Python's hasattr on list values of dictionaries always returns false?

Question

I have a dictionary that sometimes receives calls for non-existent keys, so I try and use hasattr and getattr to handle these cases:

key_string = 'foo' print "current info:", info print hasattr(info, key_string) print getattr(info, key_string, []) if hasattr(info, key_string):     array = getattr(info, key_string, []) array.append(integer) info[key_string] = array print "current info:", info 

The first time this runs with integer = 1:

current info: {} False [] current info: {'foo': [1]} 

Running this code again with integer = 2:

instance.add_to_info("foo", 2)  current info: {'foo': [1]} False [] current info: {'foo': [2]} 

The first run is clearly successful ({'foo': [1]}), but hasattr returns false and getattr uses the default blank array the second time around, losing the value of 1 in the process! Why is this?

Answer 1

hasattr does not test for members of a dictionary. Use the in operator instead, or the .has_key method:

>>> example = dict(foo='bar') >>> 'foo' in example True >>> example.has_key('foo') True >>> 'baz' in example False 

But note that dict.has_key() has been deprecated, is recommended against by the PEP 8 style guide and has been removed altogether in Python 3.

Incidentally, you'll run into problems by using a mutable class variable:

>>> class example(object): ...     foo = dict() ... >>> A = example() >>> B = example() >>> A.foo['bar'] = 'baz' >>> B.foo {'bar': 'baz'} 

Initialize it in your __init__ instead:

class State(object):     info = None      def __init__(self):         self.info = {} 

Answer 2

A dictionary key is not the same as an object attribute

thing1 = {'a', 123} hasattr(thing1, 'a') # False class c: pass thing2 = c() thing2.a = 123 hasattr(thing2, 'a') # True