr, or because of slots :-)

S.Lott · Accepted Answer · 2009-03-08 13:07:39Z

It's written in C

http://svn.python.org/view/python/trunk/Modules/datetimemodule.c?view=markup

It doesn't seem to implement setattr.

s29 · Accepted Answer · 2013-03-26 03:04:34Z

While the question has already been answered; if anyone is interested in a workaround, here's an example --

mydate = datetime.date(2013, 3, 26) mydate.special = 'Some special date annotation' # doesn't work ... class CustomDate(datetime.date): pass mydate = datetime.date(2013, 3, 26) mydate = CustomDate(mydate.year, mydate.month, mydate.day) mydate.special = 'Some special date annotation' # works

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