List attributes of an object

Question

Is there a way to grab a list of attributes that exist on instances of a class? (This class is just an bland example, it is not my task at hand.)

class new_class():
    def __init__(self, number):
        self.multi = int(number) * 2
        self.str = str(number)

a = new_class(2)
print(', '.join(a.SOMETHING))

The desired result is that "multi, str" will be output. I want this to see the current attributes from various parts of a script.

I am using Python 3.

Answer 1

>>> ', '.join(i for i in dir(a) if not i.startswith('__'))
'multi, str'

This of course will print any methods or attributes in the class definition. You can exclude "private" methods by changing i.startwith('__') to i.startwith('_')

Answer 2

>>> class new_class():
...   def __init__(self, number):
...     self.multi = int(number) * 2
...     self.str = str(number)
... 
>>> a = new_class(2)
>>> a.__dict__
{'multi': 4, 'str': '2'}
>>> a.__dict__.keys()
dict_keys(['multi', 'str'])

You may also find pprint helpful.

Answer 3

What do you want this for? It may be hard to get you the best answer without knowing your exact intent.

• It is almost always better to do this manually if you want to display an instance of your class in a specific way. This will include exactly what you want and not include what you don't want, and the order will be predictable.

  If you are looking for a way to display the content of a class, manually format the attributes you care about and provide this as the __str__ or __repr__ method for your class.

• If you want to learn about what methods and such exist for an object to understand how it works, use help. help(a) will show you a formatted output about the object's class based on its docstrings.

• dir exists for programatically getting all the attributes of an object. (Accessing __dict__ does something I would group as the same but that I wouldn't use myself.) However, this may not include things you want and it may include things you do not want. It is unreliable and people think they want it a lot more often than they do.

• On a somewhat orthogonal note, there is very little support for Python 3 at the current time. If you are interested in writing real software you are going to want third-party stuff like numpy, lxml, Twisted, PIL, or any number of web frameworks that do not yet support Python 3 and do not have plans to any time too soon. The differences between 2.6 and the 3.x branch are small, but the difference in library support is huge.

Answer 4

dir(instance)
# or
instance.__dict__

Then you can test what type is with type() or if is a method with callable().