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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?
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Definition. Python hasattr() function is used to return value TRUE after checking whether the object has the given attribute, else return FALSE. The Python hasattr() built-in function is used to check if the specified object has the named attribute present within a class or not.
It definitely can have a list and any object as value but the dictionary cannot have a list as key because the list is mutable data structure and keys cannot be mutable else of what use are they.
values() method returns a view object that displays a list of all values in a given dictionary.
Note that the restriction with keys in Python dictionary is only immutable data types can be used as keys, which means we cannot use a dictionary of list as a key . But the same can be done very wisely with values in dictionary. Let's see all the different ways we can create a dictionary of Lists.
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 = {} 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 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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