How to know if an object has an attribute in Python

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 1

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 2

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()