How to find out if object has attributes?

Question

 if groupName.group == "None":

error:

AttributeError: 'NoneType' object has no attribute 'group'

How to check if object has an attribute?

Answer 1

你想要getattr() ，你可以传递一个默认值，或者hasattr() 。

Answer 2

The error message is telling you that groupName itself is None .

In which case, there's little point in testing whether it has a particular attribute.

So you probably want something more like:

If groupName is not None:
    print groupName.group

Or, if groupName objects may not have a group attribute:

If groupName is not None:
    print getattr(groupName, 'group', None)

(Note: the last argument of getattr is a default value that can be anything you want).

Answer 3

Python follows the EAFP motto, so it is claimed to be better style just to try and handle an exception.

So try

try:
    gr = groupName.group
except AttributeError:
    complain()
else:
    if gr == "None":
        go_on()

But beware: I'm not sure if your == "None" is what you want. Maybe is None would be the thing...

Answer 4

It depends on what you call "to have an attribute".

.

hasattr()

If you want to verify if an expression Xa will return something or raise an error, you can use hasattr()

def f(x,y):
    return x+y
f.a = 'suming function'
print "hasattr(f,'a') : ",hasattr(f,'a'),'     f.a : ',f.a
print "hasattr(f,'sdjk') : ",hasattr(f,'sdjk'),'     f.sdjk : ',f.sdjk

produces

hasattr(f,'a') :  True      f.a :  suming function
hasattr(f,'sdjk') :  False      f.sdjk : 

Traceback (most recent call last):
  File "I:\all\aaaaa.py", line 26, in <module>
    print "hasattr(f,'sdjk') : ",hasattr(f,'sdjk'),'     f.sdjk : ',f.sdjk
AttributeError: 'function' object has no attribute 'sdjk'

.

__dict__

But it may happen that calling an attribute on an instance with a certain name returns something while there is not really this name in the namespace of the instance, that is to say no item name:object in the dictionary of this instance's attributes.

It's the case with a class attribute : a class attribute is returned when a call of its name is done on one of the instances of the class, while this name doesn't exist in the namespace of the instance. See example:

class Tomo:
    count = 0
    def __init__(self):
        Tomo.count += 1
    pass

ti = Tomo()
ta = Tomo()
tu = Tomo()
ty = Tomo()
print 'ty.count ==',ty.count,"       hasattr(ty,'count') ==",hasattr(ty,'count')
print 'ty.__dict__ ==',ty.__dict__,"   'count' in ty.__dict__ ==",'count' in ty.__dict__
print

result

ty.count == 4        hasattr(ty,'count') == True
ty.__dict__ == {}    'count' in ty.__dict__ == False    

So
hasattr(ty,'count') says "YES"
while 'count' in ty.__dict__ says "NO, ty doesn't have an attribute count " .

.

Then , asking hasattr(object,name) gives less precise answer on the state of the real objects in a program than asking name in object.__dict__ , though hasattr() gives more pertinent information concerning the treatment of data that is the practical aim of running a program.

So , I think that use of hasattr() is rather associated to the algorithm , and use of __dict__ rather associated to the comprehension of the underlying mechanisms of a running program.

.

dir()

Theoretically dir(object) gives the keys of the dictionnary __dict__ of the object.
But

dir([object]) Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object. If the object does not provide __dir__() , the function tries its best to gather information from the object's __dict__ attribute, if defined, and from its type object

Hence, there is the same problem with dir() than with hasattr() : it gives more attributes than the only ones being exactly in an object and not in its parent classes.

.

---

.

By the way, I presume it will be OK if you do:

if hasattr(groupName,'group') and groupName.group is None:
    ..........
    .....
    ........

It is said that it's better to use is for testing None than ==

Answer 5

In python there's a motto: Better to Say Sorry than Ask Permission. I would suggest to find out what are your needs and eventually do something like:

try:
  # do something with groupName.group

except SomeException, e: ...
  # act some other way