hasattr() vs try-except block to deal with non-existent attributes

Question

if hasattr(obj, \'attribute\'):
    # do somthing

vs

try:
    # access obj.attribute
except AttributeError, e:
    # deal with Attr         


        

Answer 1

I almost always use hasattr: it's the correct choice for most cases.

The problematic case is when a class overrides __getattr__: hasattr will catch all exceptions instead of catching just AttributeError like you expect. In other words, the code below will print b: False even though it would be more appropriate to see a ValueError exception:

class X(object):
    def __getattr__(self, attr):
        if attr == 'a':
            return 123
        if attr == 'b':
            raise ValueError('important error from your database')
        raise AttributeError

x = X()
print 'a:', hasattr(x, 'a')
print 'b:', hasattr(x, 'b')
print 'c:', hasattr(x, 'c')

The important error has thus disappeared. This has been fixed in Python 3.2 (issue9666) where hasattr now only catches AttributeError.

An easy workaround is to write a utility function like this:

_notset = object()

def safehasattr(thing, attr):
    return getattr(thing, attr, _notset) is not _notset

This let's getattr deal with the situation and it can then raise the appropriate exception.