How do you programmatically set an attribute?

Question

Suppose I have a python object x and a string s, how do I set the attribute s on x? So:

>>> x = SomeObject()
>>> attr = 'myAttr'
>>> # magic goes here
>>> x.myAttr
'magic'

What's the magic? The goal of this, incidentally, is to cache calls to x.__getattr__().

score 372 · Accepted Answer · edited Jul 09 '21 at 19:40

372

setattr(x, attr, 'magic')

For help on it:

>>> help(setattr)
Help on built-in function setattr in module __builtin__:

setattr(...)
    setattr(object, name, value)
    
    Set a named attribute on an object; setattr(x, 'y', v) is equivalent to
    ``x.y = v''.

However, you should note that you can't do that to a "pure" instance of object. But it is likely you have a simple subclass of object where it will work fine. I would strongly urge the O.P. to never make instances of object like that.

edited Jul 09 '21 at 19:40

mkrieger1

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answered Nov 12 '08 at 19:38

Ali Afshar

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Careful, however, this doesn't work in your scenario where you're creating an instance of object(). – S.Lott Nov 12 '08 at 19:42
4

Absolutely right, it doesn't. I conveniently ignored that. I would strongly urge the O.P. to never make instances of object like that. – Ali Afshar Nov 12 '08 at 19:44
3

Damn shame it doesn't work in all cases, as that would be really useful, for example, for adding the `dirty` attribute to user input... – brice Feb 28 '12 at 18:32
1

@Brice: setattr works in almost all cases. For efficiency and other reasons, 'object' is programmed so that you cannot add extra attributes to it. You can do this with your own class with the `__slots__` attribute. – dirkjot Mar 21 '12 at 22:10
4

This does not work on `int` as well. Can you explain why? (is it on all `__builtin__`'s? – Jens Timmerman Sep 05 '12 at 15:59
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@JensTimmerman: Practically speaking, because integers are supposed to be immutable. The implementation [interns](https://en.wikipedia.org/wiki/String_interning) small integers, strings, and some other builtin immutable types. If you were allowed to set arbitrary attributes on those types, this interning would break in a variety of subtle and confusing ways. – Kevin Oct 10 '16 at 23:47
One should be able to subclass `int` using the `type(cls, (int,), kwargs)` pattern? (I subclass `float` on a regular basis and override `__init__` in the custom float to add custom attributes) – cowbert Dec 12 '17 at 21:35

score 62 · Answer 2 · edited Jan 26 '12 at 04:45

62

Usually, we define classes for this.

class XClass( object ):
   def __init__( self ):
       self.myAttr= None

x= XClass()
x.myAttr= 'magic'
x.myAttr

However, you can, to an extent, do this with the setattr and getattr built-in functions. However, they don't work on instances of object directly.

>>> a= object()
>>> setattr( a, 'hi', 'mom' )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'hi'

They do, however, work on all kinds of simple classes.

class YClass( object ):
    pass

y= YClass()
setattr( y, 'myAttr', 'magic' )
y.myAttr

edited Jan 26 '12 at 04:45

Vincenzooo

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answered Nov 12 '08 at 19:41

S.Lott

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Any insight on why this does not work with instances of object()? – meawoppl Apr 07 '13 at 17:14
@meawoppl You should ask that as a [new question](http://stackoverflow.com/questions/ask) – Tobias Kienzler Apr 30 '15 at 10:43
2

can I do this with Modules, instead of Classes? – bonobo Jun 26 '18 at 14:12
The link in @jalanb's comment is 404. – Reid Nov 24 '20 at 16:47

score 23 · Answer 3 · answered Oct 28 '13 at 14:37

23

let x be an object then you can do it two ways

x.attr_name = s 
setattr(x, 'attr_name', s)

answered Oct 28 '13 at 14:37

vijay shanker

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score 2 · Answer 4 · answered Aug 28 '20 at 08:30

2

Also works fine within a class:

def update_property(self, property, value):
   setattr(self, property, value)

answered Aug 28 '20 at 08:30

d.raev

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score -2 · Answer 5 · edited Aug 09 '22 at 17:03

-2

If you want a filename from an argument:

import sys

filename = sys.argv[1]

file = open(filename, 'r')

contents = file.read()

If you want an argument to show on your terminal (using print()):

import sys

arg = sys.argv[1]

arg1config = print(arg1config)

edited Aug 09 '22 at 17:03

Cassova

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answered Jul 28 '22 at 08:49

Coder100

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Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Jul 29 '22 at 22:43

How do you programmatically set an attribute?

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