How to know if an object has an attribute in Python – Dev

The best answers to the question “How to know if an object has an attribute in Python” in the category Dev.

QUESTION:

Is there a way in Python to determine if an object has some attribute? For example:

>>> a = SomeClass()
>>> a.someProperty = value
>>> a.property
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: SomeClass instance has no attribute 'property'

How can you tell if a has the attribute property before using it?

ANSWER:

As Jarret Hardie answered, hasattr will do the trick. I would like to add, though, that many in the Python community recommend a strategy of “easier to ask for forgiveness than permission” (EAFP) rather than “look before you leap” (LBYL). See these references:

EAFP vs LBYL (was Re: A little disappointed so far)
EAFP vs. LBYL @Code Like a Pythonista: Idiomatic Python

ie:

try:
    doStuff(a.property)
except AttributeError:
    otherStuff()

… is preferred to:

if hasattr(a, 'property'):
    doStuff(a.property)
else:
    otherStuff()

ANSWER:

Try hasattr():

if hasattr(a, 'property'):
    a.property

See zweiterlinde’s answer below, who offers good advice about asking forgiveness! A very pythonic approach!

The general practice in python is that, if the property is likely to be there most of the time, simply call it and either let the exception propagate, or trap it with a try/except block. This will likely be faster than hasattr. If the property is likely to not be there most of the time, or you’re not sure, using hasattr will probably be faster than repeatedly falling into an exception block.

ANSWER:

I think what you are looking for is hasattr. However, I’d recommend something like this if you want to detect python properties

try:
    getattr(someObject, 'someProperty')         
except AttributeError:
    print "Doesn't exist"
else
    print "Exists"

The disadvantage here is that attribute errors in the properties __get__ code are also caught.

Otherwise, do-

if hasattr(someObject, 'someProp'):
    #Access someProp/ set someProp
    pass

Docs:http://docs.python.org/library/functions.html
Warning:
The reason for my recommendation is that hasattr doesn’t detect properties.
Link:http://mail.python.org/pipermail/python-dev/2005-December/058498.html

ANSWER:

You can use hasattr() or catch AttributeError, but if you really just want the value of the attribute with a default if it isn’t there, the best option is just to use getattr():

getattr(a, 'property', 'default value')