可以动态地向对象添加属性吗？

Question

I would like to create an object then add attributes to the object on the fly. Here's some pseudocode EX1:

a = object()
a.attr1 = 123
a.attr2 = '123'
a.attr3 = [1,2,3]

EX2: the first page of this PDF

In Python is it possible to add attributes to an object on the fly (similar to the two examples I gave)? If yes, how?

Answer 1

If you are using Python 3.3+, use types.SimpleNamespace :

>>> import types
>>> a = types.SimpleNamespace()
>>> a.attr1 = 123
>>> a.attr2 = '123'
>>> a.attr3 = [1,2,3]
>>> a.attr1
123
>>> a.attr2
'123'
>>> a.attr3
[1, 2, 3]

Answer 2

In Python is it possible to add attributes to an object on the fly (similar to the two examples I gave)?

Yes.

If yes, how?

You could do:

class AttrHolder:
    pass

a = AttrHolder()

a.attr1 = 123
a.attr2 = '123'
a.attr3 = [1,2,3]

Or even something truly awful like:

import email #my choice of module is arbitrary

email.random_attribute = 'hello'

The unasked question: should you be doing this? Probably not. You're just using these things as stand-ins for a proper dict . If you want named attributes, consider a namedtuple .

A = namedtuple('Attribute_Holder',['attr1','attr2','attr3'])
a = A(123,'123',[1,2,3])

This provides a (loose) contract for A , and gives a nice repr :

In [70]: print(a)
Attribute_Holder(attr1=123, attr2='123', attr3=[1, 2, 3])

Or just use a dict . That's what they're for.

In fact, that's what you're doing when you 'add attributes on the fly', just with an unnecessary layer of abstraction. Consider, using the above AttrHolder :

In [77]: a = AttrHolder()

In [78]: a.__dict__
Out[78]: {}

In [79]: a.hi = 'hello'

In [80]: a.__dict__
Out[80]: {'hi': 'hello'}

You're using a dict whether you like it or not!

Answer 3

Here is an example:

class Test:
    pass
for mark, name in enumerate(("attr1", "attr2", "attr3")):
    setattr(Test, name, mark)
print Test.attr1
print Test.attr2
print Test.attr3

output:

0
1
2

So, the answer is "yes". Although it might not be such a good idea (could lead to problems such as losing track of the attributes or something).

Answer 4

Um, am I missing something? Take the following example:

class C():
   def __init__(self, x):
       self.myX = x

c = C(42)
c.attr1 = 123
print c.myX
print c.attr1

What seems to be the problem?

Answer 5

I use dictionaries for the exact purpose that you are describing (at least from my understanding of the question)...

dict = {}
dict["attr1"] = 123
dict["attr2"] = '123'
dict["attr3"] = [1,2,3]

dict.get("attr1")

Answer 6

If you really want to add attributes to an anonymous instances of an anonymous class , you could write:

>>> obj = type('obj', (), {})()
>>> obj.attr1 = 123
>>> obj.attr2 = '123'
>>> obj.attr3 = [1,2,3]

But, really, don't do this!